Corn (Zea mays L.) Production in a Grass/CIover Living Mulch System
by
Philip R Greyson
Submitted in partial fùlfillment of the requirements for the degree of Master of Science
at Nova Scotia Agricultural College Truro, Nova Scotia, in cooperation with Dalhousie University Halifax, Nova Scotia
March, 1998
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TABLE OF CONTENTS
LIST OF TABLES vii LIST OF FIGURES LIST OF ABBREVIATIONS xi ABSTRACT xii .- - ACKNOWEDGMENTS xii1
BJTRODUCTION 1 LITERATURE REVIEW 2.1 Corn
2.2 Living Mulches 2.2.1 Mulch suppression 2.2.1.I Chernical suppression 2.2.1.2Mechanical suppression 2.2.1.3 Combined mechanical and chernical suppression 2.3 Soil Physical Properties 2.3.1 Soil moisture
2.3 -2 Soi1 temperature 2.3.3 Soil nitrogen response OBJECTIVES METHODOLOGY
4.1 Background and Experimentai Design 4.2 Treatment Factors
4.2.1 Living mulch treatments 4.2.2 Control treatments
4.2.3 Nitrogen treatments 4.2.4 Imgation treatments 4.2.5 Straw mulch treatments
4.3 Field Preparation and Sampling 4.3.1 1995 Protocol 4.3.2 1996 Protocol 4.4 Statisticai Analysis 4.4.1 Yield and mulch analysis 4.4.2 Irrigation andysis 4.4.3 Frost analysis
4.5 Meteorological Conditions RESULTS
5.1 Corn Yields and Harvest Index 5.2 Imgation 5.3 Corn Growth Parameters
5 -4 Living Mulch 5.4.1 Mulch totals (between-row) 5.4.2 Mulch totals (within-row)
5.4.3 Living mulch composition
5.5 Frost Effects 5.6 Soil Moisture DISCUSSION 6.1 Corn Yields
6.2 Nitrogen Effects
6.3 Imgation 6.4 Soil Moisture 6.5 Frost
6.6 Straw Mulch 6.7 Methodology Improvements 6-7Conclusions LITERATURE CITED LIST OF TABLES
Table 1 Summary of treatrnent combinations including treatment designation, nitrogen rate and mulch suppression description
Table 2 Schedule of irrigation and average amount of irrigation water (L) applied to CONTROL and TILWHERB treatments in 1995 and 1996
Table 3 Surnrnary of field activities for 1995 and 1996
Table 4 Average total corn yield (kg DM ha-') and harvest index (HI) for 1995 and 1996 as affected by mulch suppression and nitrogen (N"). 39
Table 5 Total corn yield (kg DM ha-') averaged over rnulch suppression treatments in 1995 and 1996 under imgated and rainfed conditions. 40
Table 6 Final corn heights and number of days £tom seeding to silking for 1995 and 1996 as affected by mulch suppression and nitrogen (N). 4 1
Table 7 Between-row mulch dry matter (kg DM ha-' ) on five sampling dates as afEected by mulch suppression and nitrogen (IV) in 1995.
Table 8 Between-row mulch dry matter (kg DM ha-' ) on four sampling dates as affected by mulch suppression and nitrogen (N) in 1996
Table 9 Within-row mulch dry matter (kg DM ha" ) on five sarnpling dates as afEected by mulch suppression and nitrogen (N) in 1995.
Table 10 Within-row dry mattw(kg DM ha-' ) on four sampling dates as af5ected by mulch suppression and nitrogen (N) in 1996.
Table 1 1 Mulch composition (%) on two sampling dates as affected by mulch suppression and nitrogen (N) in 1995.
Table 12 Mulch composition (%) on four sampling dates as afTected by mulch suppression and nitrogen (N) in 1996.
Table 13 Frost damage scores as affected by mulch suppression and nitrogen (N) for the fiost event on June 22, 1996.
vii Table 14 Soi1 resistance to penetration (bars) under mulch and no mulch (CON'TROL) in October 1996. LIST OF FIGURES
Figure 1 Layout of expenmental plots in the Brookside field in 1995 and 1996 (See Table 1 for treatrnent definitions.). Experimentai units are 8 rn X 3.1 m.
Figure 2 Average May-September mont hly precipitation (mm) for 1995 and 1996 in Brookside, N.S. compared to 30 normal.
Figure 3 Daily maximum temperatures fiom seeding to first Auturnn fiost in 1995 for Brookside, N.S.
Figure 4 Daily minimum temperatures from seeding to first Autumn fiost in 1995 for Brookside, N. S.
Figure 5 Daily precipitation fiom seeding to first Auturnn frost in 1995 for Brookside, N. S.
Figure 6 Daily maximum temperatures from seeding to first Autumn fiost in 1996 for Brookside, N.S.
Figure 7 Daily minimum temperatures fiom seeding to first Autumn frost in 1996 for Brookside, N.S. and showing the fiost event of June 22.
Figure 8 Daily precipitation fiom seeding to first Auturnn frost in 1996 for Brookside, N. S.
Figure 9 Corn development for mulch and no-mulch treatments in 1995 as afEected by heat unit accumulation. Error bars are equal to + 2 S.E.
Figure 10 Corn development for mulch and no-mulch treatments in 1996 as afEected by heat unit accumulation. Error bars are equal to t 2 S.E.
Figure 1 1 Volumetric soil moisture (0-1 5 cm soil depth) under mulch and conventional tillage measured during the 1996 growing season.
Figure 12 Volumetric soil moisture ( 15-25 cm soil depth) under mulch and conventional tillage measured during the 1996 growing season. Figure 13 Volumetnc soil moisture (50 cm soil depth) under mulch and conventional tillage measured during the 1996 growing season. 5 1 LIST OF ABBREVIATIONS AWC available water capacity CD calendar day CHU con heat unit DM dry weight of four harvested plants (kg) DMK kemel dry matter (kg) DMWP whole plant dry matter (kg) FC field capacity FDS fiost damage score HI harvest index HRL harvested row length (m) LA leaf area (m) LAI leaf area index L& incoming longwave radiation LT reflected longwave radiation N nitrogen -03 ammonium nitrate m3 amrnonia PDM proportion dry matter PW proportion of minimally damaged plants pMOD proportion of moderately damaged plants pSEV proportion of severely damaged plants PWP permanent wilting point RCBD randornized complete block design RW row width (0.76in) SM soi1 moisture s3c incoming shortwave radiation ST reflected shortwave radiation TDR time domain reflectometry wwt wet weight of four plants ww2 wet weight of plants in harvested area
Treatment factors
TILL tillage treatment consisting of a 30 cm wide tilled stnp
HERB herbicide treatment consisting of a 30 cm wide herbicide application
TILL/HERB combination of a 30 cm wide tilled strip and a 30 cm wide herbicide band
CONTROL conventionai tillage and weed control
STRAW treatment with a straw mulch placed over the living mulch Abstract
Living mulches are low growing swards of vegetation that are used to reduce soil erosion, to conserve soil moisture, and to control weeds when grown with row crops. They may, however, also compete with the pnmary crop for light, nutnents and water. Therefore they muR be suppressed so that the pnmary crop is not severely affected. They have been used primarily in vegetable crop production, but numerous attempts have been made to use them with field crops such as maize and wheat. The current study examined the effects of difTerent methods of mulch suppression on silage corn (Zra muys L.) in Truro, N.S. during 1995 and 1996. Silage corn was grown in a living mulch composed of white clover (Tnfoiizm repens L.) and mixed grasses. The mulch was suppressed with herbicide bands (giyphosate and atrazine), tilled strips, or a combination of both. Corn was planted into the suppressed strips and the mulch in between the rows was mowed early in the season to reduce competition. Control plots with conventional tillage and herbicide use (no rnulch) were also planted. Where both methods of suppression were used on the mulch, corn yields were not significantly different (p>0.05) fiom yields in the unmulched control plots in 1995, but were reduced by 40% in 1996. Where only one method of mulch suppression was used (either mechanical or chemical) corn yields in plots with living mulch were reduced by 39 to 72% compared to the unmulched control plots. Yields in the mulched plots were 27 % lower in the second year of the study than in the first year. This reduction rnay have been due to the efects of a fiost in late June of 1996 which caused signifiant damage to corn in the mulched plots, but little darnage in the unmulched plots. Higher soi1 moisture levels in the mulched plots may have caused lower soil temperatures, which in turn made the corn in the mulched plots prone to fiost damage. In addition to the increased risk of frost damage, the living mulch also delayed corn emergence and development. While there may be some reductions in herbicide use with living mulch, the nsk in cool temperate climates of delayed development and reductions in yield rnay Iimit the use of living mulches to cool season crops. Acknowledgments 1 would like to thank a number of individuah and organizations for support given to me during the course of this projea. First, acknowledgrnent must be given to the National Science and Engineering Research Councii (NSERC) for financial support given through an NSERC research gant. Much gratitude is extended to my supervisor, Dr. Ralph Martin, who was wiUing to take me on as a graduate student and was able to consistently offer valuable advice, criticism and encouragement. Thanks are also extended to the members of my advisory comrnittee, Dr. Rob Gordon, Dr. Gary Atlin, and Dr. Bert Christie, for valuable design and editoriai assistance. I would also like to thank Dave Langille for technical assistance in the use of the WSsoi1 rnoisture meter and the cone penetrometer and Chns Donkin for his help with the weather station equipment. Statistical analysis of the project would have been impossible without the patient assistance of Dr. Tess Astatkie. Thanks to the many people who helped with the tedious but essential tasks of botanical separations and harvesting: Nathan, Brook, Kim, Shauna, Amy. and Tracy. Many thanks to Paul Muto, Bill Crooks, Sarah Green, and Yela Zahirovich for a sometirnes jaundiced but aiways humorous view of graduate studies. Speciai thanks to my parents, Dorothy and Richard Greyson for their support of my rather circuitous scholastic career and finally, 1 thank Heather Wilkinson and Sylvan Greyson for their patience and love.
siii 1.0 INTRODUCTION
As a Iivestock feed, corn (Zea mays L.) is one of the most nutritious and palatable crops available as a livestock feed in temperate areas. It generally outyields many other forage species, making it attractive for livestock producers (Watson, 1988). A relatively short, cool growing season makes the production of corn in Atlantic Canada risky.
Despite this risk its value as a feed, the development of early maturing hybrids, and changes in agronomie practices make it a worthwhile crop for fmers. Approximately
7000 ha of corn are planted annually in the Maritime provinces, with roughly equal proportions of land planted to grain and silage (Nova Scotia Agricultural Statistics, 1996).
Due to the climatic limitations of the Maritime provinces, it is essential that other limiting factors be minirnized to ensure adequate yields. Recornmended growing practices according to the Atlantic Provinces Field Crop Guide (1991) include primary tillage with a moldboard or chisel plough, and secondary tillage with either disc harrows or 'S' the harrows. Weed control is achieved with both tillage and herbicides. No-tiil production is being used by some farmers but is generally lirnited to coarser soils; under no-till regirnes, soils tend to remain cool late into the spring and therefore delay planting (Mock and
Erbach, 1977). With the short growing season, early seeding is necessary for optimum yields.
No-till planting is used to reduce soil erosion, to maintain soi1 organic matter, to increase soil water-holding capacity, and to reduce nutrient leaching dunng winter months
(Blevins et al, 1977; Hall et al., 1984). A reduction in tillage cm have a number of disadvantages however, primarily in relation to weed growth and cornpetition (Liebman and Dyck 1993). Without tillage crop yields can be severely limited if no other weed suppression methods are used. Chernicd means of weed suppression typically replace mechanical methods in no-till systems. This reliance on herbicides has been questioned due to herbicide residues in soil, water, and food, herbicide resistant weed populations, and the effects of herbicides on non-target organisms (Smith, 1982; Lym and Messersmith,
1988; Neher, 1992; Heap et al., 1993). An alternative method of soil conservation and weed control is the use of living mulches.
Living mulches involve the use of an inter-row cover of low growing plant species to suppress weed growth and to reduce runofhnd soi1 erosion (Regnier and lanke, 1990).
With a permanent cover on the ground, runoff cm be reduced by 86 to 98%. and erosion can be reduced by 93 to 100% compared to losses fiom tilled ground (Hall et al., 1984).
In addition, herbicide losses in the runoff water can also be significantly reduced. Living mulches have been assessed for both tropical (Akobundu, 1982; Bridgernohan and
Brathwaite, 1989) and temperate regions (Kurtz et al., 1952; Elkins et al., 1983; Hartwig,
1988; Echtenkamp and Moomaw, 1989; Grubinger and Minotti, 1990; Ilnicki and Enache,
1 992; Abdul-Baki and Teasdale, 1993; Jones and Clements, 1993). Studies of relative competitiveness with both single and multiple species swards have been reported (Kurtz et ai., 1952; Nicholson and Wien, 1983; DeGregorio and Ashley, 1985). While many non- legumes have been evduated as living mulches, legumes tend to be preferred because of their reduced nitrogen 0requirements and lower cornpetition for soil N (Akobundu,
1983; Elkins et al., 1983; Regnier and Janke, 1990). As with no-till planting, the benefits attnbuted to living mulches are: reduced soil losses fiom erosion, increased water holding capacity and water infiltration, reduced nutrient and pesticide losses from runoff, and a
possible reduction in economic costs for the fmer(Gmbinger and Minoni, 1990; Jones
and Clement, 1993).
The primary constraint determinhg the success and adoption of living mulch systems is the extent of competition between the primary crop and the mulch (Wiles et al.,
1989). In addition to cornpetition for nutrients, light, and water, ailelopathy has been suggested as a possible limiting factor with living mulch systems (Fischer and Burrill,
1993). The terrn "interference" has been used to describe both competition and allelopathic effects. Interference between the crop and living mulch can reduce both yield and quality; therefore the goal of much of the research in living mulches has been to minimize this interference by suppressing the mulch. For living mulches to be an acceptable alternative for farmers, this suppression must be practical and economical. 2.0 LITlERATURE REVIEW
2.1 Corn
Corn is one of the most important crops in the world and is grown for human food, livestock feed, and many industrial purposes (FAO, 1988; Watson, 1988).
Production in 1987 was over 450 million tonnes, with over 50% of that grown for livestock feed (FAO, 1988). Over 90% of the corn production of the United States goes to feeding livestock and the North Amencan crop accounts for approxirnately 40% of the world production (FAO, 1988).
Although the exact origins are unknown, con probably originated in Mexico
(Benson and Pearce. 1987). Due to its sub-tropical to tropical origin it tends to be poorly adapted to cool growing seasons. The majority of corn in the world is grown between 30 and 55' latitude but little is grown at latitudes higher than 47' (Shaw, 1976). It generally requires a growing season of 120- 180 frost-free days and a mean rnid-season temperature of no less than 19'~. While there are attempts at developing cold tolerant varieties, corn as a crop is still very sensitive to low soil temperatures and cool air tempentures. Frost cm severely damage the growing plant and limit yields (Duncan and Widholm. 1994).
Traditionally, the growing of corn has been somewhat problematic for ecological reasons.
Wide row spacings have made soil erosion and residue runoff a problem in some corn growing area. As a crop. corn is generally quite cornpetitive for water and nutrients
(Duvick, 1984; Tollenaar et ai., 1997). This fact, in addition to its importance as a crop, make it attractive for use with living mulches where cornpetition may be a lirniting factor. 2.2 Living Mulches
An effective living mulch system must conserve soi1 while not diminishing the
quality and quantity of the associated crop. The mulch layer must be periodicaily
suppressed in some way because an unsuppressed mulch competes strongly with the crop
and causes severe yield reductions (Elkins et al., 1983; Wiles et al., 1989; Grubinger and
Minotti, 1990; Fischer and Burrill, 1993; Augustin, 1994). Nicholson and Wien ( 1983),
Elkins et a[ ( 1983) and Echtenkamp and Moomaw ( 1 989) found an inverse relationship between mulch yields and corn yields as greater control of the mulch resulted in higher corn yields. The method and timing of rnulch suppression has been the objea of much of the research.
2.2.1 Mulch suppression
Complete or partial suppression of the mulch is generally accomplished by chemicai or mechanical methods, or a combination of the two. Broadcast applications of herbicides may be used to either kill the mulch outright or suppress it sufficiently to allow the primary crop to establish itself In the latter case, regrowth of the mulch creates a persistent living cover on the gound. Herbicides may also be applied in bands dong the planting rows to selectively control mulch growth within the crop row. Mechanical suppression typically involves mowing or some form of tillage.
2.2.1.1 Chernical suppression
Herbicides have been used to suppress the mulch in a number of studies (Adams et al.. 1970; Elkins et al., 1983; Hartwig, 1988; Echtenkarnp and Moomaw, 1989; Fischer and Burrill, 1993). The results have been mixed and depend on the mulch species used. HamKig (1988) reported that corn grown in Pemsylvania, USA, in a herbicide suppressed crownvetch (CoroniIIa vmia) mulch consistently outyielded corn without the mulch. The higher yields in mulch were possible only der an establishment period of about six yean during which corn yields in the crownvetch mulch were lower than yields with no mulch.
The lower corn yields in the first six years were thought to be due partly to a policy of reduced weed control intensity to allow for the establishment of the crownvetch. Mer that period it was ~ggestedthe crownvetch provided a positive influence upon the growth and yield of the corn. A pre-plant application of atrazine (2.2 kg ha-') combined with a pre-emergence application of atrazine and metolachlor (2.2 and 1.4 kg ha-', respectively) and dicamba (0.22 kg ha-' post emergence) was used to suppress the mulch.
These rates allowed for sufficient regrowth of crownvetch to cover the field the following year. Mer the six year establishment penod corn in crownvetch outyielded corn under conventional tillage by 10-50 % for at least six years. Adams et a/. ( 1970) found an application of maleic hydrazide (4.5 kg ha-' and 9 kg ha-') to be ineffective in suppressing
Coastal bermudagrass (CynodondacqIlon (L.) Pers.) sufficiently to allow adequate corn yields in Georgia. Reductions in corn yields were 2740% compared to yields under conventional management. In an evaluation of various species mixtures for use as a living mulch in Nebraska, Echtenkamp and Moomaw (1989) found that corn yields in some of the living mulches tested did not differ from yields in bare ground plots. In that study, mixtures of herbicides were used to suppress the mulch. Corn yields in 1986 in al1 mulch mixtures were the same as the bare ground control, but in 1987 there were reductions in corn yield in plots with Ladino clover (Triffilrmrepens L.) and Chewing's fescue (Festzrca mbra var. cornmutatu). In these plots there was littie suppression of the rnulch
and mulch yields were highest.
Using chernical mulch suppression in Illinois, Elkins et al. ( 1983) found that good
corn yields could be obtained while up to 60% of the living mulch survived at harvest. In this nudy the authors used eight combinations of herbicides and growth regdators on mulches of tdl fescue (Festuca arzcndinacea Schreb.), orchardgrass (Dactylis g[omerata
L.), smooth bromegrass (Bromzrs inermis Leyss.) and alfdfa (Medcago miva L. ).
Herbicide was applied either in bands 23 cm wide or broadcast. With 60% bromegrass cover remaining at harvest, corn grain yields were 8279 kg.ha", and at 10% cover remaining, yields were 10,097 kg. ha-'. Grass mulches were found to be easier to maintain than legumes and high corn yields were possible. Alfdfa sods were killed by most combinations of the herbicides, but corn yields were high (7965 to 12,104 kg.hae1).
In a comparison in Georgia of yields of corn grown in living and dead mulches,
Box et al. ( 1980) used herbicides to completely kill (CK) ta11 fescue sod or kill stnps (SK) in the sod in which corn was planted. Corn grain yields were higher by approximately
20% in the CK treatments compared to the SK treatrnents.
2.2.1.2 Mechanical suppression
Reducing the growth of the mulch through mowing and partial tilling alone may not be sufficient to ensure adequate mulch suppression. Grubinger and Minotti (1 990) planted sweet corn in 50 cm wide arips that had been tilled in a white clover sward in
New York. They found early rnowing of the mulch done, either 2 or 5 times during the season, did not improve sweet corn yields compared to yields from unsuppressed clover plots. The mower used in the study by Gmbinger and Minotti cut the sward down to 15 cm and removed the clippings. It was suggested that this moderate defoliation coupled with the removal of nutrients may not have reduced the competitiveness of the clover sufficiently, and may actually have increased the clover's ability to compete with the corn.
However, both Vrabel(1983 as quoted in Grubinger and Minotti (1990)) and Peters
(1986 as quoted in Gmbinger and Minotti (1990)) working in New York State, found mowing white clover 3 times resulted in corn yields similar to those obtained using clean cultivation or atrazine-suppressed clover. Gmbinger and Minotti (1 990) reported that partial rototilling of the white clover living mulch 6 weeks after corn emergence produced yields of 5.6 t ha-' compared to 5.4 t ha" fiom clean cultivated plots.
In a study in Illinois, Kurtz et a!. (1952) grew field corn in a mechanically suppressed living mulch. They evaluated 10 Iegumes and 5 grasses for their ability to reduce weed infestation and for stand longevity. The mulches were suppressed by initially tilling 50 or 75 cm wide strips in the sod. Corn was planted in the tilled strips and early in the season the inter-row mulch strips were mowed. At higher levels of applied nitrogen
(262 kg ha*') corn yields of 97, 90, and 83% of the control, in mulches of sweet clover
(Melilotirs sp. ), Timot hy (Phletïmpratense L. ), and ryegrass (Loliiim perme ), respectively, were possible.
2.2.1.3 Combined mechanical and chemical suppression
Some success with living mulches has been reported where both methods of mulch suppression have been used. Augustin (1994) found no significant difference between corn yields fiom bare ground control plots and living mulch plots in which the mulch had been tilled in strips and suppressed with a band of herbicide. Yields were over 1 1,000 kg
DM ha-' in both treatments. Fischer and Burrill(1993) report no significant difference in sweet corn yields between mulch and conventional control treatments in Oregon. In that study the mulch had been suppressed with strip tillage (10- 15 cm) and a 15 cm band spray of atrazine and alachlor ( 1 .7 and 2.2 kg ai. ha-', respectively).
2.3 Soil Physical Properties
2.3.1 Soil moisture
Competition for soi1 moisture (SM) between the crop and the mulch is potentially an important limitation to the success of a living mulch system. While competition at al1 times of the growing season may be detrimental to yields, water stress at time of flowering rnay be especially crucial. In systems using com, this period would be roughly from 6 weeks before silking to 3 weeks after (Dale and Shaw, 1965). Atmospheric demand for moisture is generally greater due to higher air ternperatures, and moisture-sensitive physiological processes related to yield are occurring at this tirne. At this stage the nurnber of ovules that can be fertilized is determined (Shaw, 1976). Barnes and Woolley
(1 969) reported a 6 to 8% reduction in yield due to water stress a few days before tasselling, and Claassen and Shaw ( 1970) estimated potential yield reductions due to moisture stress at the time of silking at 6 to 8% per day of stress.
For living rnulch systems, the primary concem is whether evapotranspiration fkom the mulch layer will be greater than evaporation of water from a bare field. The rate of evapotranspiration will depend on many factors, including: rooting depths of competing species, the leaf area index (LAI) of the mulch and the crop, the differing albedos of a green sward and bare soil, and levels of soil orgMc matter. In a study examining water use by various sward species in an orchard, Toenjes et ai. (1 956 as quoted in Nicholson and Wien, 1983) found grasses used less water than legumes. They reported that
Chewing's fescue and Kentucky blue gras (Pw pratensis L.) absorbed water primarily from the top 20 cm of the soil while white clover used soil water in the top 100 cm.
Soi1 organic matter Seasthe arnount of moisture available to the crop. Blevins et al. (1977) found that soil under a dead mulch contains higher organic carbon levels in the top 5 cm of soil than conventionally tilled soils. At depths below 15 cm there was no difference in organic matter contents between conventional and dead mulch plots.
Moschler et al. (1975) also report higher organic matter content in no-till situations compared to conventional tillage. There is a possibility that the higher organic matter levels in soils with living or dead mulches would increase the available water capacity
(AWC). The consensus has been that organic matter doesn't generally increase the AWC
(Macke and Mehuys, 1985). With increasing soil organic matter, a soil's field capacity
(FC) increases but the permanent wilting point (PWP)also increases. However, Hudson
( 1994) shows that the rate of increase for PWP with increasing organic matter is less than the rate of increase in field capacity, resulting in an increase in AWC. Averaged across soil texturai groups, an increase in organic matter content from 1% to 3% can almost double the AWC. Moore et al. (1 989) suggested that the positive effects of additions of organic matter are temporary due to decomposition, and either annual additions must be made, or in the cases of permanent living mulches, the benefits may not be apparent in the first few years of the sward's existence. In a study exarnining soil surface strength and soil permeability Folomnso et a!.
(1 992) reported that cover crops can increase steady infiltration rate and cumulative water
intake by as much as 147% and 10 1%, respectively, when cornpared to bare soil. They
suggested that these increases resulted fiom better soil aggregation, increased soil organic
matter, and reduced surface crusting.
These factors (increased organic matter content, increased infiltration, decreased
run-off and surface crusting) al1 should contribute to higher SM levels under living
mulches. Studies where moisture readings are measured show that SM is higher under
living and dead mulches compared to moisture levels under conventional tillage. Soi1
moisture recharge is faster after a rainfall where there is a mulch cover on the surface and
run-off is reduced (Hill and Blevins, 1973; Box et al., 1980; Hall et al., 1984).
Box et al. (1980) measured soi1 water potential under corn planted in completely
killed and strip killed tall fescue sod over six years of continuous cropping. Five
irrigation treatments were examined. They found no difference in soil water potentials for
the 0-60 cm depth range between a dead mulch and a living mulch. This was true in both
imgated and rainfed conditions.
Hill and Blevins (1973) planted no-till corn into a killed sod. They found that,
over three years, SM deficiencies were always pater under conventionally tilled plots in
the top 50 cm of the soil profile. Below 50 cm, the differenceswere not significant.
Water losses in the conventional plots fiorn seeding to 40 days after seeding were from 8 to 27% higher than in the no-till plots. Once the canopy had developed fiilly, the water
losses between the two treatments were equal, but due to the greater loss early in the season, SM was consistentiy higher in the no-till plots for the entire growing season.
Jones et al. ( 1968) also reported SM levels were higher under killed sod in contrast to bare soil down to a depth of 45.7 cm. hiring the six years of the study, the average SM was 1.8 to 3 1.2 mm greater in the mulch treatment than in the conventionally tilled treatments.
Using various legume and gras species as living mulches, Kurtz et aï. ( 1952) evaluated differences in corn yields and water use arnong the treatments. Soil moisture readings were taken at 25 and 50 cm depths throughout the season. At the 25 cm depth,
SM levels declined more rapidly under the living mulch treatments than under the conventional treatrnents, but aiso recovered more quickly after a raiddl event. This was attnbuted to the increased percolation of water into the soil layer under a mulch layer.
The authors also showed that SM levels at the 50 cm depth fluctuated much less than at
25 cm for both conventional and mulch treatments, but that there was no recharge of soil moisture under conventionally tilled plots. Soil moisture at the 50 cm depth under the living mulch gradually declined throughout the growing season, but there was a substantial recharge of soil water after a rainfdl.
While SM levels are consistently higher under mulches than under bare soil, it is not clear whether this increased availability of water confers any benefit to the prirnary crop. Studies reporting crop yields with living mulches under irrigation or where there has been severe water stress have show inconsistent results. Hartwig (1 990) reported yield advantages in Pemsylvania of 20 to 55% for corn grown in a living mulch of crownvetch compared to no mulch. These higher yields occurred even in a season when June and July rainfall was the lowest in 100 years and no additional moisnire stress was observed due to the presence of the living mulch. Lake (199 1) found no yield improvement fiom irrigation for corn grown in living mulches in Wisconsin. Vrabel(1983 as quoted in Grubinger and
Minotti, 1990), however, reported reduced yields due to an extended dry period.
Nicholson and Wien (1983) in New York noticed severe drought stress symptoms in sweet corn dunng July. The symptoms of drought stress were least in the bare ground control and varied in other plots and were thought to be dependent upon the mulch species. They found that the more prornising living mulches were those species that were naturally low growing and less vigorous, specifically Chewing's fescue, Kentucky bluegrass and wild white clover. Syrnptoms of drought stress (flagging and rolled corn
Ieaves, stunted corn plants and necrotic leaf tips) were prominent in colonial bentgrass
(Agrostis temis) and red fescue (F. mbra) plots. In two years of a six year study, Jones et al (1968) found corn yield increases of about 1500 kg ha-' in plots with a killed sod mulch compared to yields fiom conventional plots. They attributed much of this yield increase to the higher SM resulting frorn the presence of the mulch. In other years, the yields of mulched and conventional treatments did not differ significantly.
In their six year study of continuous corn grown with living and dead mulches,
Box et al. (1 980) reported higher grain and stalk yields in irrigated treatments compared to rainfed conditions. Irrigation increased grain yields by approximately 1500 kg ha-'.
2.3.2 Soil temperature
Soil temperatures are generally lower under a mulch layer (Adams et al., 1970;
Mock and Erbach, 1977; Horton et al., 1994) and lower soi1 temperatures tend to delay and inhibit corn emergence (Adams et ai., 1970; Duncan and Widholm, 1994). In a study
of corn grown in sod Adams et ai. (1970) found soil temperatures below the sod to be on
average 1°c lower than temperatures in clean tilled plots. Since corn growth and
development is dependent upon relatively high soil and air temperatures (Duncan and
Widholm, 1994). lower soi1 temperatures could limit the use of both no-till and living
mulches for corn production to soils with better drainage.
2.3.3 Soii nitrogen response
Under no-till and living mulch situations there is some concern that since applied
nutnents are not incorporated into the soil, the availability of some nutnents may be
lirnited (Box et ai., 1980). Total N stores may be higher with no-till and living mulches
compared to fiequently tilled soils (Beale et al., 1955: Blevins et ai., 1977), but inorganic
sources of N may be lower because decomposition of organic matter and N mineralisation
rates tend to be lower in undisturbed soils, primarily due to the reduced Oz levels (Box et al., 1980; Needham., 1983). Higher N rates are often applied to crops in these situations
as a resutt.
In addition to the concem about lirnited availability of added nutnents, there is the
possibility that living mulches will compete with the primaqr crop for nutrients (primarily
N), and cause a reduction in yield. In Switzerland, Fei1 et al. (1 997) measured leaf N- contents of corn grown in an Italian ryegrass (Lolirrm rnzrltrflonim Lam.) mulch, and reported lower 1eafN concentrations in corn from plots with a mulch cornpared to corn grown with conventional tillage. Silage dry matter yields were significantly lower in the
mulch plots and this reduction in yield was attributed to cornpetition for N. With sufncient suppression or where high proportions of legumes are used, however, cornpetition for N may be minimal (Blevins et al., 1977; Akobundu, 1982; Grubinger and
Minotti, 1990). In a study of living mulch use in the tropics Akobundu (1982) found that corn grain yields could be maintained at high levels (2.5 to 3.5 t ha*')with a leguminous living mulch (Psophocarpispalzitris) over 4 successive crop ping seasons. Y ields in conventional till plots declined to below 2 t ha-' after the second season even with additions of 120 kg N ha-' while yields in plots with the mulch remained above 2.5 t ha-' with lower additions of N-fertilizer ( O and 60 kg N ha-'). Peters ( 1986 as quoted in
Grubinger and Minotti) reported that yields of corn grown in a herbicide-suppressed white clover mulch were highest when no N was added and were higher than yields under clean cultivation even with the addition of N fertilizer. In another study in New York State
(Grubinger and Minotti, 1990), corn yields from a mulch of clover suppressed with rototilling once early in the growing season were as high as yields from bare ground plots and corn IeafN levels were higher in the plots with suppressed clover.
Kurtz et al. (1952) found that at intermediate N levels (1 14 kg ha-') yields of corn were higher in plots with leguminous mulches than with grass mulches. Corn grown with a leguminous living mulch yielded between 90 and 1 12% of the conventional control while yields with grass mulches were between 7 1 and 84% of the control. When no nitrogen was added yields of corn with leguminous mulches ranged from 29 to 84% of the control, and yields from the grass mulched plots ranged from 13 to 40%.
For the primary crop to derive any benefit in the forrn of nitrogen from the associated legurne in a living mulch there must be some suppression of the legume. Suppression (defoliation, shading) enhances the Ioss of root nodules (Chu and Robertson,
1974; Simpson, 1 976) and this wouid increase the amount of N available to the primary
Crop. 3.0 OBJECTIVES
The primary purpose of this investigation was to assess the effect of various factors on the growth and development of corn grown in a grasdclover living mulch system.
The specific objectives were:
1. To test the effect on the yield of silage corn of roto-tilling and/or herbicide in a strip as methods of mulch suppression;
2. To test the effect on the yield of silage corn of no mulch suppression and a straw rnulch;
3. To test the effect of imgation on the yield of corn grown in a living mulch and on the biomass of the living mulch;
4. To test the effect of two levels of N on corn yields in a living mulch. 4.0 METHODOLOGY
4.1 Background and Experimentai Design
The study was conducted in 1995 and 1996 at the Brookside field of the Nova
Scotia Agricultural College, Tmro, Nova Scotia (45' 14' N, 63') 19' W, 40 m). The site is located on a Woodville sandy loarn (orthic humo-ferric podzol) soi1 (Nova Scotia Soil
Survey, 1991 ). The local area receives an average of 2200 corn heat units (CHU'S) annually (Bootsma et al., 1992). An established white clover population was present in the sward, which was dominated by Pasture grasses (primarily Poapruterzse, Dactylis glomeruta, PhIez~mpratense, Festm-z an~~~diinacea,A lopeczin~s pratemis) and vari ou s weed species.
Silage corn was grown in a living mulch which had been mechanically or chernically suppressed. The design was a 5x2 factorial arrangement in a randomized complete block design (RCBD). There were five levels of mulch suppression, two levels of nitrogen (N) application and 8 blocks.
Treatments in which there was no prior mulch suppression (Table 1) were unsuccessful. Emergence rates in these treatments were low and corn growth and development were severely delayed compared to other treatments; therefore they were abandoned early in July, 1995 and not repeated. The design was then analyzed as a 4x2 factorial with mulch suppression and N as the main factors. In 1996 an additional mulch suppression treatment was added to replace the abandoned treatments of 1995 and the design was analyzed as a 5x2 factorial. The treatrnent combinations are outlined in Table
1 and the layout of the experimental field is shown in Figure 1. Irrigation was carried out ody on ïILL/HERE%and CONTROL treatrnents in four of the eight blocks and analyzed as a split plot arrangement. Individual plots were 8 m X 3.1 m (includes 6 cm between plots) in size (24.8 m2). Four corn rows, 0.76 m apart, ninning North-South, made up a single experimental unit. Grass buf5er strips (3 m wide) were present between blocks.
Table 1. Summary of treatment combinations including treatment abbreviation, nitrogen rate and mulch suppression description.
Treatment Nitrogen Mulch Suppression (kg ha-') NOSUPP-ON O no mulch suppression -abandoned July 1995 NOSUPP-1 1SN 115 no mulch suppression -abandoned July 1995 TILL-ON O 30 cm tilled strip
TILL- 1 I SN Il5 30 cm tilled strip
KERB-ON O 30 cm herbicide band
HERB- 1 1SN 115 30 cm herbicide band
O 30 cm herbicide band on top of 30 cm tilled strip TILL/HERB- 1 1SN 115 30 cm herbicide band on top of 30 cm tilled strip CONTROL-ON O conventional tillage and weed suppression
CONTROL- 1 1SN 115 conventional tillage and weed suppression
STRAW-ON O straw mulch placed over corn rows- 1996
STRAW- 1 1 SN 115 straw mulch placed over corn rows- 1996 Figure 1. Layout of experimental plots in the Brookside field in 1995 and 1996 (See Table 1 for treatment definitions.). Experimental units are 8 m X 3.1 m.
Block 4 I -- TILL' NOSUPP HERB 1 COhiOL TILL HERB TILL
HERB SCUW
O-N O-N O-N O-N 1 15N Irrigakd
Block 3
CONTROL CONTROL HERB TIW nu HERB mu HERB HERB HERB
115'1 O-N 1 lm 115N O-N O-N O-N 1 In
Block 2 - -
TILL1 TILL TILL TIW NOSLTP tIERE3 COhTROL HERB NOSLTP
HERB HERB STRAW STRAW
O-N 115N O-.i 11% O-N O-N O-N l l5S 1 l5N
Block 1
TILL HERB NOSUPP TILL NOSLTP TILU HERB nu
STRAW STRAW HERB HERB
O-N l l5N O-N 11% 1 ISN 1 15N O-N O-N O-N
irrigatçd irripted Figure 1. (continued) Layout of expenrnental plots in the Brookside field in 1995 and 1996 (See Table 1 for treatment definitions.). Experimental units are 8 m X 3.1 m.
Block 8
HERB CONTROL NOSUPP TILU TILL HERB NOSUPP
STIUW HERB mw
O-N O-N O-x 1 l5N O-N 1 lm 1 l5N
Block 7
TILL 'JOSüPP TILL HERB TILL'
STRAW HERB HERB
O-N 1 l5N O-N
[mgatrd [rrigated
1Block 6 TILL' NOSUPP TILL TILL TILL' CONTROL NOSL'PP COYTROL HERB
HERB !XRAW HERB STRAW
O-N O--u' 1 lSN O-N 1 I5N O-N 1 l5N 11m O-N
Irrigtrd Irrigated hgated Imgateii
Block 5
CONTROL TIW TILL HERB NOSL'PP COEUrrROL HERB
HEM mw
115N O-N O-N O-N 1 l5N O-N 1 ISN 4.2 Treatment Factors
4.2.1 Living mulch treatments
In the living mulch treatrnents (TILL,HE=, and TILWHERB) mulch suppression was achieved with either a 30 cm tilled strip or a 30 cm wide band of herbicide, or a combination of the two. For mechanical suppression a tractor mounted roto-tiller was used to cut 30 cm wide strips in the sod before seeding. By removing certain blades on the roto-tiller it was possible to till two rows at once. The distance between strip centers was 76 cm. Two passes with the roto-tiller were required to incorporate the sod into the soil. The second pass was offset from the first by 10 cm as the width of the tilled strip was only 20 cm. For chernical suppression a pre-emergence herbicide band ( 1.5 kg ai. ha-' atrazine and 1. I kg aiha-' glyphosate) was applied within a 30 cm band dong the corn rows using a hand sprayer, thus reducing the application rate over the whole plot by 60%.
The corn was then planted into thesç suppressed strips.
4.2.2 Control Treatments
The control treatments (CONTROL-ONand CONTROL-11SN) were ploughed and disced. Weed suppression consisted of a pre-emergence herbicide application ( 1-5 kg a.i. ha-' atrazine and 1.1 kg a.i. ha*' glyphosate) applied over the entire 8 rn X 3.1 rn plot.
Weed suppression continued throughout the growing season with hand weeding.
4.2.3 Nitrogen treatments
Additions of N were made at the recommended rate (1 IS kgN ha-' in both years) to half of the plots in the form of ammonium nitrate (N&N03). Other plots received no additionai N. The fertilizer was applied on the soil surface adjacent to the corn rows and covered with a layer of stmw to lirnit ammonia (NH,) volatilization. The added stmw was approximately 1-2 cm deep and added to al1 treatments. It was not incorporated and so shouid not have increased denitrification.
4.2.4 Irrigation treitments
Resdts from a similar study the previous year (Augustin, 1994) and observations of eariy corn development in 1995 suggested that the TILL/KERB treatment would probably be the more successfid of the mulch treatments and so irrigation was only applied to TILL/HERB and CONTROL in order to provide a comparison of imgation effects between the more productive mulch treatment and the control. Imgation was applied to these treatments in four of the eight blocks (Fig. 1). It was estimated that field capacity (FC) was at approxirnately 30% volumetric soi1 moisture (SM) and therefore irrigation water was added to bring SM up to that level. In late October, 1996, a series of
SM measurements were performed using an IRAMS Soi1 Moisture Meter (Foundation
Instniments Inc., Nepean, Ont.) which estimates SM based on the time domain reflectomeûy (TDR) method (Topp et al., 1980). Measurements confirmed a FC of approximately 3 1%.
During each field season SM measurernents over the top 15 cm of the soi1 profile were performed as fiequently as twice a week using the IRAMS meter. An average of six random measurements was obtained for each plot. Since the ha1 harvestable area was only 4 m2 in the center of each plot, the imgation was lirnited to an area surrounding the ha1harvested area using a trickle irrigation system. The irrigation system covered an area of 14 m2 of the plot. It was assumed the irrigation water would evenly disperse laterally throughout this area and therefore 1 .O L of imgation water was assumed to add
0.071 mm of depth. Based on a soi1 depth of 15 cm, this would increase the SM by
0.05%. Using this relationship the number of litres of water required to increase the SM to FC was calcuiated as (1):
I = (30% - SM%) * 20L (1) where 20 L is the volume of irrigation water required to raise SM by 1%. A summary of the irrigation schedule is shown in Table 2.
Table 2. Schedule of imgation and average amount of irrigation water (L) applied to CONTROL and TILWHERB treatments in 1995 and 1996.
Date Treatrnent Amount Depth (LI (mm) August 3 CONTROL 360 25 -6 August 12 CONTROL
TILL/HERB 360 25 -6 TILUHERB
August 12 CONTROL L 08 7.7 August 17 CONTROL
TILUHERB 76 5.4 rumm
Aupst 22 CONTROL 120 8.5 August 23 CONTROL
TILWHERB 105 7.5 TILL/HERB
Sept. 5 CONTROL 125 8.9 August 3 1 CONTROL
TILWHERB 102 7.2 TIL WHERB
Sept. 9 CONTROL
TILUHERB 4.2.5 Straw mulch treatments
In 1996 a treatment was added to the plots which had no mulch suppression
(NOSUPP)the previous season and had been abandoned. Straw was used to help suppress the living mulch within the corn rows dera strip was tilled in the sward. In these plots on July 8, a mulch of straw, 8-10 cm in depth, was placed approximately 15 cm on either side of the corn rows in al1 blocks. The corn seediings at this time were between
10 and 15 cm hi&. The straw was placed on both sides of the rows as close to the corn as possible. On July 18 the straw mulch was chopped and blown against the corn using a lawn mower. Haif of these treatrnents received the recomrnended level of N and the other half received no additional N.
4.3 Field Preparation and Sampling
A summary of al1 field activities for both years is show in Table 3.
Ail treatments received recommended levels of P20r and K20 (30 kg ha-' P205 as triple-super-phosphate in both years and 95 kg ha-' and 40 kg ha-' K20 as KCL in 1995 and 1996, respectively). The fertilizer was applied by hand dong the corn rows at a distance of approximately 4 - 6 cm. Seeding was performed with a no-till seeder using
Pioneer 3979 (2325 CHU'S requirement) seed at a density of 68,000 seeds ha-'.
Mowing of the living mulch between the corn rows was performed in al1 treatments to reduce cornpetition with the corn. Mowing was performed with a "string trimmer" when the average sward height reached 12 cm. Seven to ten measurements of the sward height in each mulch plot were made at least twice a week. The living mulch was then mowed down to less than 3 cm. Mowing between the corn rows ceased when the corn canopy closed in the CONTROL treatments. In both years the canopy closed in the CONTROL treatments 2-6 days before the canopy closed in the mulch treatments.
Table 3. Summary of field activities for 1995 and 1996.
Ploughing, discing of May 22 May 17 CONTROL roto-tilling TILL, HERB, May 25 June 3 TILLrnRB, and STRAW
Seeding May 26 June 6
Mowing of mulch in June 5, 16, 28, July 1 1, and hne 17, July 5, and 18 between corn rows 20
Applied herbicide to HERB, June 1 June 11 TILLrnRB, and CONTROL
Applied N. P, and K June 6 June 20
Irrigation started Aupst 3 August 12
Auturnn killing frost September 14 September 24
Harvest September 30 October 20
Measurements of corn plant height and corn growth stage (Hanway, 1989) were recorded 2-3 times week-' throughout the growing season until more than 50% of the plants in a plot had reached the silking stage. Measurernents were done on 10 plants in each plot with the averages recorded.
Two samples of mulch biomass were obtained before each rnowing using a 0.2 m
X 0.5 m (0.1 m2) quadrat and hand shears. For one sample the quadrat was placed perpendicular to, and over a corn row. The second sample was taken with the quadrat piaced paraltel to and between two corn rows.
Al1 samples were dried and weighed. The first and last perpendicular samples of the growing season were separated by hand into grass, clover, and weed (residual) before drying and weighing.
Sarnples of the corn biomass (2 plants) were also obtained before each mowing from the same area as the first mulch sample. Corn leaf area was measured with a Li-Cor
3 100 leaf area meter ( Li-Cor Inc., Lincoln, NE) and LN calculated based on the following:
where LA is the measured leaf area (mZ),RW is the row width (0.76 m) and HRL represents harvested row length (m). The sarnples were then dried and weighed.
For harvesting, ail corn plants within an area of 4 m2 (1.52 rn X 2.63 m ) in the center of each experirnental unit were removed. The fiesh weights of al1 plants from this sample were recorded, as well as the khweights of four randornly selected plants.
These four plants were dried, the kemels separated from the cob, and the weights of the plants and kemels measured. Harvest index (HI) was calculated for the sample of four plants based on the following: DM Hr = Dm
where DMK is the dry weight of the kemels (kg) and DMWP is the dry weight of the
whole plant (kg). The proportion of dry matter @DM) was calculated using:
where DM is the dry weight of the four plants (kg) and WWi is the weight of the four
plants at harvest (kg). From this the total yield (kg DM ha-') was calculated using:
Yield = pDM * WW, * 2500 (5)
where WW2 is the weight of the plants at harvest fiom the harvested area in each plot (kg
0.0004 ha-')
Mulch samples were also taken at this tirne and separated, dried and weighed.
Meteorological data including rainfall (Mode1 60 18-B tipping Bucket Rain Gauges,
Weathertronics, Sacramento, CA), air temperature (Cu/Co thermocouples), solar radiation
(LI-ZOOSZ. Li-Cor Inc., Lincoln, NE) and relative humidity (RH)(oniy in 1996) were
measured at the site. Data was recorded every 60 s and averaged hourly by a CR 1 O
dataiogger (Campbell Scientific Corp., Logan, UT). Temperature, solar radiation, and RH sensors were positioned approximately 1.5 m above the ground. From temperature data collected at the site daily CHU'S were calculated using:
CHU = Kun + y,, 2 in which:
Y,, = (T,, - 4.4' C) and:
Y,, = 3.33(~,, - 10'~)- 0.064(~,, - 100~)'
where T- and T,, are the daily minimum and maximum temperatures, respectively
(Brown, 1969).
4.3.1 1995 Protocol
In early April30 soil subsamples from 0-30 cm depth were collected and mixed to
form a bulk soil sample. hdysis was performed by the Nova Scotia Department of
Agiculture and Marketing Analyticd SeMces Laboratory in Truro for nutrient content,
pH, and organic matter.
The experimental site was grazed by cattle in rnid-May and mowed on May 23.
On May 22 the two control treatments were prepared by ploughing, discing and
harrowing. The treatments TILL and TILLMERB were roto-tilled with 30 cm strips on
May 25. Seeding of al1 plots was performed on May 26.
Herbicide was applied with a hand sprayer to treatments HERB, TILL/HERB, and
CONTROL on June 1. Fertilizer (N, P, and K) was added on June 6. Mowing of mulch
between the corn rows was done on lune 5, 16, 28, luly 1 1, and 20. The first rnowing
occurred before any corn had emerged and as a result no sarnples were taken.
In blocks 7 and 8, three plots were abandoned in mid-July due to flooding
(TILUHERB-ON in block 7, HERB-ON and TILL/HERB-ON in block 8).
Corn samples were obtained before the four final mowings. The first two sarnples of the season were taken with the 0.1 m2 quadrat placed parallel to and over the corn row, while the final two were taken with the quadrat positioned perpendicular to the corn row. These samples were obtained from areas near the ends of each plot which were not part of the final harvestable area.
Corn developrnent and average height for each expenmental unit were recorded from emergence to silking. At least twice weekly the average height and growth stage of
10 randomly selected plants in each plot was obtained. Height and growth stage observations ceased when anthesis and silking had been reached by an average of 10 plants within each plot.
Watenng of imgated plots began on August 3. The corn was harvested on
September 30.
4.3.2 1996 Protocol
The CONTROL subplots were ploughed on May 17 and disced on May 29.
Treatments TILL, TILLMERB, and STRAW were roto-tilled with 30 cm strips on June
3. Seeding of al1 treatment plots (except for 6 which were too wet for the tractor) was done on June 6. These 6 plots were hand planted the following day. They were TILL-ON
(Bl. 1), TILLMERB-ON (BI. 2). CONTROL-ON (Bl. 5), TILL/HERB-ON (BI. 7), ad
TLWHERB-ON and STRAW- 1 I SN (BI. 8).
Al1 plots received the insecticide tefluthrin (Zeneca Agrochemicals) during seeding.
Treatments receiving herbicides were sprayed on June 1 1.
The experimental area was mowed on May 22 and June 3. Ail living mulch treatments were then mowed on June 17, July 5, and 18. Mulch samples and corn samples were collected on July 5 and 18. Additional mulch samples were taken on August 14 and corn sarnples on August 20 (Corn had jua emerged on June 17 and so no samples were collected at the first mowing).
Fertilizer was applied on June 20. Al1 treatrnents received additions of P and K
(amounts outlined above) but only treatments requinng N (as outlined in the design) received the recommended N.
On June 19 SM rneasurements comrnenced using the TDR method for three depths
(0- 1 5, 15-25, and 50 cm). Measurements were taken at least three times weekly with a
Tektronix 1502C Metallic Cable Tester (Tektronix Inc., Beaverton, OR). Stainless steel probes 15 cm in length were buried in 8 plots (4 with living mulch and 4 with conventional tillage) at the 3 depths (perpendicular to the soil surface for the 0- 15 cm depth, at an angle of 45" for the 15-25 cm depth, and parallel to the soil surface for the 50 cm depth).
On the night of June 22 the field site was hit by frost (- 1 SOC) and many seedlings were severely darnaged. A frost damage rating was recorded for each plant on July 1.
Corn growth stage readings and height measurements commenced on July 6.
For the STRAW treatments, a mulch of straw, 8- 10 cm in depth, was placed approximately 15 cm on either side of the corn rows in al1 blocks on July 8. The corn seedlings were between 10 and 15 cm tall when the mulch was added.
The month of July was unusually wet in 1996 (see Figure 2) and therefore irrigation was not started until August 12 (Table 2). It was done in the szme manner as
1995. Due to either poor germination or severe fkost damage, a number of plots were abandoned. These were CONTROL-ON (BI. 2), STRAW-1 1SN (BI. 3 .), STRAW-ON &
N, TILL-ON & N (BI. 4), and STRAW-ON& N and TILLMERB-ON (BI. 8).
Hawesting was done on Oaober 10 using the sarne methods as in 1995.
Penetrometer readings using a cone penetrometer (Star Quality Sarnplers, Edmonton, Al.) were recorded for al1 plots in late Oaober of 1996 to mesure any differences in resistance to penetration between mulch plots and control plots. Two readings were obtained from each plots for four depths (0-5 cm, 5- 10 cm, 10-20 cm and 20-30 cm).
4.4 Statistical Analysis
4.4.1 Yield and mulch analysis
The mode1 used for analyzing both the corn parameters and the mulch weights was:
Yijk=p + Pi + Tj +YI+ tyji, f~ijk, where p represents the overall mean, pi represents the eEect of block, Tj represents the effects of the mulch suppression (4 levels in 1995 and 5 in 1996), yk represents the effect of N (2 levels), qjk represents the effect of the interaction of mulch suppression and N, and Eij~represents the error of al1 treatments and blocks. Since there were missing data points due to unsuccessful plots, the LSMEANS statement was used to generate estimates of the means and the PDFF fiinction used to determine differences between treatments where main effects were found to be significant (SAS Inaitute Inc., 199 1). Interactions were assessed but only reported when significant (pcO.05). Results from the imgated plots were not used in this analysis. 4.4.2 Irrigation analysis
Imgated and rainfed units formed whole plots of a split-plot 2X2X2 factorid
arrangement of N and mulch. Nitrogen and mulch had two levels each (O and 1 15 kg ha-'
,living mulch and no living mulch, respectively). Treatments TILL/HERB and
CONTROL were used in the analysis of this design. The statistical model used for its analysis was:
Yijkl = + pi + rj + (Pr), + yk + al + (~a)ki+ (pq{)ik + (pa)ii +@yak + Eijkl ( 10)
In this model p represents the overall mean and 0, r, y, and a refer to the effects of block, imgation, N, and living mulch, respectively. Error terms used for assessing the significance of the main factors and their interaction were the respective mean square of the interaction of block and the tested factor.
4.4.3 Frost analysis
A visual score based on the degree of foliar damage was given to each plant in every plot. Categories of damage were 'severe', 'moderate', and 'minimal'. Using the relationship:
FDS = 3 * pSEV + 2 * pMOD + pMIN (1 1) where FDS is the frost damage score, pSEV is the proportion in each plot of severely damaged plants, pMOD is the proponion of moderately damaged plants, and pMM is the proportion of minimdly damaged plants, it was possible to derive a 'fiost damage' score for each plot ranging from 1 to 3. If al1 plants in a plot were severely damaged the score for that plot would be 3.0. The resulting kost scores were used in the analysis comparing the amount of damage between different treatments. In the analysis of these scores the
following mode1 was used:
Yijk= + + pj + Eijk U2)
As before, represents the overall mean, s represents the effea of block pj represents
the effects of the treatment on that plot, and ~ijirepresents the error of al1 treatments and
blocks.
4.5 Meteorological Conditions
Average monthly rainfall for 1995 and 1996 compared to the 30 year normal for
Tmro is presented in Figure 2. Rainfall during both growing seasons was higher than
average (1 8 and 3 8% higher in 1995 and 1996, respectively) but showed somewhat
different distribution during the two years (Nova Scotia Department of Agriculture and
Marketing, 1995; 1996). In both years, rainfall was higher than average in July (57 and
1O 1% higher in 1995 and 1996, respectively), which is a critical period for corn
development. However, rainfàll was lower than average in August both years (35 and
7 1% lower for 1995 and 1996, respectively). Precipitation in June 1995 was 1 10% above
average but 33% lower than normal in 1996. In September rainfall was 23% below
normal in 1995 and 199% above normal in 1996. Maximum, minimum and average daily air temperatures and precipitation fiom the study site for 1995 and 1996 are shown in
Figs. 3-8. The study site received 2237 CHU'S between seeding and the first Autumn frost in 1995 and 2186 CHU'S in 1996. Figure 2. Average monthly (May-September) precipitation (mm)for 1995 and 1996 in Brookside, N.S. compared to 30 year normal. Figure 3. Daiiy mdumtemperatures hmseedhg to first Autumn frost in 1995 for Brookside N.S.
Figure 4. Daily minimum temperatures fiom seeding to first Autumn frost in 1995 for Brookside. N.S.
Figure 5. Daiiy precipitation fiom seeding to first Autumn frost in 1995 in Brookside. N.S. II,, Figure 6. Daily maximum temperatures hmseeding to first Autumn fiost in 1996 for Brookside. N.S.
O!..,, ,.,
Figure 7. Daily minimum temperatirres from seeding to fmt Autumn frost in 1996 for Brookside, N.S. and showing the fiost event of June 22) 1 -l.sOc lune 22
Caiaidar day
Figure 8. Daily precipitation from seeding to first Autumn fron in 1996 in Brookside, N.S. 5.0 RESULTS
5.1 Corn Yields and Harvest Index
In both 1995 and 1996, highest yields were in treatments without mulch
(CONTROL)or where the living mulch had been suppressed the most (TILUHERB)in
1995) (Table 4). In 1995 there was no significant difEerence (p>0.05) between the conventionally tilled treatments (CONTROL = 10362 kg DM ha-') and the highest yielding mulch treatment (TILUHERB = 8006 kg DM ha-'). The lowest yields in 1995 were for the living mulch treatments in which there was only one method of suppression
(TILL and HERB). Yields for these treatments were 5 5 and 62% of the control treatment yields, respectively. There were no interactions between mulch suppression treatment and
N application.
Al1 mulch treatments in 1996 showed significant reductions (pc0.05) in yield compared to CONTROL plots (1 1583 kg DM hae'). Yields for living mulch treatments
TILL/HERB, HERB,and STRAW were 59, 52, and 46%. respectively of the conventional treatrnents. The mulch treatment with only strip tillage as mulch control
(TILL)showed a 77% reduction in yield.
Mulch suppression had no effect on HI in either year but N had a positive effect in
1996. The average Hi for treatments with N was 0.34 and without was 0.23. Treatments
TILL-ON and HEM-ON both had highiy variable kemel weights and hence variable HI'S.
The HI'S for these treatments ranged between 0.04 and 0.37, probably as a result of excess cornpetition form the mulch in some of the plots. Table 4. Average total corn yield (kg DM hi') and harvea index (HI) for 1995 and 1996 as affecteci by mdch suppression and nitrogen 0.
Treatment * Total yield (kg DM ha-') HI
1995 1996 1995 1996
TILL 5671 c 2705 c -31 a 25 a
HERB 6369 bc 6047 b -32 a -28 a
TTLL/KERB 8006 ab 6844 b .35 a -31 a
CONTROL 10362 a 11583 a -35 a -30 a
STRAW 5351 b -28 a
Nitrogen
O-N 6049 b 4901 b .32a .23 b
*See Table 1 for treatment definitions. Means in same year in sarne category with different letter are significantly different (p Irrigation had no effect on total yield, HI, or any measured parameter (Table 5). Table 5. Total corn yield (kg DM ha-') averaged over mulch suppression treatments in 1995 and 1 996 under irrigated and rainfed conditions. Total yield (kg DM ha-') 1995 1996 Rainfed 10009 a 9663 a Imgated 10579 a 9508 a Means in same year with same letter are not significantly different (p0.OS). 5.3 Corn Growth Parameters Effects of treatments on corn heights and days to silking were similar to those for yield (Table 6). The tallest and fastest matunng treatments were those without mulch. In 1995, the mulch treatment with two methods of suppression (TILL/KERB ) matured to the silking stage in the same number of days as CONTROL (76 days). Other mulch treatments reached the silking stage 4 to 6 days later than CONTROL. The control treatments silked in approximately the sarne number of days in 1996 as 1995 while the mdch treatments were delayed by 5 to 12 days. Figures 9 and 10 show corn development as it is affected by CHU accumulation in both years. There was no sigruficant difference in growth stage attained by the corn between the mulched treatments and the conventional control on any day dunng 1995 @>O. 05). In 1996 however, development was significdy delayed (p 400 more CHU'S to reach the si1king stage. Table 6. Finai corn heights and number of days from seeding to silking for 1995 and 1996 as affiected by muich suppression and "trogen 0. Treatment* Final heights (cm) Days to silking TILL 155 c 133 c 81.8 a 88.4 a HERB 171 b 157b 79.7 ab 8 1.3 bc TILLJHERB 184 ab 172 b 76.1 bc 80.5 c CONTROL 192 a 199 a 75.8 c 76.3 d STRAW 157 b 83.8 b Nitrogen O-N 164 b 150 b 79 a 84 a *See Table 1 for treatment definitions. Means in same year in same category with different letter are significantly different (p<0.05). Figure 9. Corn development for mulch and no-mulch treatments in 1995 as affected by heat unit accumulation. Error bars are equal to + 2 S.E. +No mulch -0- Mulch Corn heat units Figure 10. Corn development for mulch and no-mulch treatrnents in 1996 as afEeaed by heat unit accumulation. Error bars are equal to t 2 S.E. * No mulch -+Mulch Corn heat units 5.4 Living Mulch 5.4. L Mulch totals (between-row) Generally there were few differences between treatments in the mulch DM on each sampling date (Tables 7 and 8). On July 18 and September 29, 1995, HEM had mulch levels 50 and 67% higher, respectively, than mulch levels in the other mulch treatments. This was aiso observed on July 17, 1996. Table 7. Between-row mdch dry matter (kg DM ha-' ) on five sampling dates as aEeaed by mulch suppression and nitrogen (N) in 1995. Treatment * Sampling Date June 15 iune 27 July 8 July 18 Sept. 29 TILL 1451 a 870 a 1117a II41 b 2150 b HERB 1522 a 840 a 1315 a 1810 a 3398 a TILLMERI3 144 1 a 802 a 1089 a 1263 b 1910 b Nitrogen 115kgPJhi' 1353a 861 a 1218 a 1444 a 2418 a O-N 1590 a 813 a 1129 a 1365 a 2554 a * See Table 1 for treatment definitions. Means followed by same letter on the same sampling date and in the same category are not significantly different (p>0.05). There were no differences (p0.05) between treatments in the first and last sampling dates in 1996 (July 5 and October 16). Mid-season (July 17 and August 14) mulch sarnples were larger (pCO.05) in HERB again, but TUwas also comparable to HERB on Augun 14. The treatments with the straw mulch had lower living mulch levels during the mid- season compared to HERB. Table 8. Between-row mulch dry matter (kg DM ha-' ) on four sarnpling dates as affiected by mulch suppression and nitrogeen 0 in 1996. ~reatment* skpling Date July 5 July 17 Aug. 14 Oct 16 TILL 1028 a 834 b 1936 a 2244 a HERB 1468 a 1249 a 2023 a 1999 a TILLMERB 1060 a 878 b 1663 b 1728 a STRAW 1218 a 942 b 1467 b 1850 a Nitrogen 1 15 kgN ha-' 1283 a 1029 a 1790 a 1901 a O-N 1105 a 922 a 1755 a 2010 a * See Table 1 for treatrnent definitions. Means followed by same letter on the same sarnpling date and in the same category are not significantly different (p>0-05). 5.4.2 Mulch totals (within-row) Due to a high variance within and between blocks, within-row mulch yields do not reveal many trends (Tables 9 and 10). However, on the first sarnpling date (June 15) HERB had higher levels (p<0.05) of living mulch DM within the corn rows than al1 other treatments. This was seen to a lesser degree on the first sampling date (July 5) in 1996. Treatment HERB in 1996 had mulch dry weights significantly higher (pC0.05)than Table 9. Within-row mulch dvmatter (kg DM ha-' ) on five sampling dates as afEected by mulch suppression and nitrogen 0 in 1995. Treatment * Sampting Date June 15 June 27 July 8 July 18 Sept. 29 HERB 727 a 783 a 1105 a 1337 b 1813 b Nitrogen O-N 446 a 523 a 1022 a 1321a 2004a * See Table I for treatment definitions. Means followed by sarne letter on the same sampling date and in the same category are not significantly different (p>0.05). On July 18, 1995 TILL showed the greatest mulch DM. In 1996 this sarne relationship was dso observed on July 17 and August 14. On both these dates the treatment with tillage only as the method of mulch suppression (TILL)had mulch DM levels higher than other treatrnents(p<0.05). On July 8 and September 29 in 1995, TILL/HERB had lower levels of mulch in the corn rows than ail other treatments. By the end of the 1996 season there were no significant differences (p>0.05) between treatments for within row mulch weights. Table 10. Within-row dry matter (kg DM ha-' ) on four sampling dates as affected by mulch suppression and nitrogen (N) in 1996. Treatment * Sampling Date July 5 July 17 Aug. 14 Oct 16 TILL 463 ab 735 a 1815 a 2421 a HERB 559 a 519 b 1269 b 1914 a TILL/HERB 305 b 376 b 1 140 b 1754 a STRAW 607 a 413 b 1036 b 1533 a Nitrogen 1 15 kg~.ha-' 526 a 568 a 1246 a 2077 a O-N 441 a 453 b 1384 a 1734 a * See Table 1 for treatment definitions. Means followed by same letter on the same sampling date and in the same category are not significantly different (p>O.OS). 5.4.3 Living mulch composition The percentages of clover and weeds in the living mulch from the within-row sarnples for both years are show in Tables 1 1 and 12. The remainder of the mulch was corn posed of various grass species including Poa pratense, Dactyiis glomerata. PPh(ezmz pratense, Festtica an~ndinacea,Alopeczcms pratensh. There were a number of areas within the smdy site with large concentrations of quack grass (Elytrigin repens (L.) Nevski). Clover composition ranged from a low of less than 3% to aimost 20%. In 1995 HERB had significantly higher (p<0.05) clover proportions than TILL and TILLMERB. There were no significant differences (pz0.05) in the proportion of weeds in the separate treatments in 1995 even though there was a wide range of values (4.1 to 23.0?40) Table II. Muich composition (%) on MO sampling dates as Bected by mulch suppression and nitrogen (N) in 1995. Treatment* July 8 September 29 clover mSS weeds i clover gras weeds TILL 6.0 b 87.9 a 6.1 a HERB 18.3 a 77.6 a 4.1 a TILWHERB 6.0b 78.9 a 15.1 a Nitrogen 115kgNha-' 10.1 a 81.7 a 8.2 a 11.0 a 75.9 a 13.1 a O-N 10.0 a 81.5 a 8.5 a 9.5 a 71.7 a 18.8 a * See Table 1 for treatment definitions. Means followed by same letter on the sarne sarnpling date and in the same category are not significantly different (p>O.05). Clover content in the sward 1996 (Table 12) was lower than in 1995 but again there was a wide range between treatments (2.5 to 15.0%). There were no significant differences (p>0.05) between treatments. The addition of N had no effect on the amount of clover in the sward, Table 12. Mulch composition (%) on four sampling dates as affected by mulch suppression and nitroyen (N) in 1996. Treat ment * July 5 July 17 Aug 14 Oct 16 clover grass weeds clover gras weeds clover grass weeds clover grass weeds TILL 5.2 a 65.9 a 28.9 a HERB 11.9 a 77.9 a 10,2 a TILLMERB 6.2 a 72.3 a 21.5 a STRAW 4.6 a 85.8 a 9.6 a Nitrogen 1 15 kgN ha" 8.0 a 75.0 a 17.0 a O-N 6.0 a 75.9 a 18.1 a * Sec Table 1 for treatmcnt dcfinitions. Means followed by same letter on the same sampling date and in the same category are not siynificant (p>0.05). While clover percentages were generdy lower in 1996 than in 1995, weed percentages were higher. The range in values of weed percentages was high (9.6 to 34.6%) but there were significant differences (p 5.5 Frost Effects Al1 living mulch treatrnents exhibiteci more frost damage (p<0.05) than the conventionally grown treatments (Table 13). Treatments without mulch (CONTROL)had a frost damage score of 1.56 while the mulch treatments had scores ranging fiom 2.14 to 2.26. In this analysis TILL and STRAW were combined since the straw mulch had not been applied by the June 22 froa. Table 13. Frost damage scores as affected by mulch suppression and nitrogen (N) for the frost event on June 22, 1996. Treatment * Score TILWSTRAW 2.26 a HERB 2.21 a TILL/HERB 2.14 a CONTROL 1.56 b Nitrogen Tl 5 kgNW'- 2.1 a O-N 2.0 a * See Table 1 for treatment definitions Means followed by same letter are not significaotly different (p0.05 ). 5.6 Soii Resistance to Penetration Differences in soi1 resistance to penetration were found only between 10 and 20 cm (pc0.05, Table 14). There were no differences found between the mulch treatments and the bare ground treatments for any other depth. Table 14. Soil resistance to penetration (bars) under mulch and no mulch (CONTROL) in October 1996. Depth Soil resistance to penetration (bars) Mulch 2 a 10 a 26 a 44 a No mulch 2 a 6 a 12 b 42 a Means followed by same letter are not significantly different (p>0.05 ). 5.6 Soil Moisture The results frorn the SM measurements are shown in Figures 1 1- 13, dong with daily precipitation. Up until about July 18, 1996 (C.D. 199) there were little differences in SM between the two lower depths (15-25cm and 50 cm) between the mulch and no mulch. Frequent Ructuations in SM occurred in the surface layer (0-15 cm) where the greatest differences between mulch and clean cultivated plots were evident. At al1 depths (0-15, 15-25,and 50 cm), the difference in SM between mulch plots and clean cultivated plots increased in late July and into August. Figure 11. Volumetric soi1 moisture (0-15 crn soi1 depth) under muich and conventional tülage manuexi during the 1996 growing season. Figure 12. Volumetric soil moisture (15-25 cm soil depth) under mulch and conventionai tillage measured during the 1996 growing season. r ZOO I l i Figure 13. Volumetric soi1 moisture (50 cm soil depth) under rnulch and conventional tillage measured during the i 996 growing season. 6.0 DISCUSSION 6.1 Corn Y ields With sutficient suppression of a living mulch, it is occasionally possible to achieve corn yields comparable with conventional tillage methods. In 1995, yields from the TILUHERB treatment were not significantly different (p>0.05) from the CONTROL yields. A similar local study in 1994 had yields of 1 1,573 and 1 1,947 kg DM ha-', for identical TILUHEiU3 and CONTROL treatments, respectively (Augustin, 1994). Where only one method of mulch suppression was used, or none at dl, corn yields were reduced significantly (p<0.05). The lower yields in the mulch treatments in 1996, however, may be more indicative of the difficulties of using living mulches in locations with relatively short, cool growing seasons. In 1996 there was a 40% reduction in corn yield in the TILUHERB compared to the CONTROL. The living mulch afFected the growth and development of corn in a number of ways. According to observations, germination and emergence, as well as phenological development, were delayed, and LAI and final height were reduced compared with conventional methods. Corn grown with a living mulch was also severely damaged by the June 22 frost. These delays in development and the damage due to ffost resulted in a reduction in final yield. Much of this developmental delay may be attributable to a reduction in soil temperature. Although soil temperature was not measured in this study, numerous authors have reported lower soil temperatures under mulches and in no-till conditions (Adams et al., 1970; Mock and Erbach, 1977; Horton et al., 1994). Adams et al- ( 1970) showed a difTerence of at least 1'~lower at 15 cm depth under a suppressed sod compared to conventional plots. It was suggested that this difference was due to a possible insulating effect of the sod. Unger (1 978) recorded soil temperatures of 3-5 'C lower at 10 cm depth under 12 t ha-' of applied mulch compared to bue soil. Horton et al. (1 994) showed mulched soils may be as much as 8 OC lower than bare soils at the 2.5 cm depth. DiReremes were greatest as the bare soil started to dry. Soi1 moisture plays a dominant role in determining soil temperature. Since soils in this region generally are saturated early in the growing season, evapotranspiration and drainage will have a large impact on the warming of the soil. Soil evaporation (E) is govemed by processes expressed in the equation: where A is the slope of the saturation vapour pressure curve, R, is the net radiation, G is soil heat flux, p is air density, C, is the specific heat of dry air at constant pressure, e, and e, are the vapour pressure at the evaporating surface and of the atrnosphere, respectively, r,, is aerodynamic resistance, L is latent heat of vaporization, y is the psychometric constant, and r, is surface resistance (Monteith, 1975). Mulches and surface residues will affect a number of the variables that control E, primarily R,, , G, and r,. Net radiation is the baiance of energy resulting From incoming shortwave (s$) and longwave (L$) radiation and outgoing radiation (and L?). Incoming radiation will be the same for both mulched soils and bare soils but reflected shortwave (s?) will generally be higher nom a mulch layer since the albedo of a living mulch is usually higher than that of a bare wet soil (Oke, 1987). This results in a lower % for mulched soils. The r, is a funftion of windspeed at the soil surface. Crop residues have been shown to significantly reduce surface wind speed compared to bare soil (Heilman et al., 1992). It is probable that living mdches would have a similar effect. Net radiation will be lower and r, will be higher where there is a mulch layer, and therefore soils will be wetter. If soils under a mulch do not dry as quickly as bare soils, G, the soil heat flux will be affected since it is influenced by the higher heat sotrage and transfer capacity of water. Heat in a wet soil will be dispersed more evenly throughout the soil profile (Steiner, 1994). This will slow wanning of the soil in the spring. If mulched soils are slow to wann up, the VPD directly above the soil will be lower, Merreducing evaporation from the soil. Soi1 rnoishw readings in 1996 began on June 28 (Calendar day (CD) 180) and at that thethe SM levels in the mulch plots were 6% higher than the conventional plots. However, the difference in SM, and therefore soil temperature may have been even greater at the depth the seed was planted. Measurements were an average between O and 15 cm, but the seed was placed at a depth of about 1-3 cm. Evaporation From this level would have been greater and soi1 temperatures in this seed zone could be expected to be higher in the conventionally tiiled plots. This would account for the 5-9 day delay in corn emergence observed between conventional and mulch plots. Adams et al. (1 970) reported a delay in emergence in mulch plots. They suggested that the delay in emergence contributed to the 7-20 day delay in tasselling seen in plots with suppressed mulch compared to the conventional treatments. Reduced soil temperatures in the mulch plots was thought to be the cause of this delay in germination and emergence. Early in the growing season it could be expected that the two mulch treatments in which tillage was used to suppress the rnulch (TILL and TILL/HERB) would be similar in terms of soil warming. It would also be expected that the one mulch treatment with only herbicide to suppress the mulch would be delayed compared to the strip tilled plots since there was no turning of the soil in the corn row. The corn developrnent results in 1995 suggest a dEerence between the tilled treatments and the HEM treatments but in 1996 the frost made any differences in corn development difficult to analyze. The difference appears to be minimal. The delay in germination and ernergence in mulch treatments may have been compounded by competition from the mulch throughout the growing season, resulting in the lowered yields in the mulch treatments compared to the bare ground. Cornpetition dunng the early growing period for the corn (the 4-6 leaf stage) would be primarily for light. Since SM through that period appeared adequate (and higher in the mulch treatments than in the conventionally tilled controls), cornpetition between the corn and the mulch for water would have been unlikely. Just before each mowing, the average mulch height was 12 cm and up untii this stage the corn was less than 13 cm in height. In both years, the mulch in between the corn rows was mowed three times before the corn reached 13- 15 cm. Shading of the corn early in the season may have contnbuted to the delay in development. Cornpetition for nutnents wouid have been minimal early in the season as the fertilizer had been recently applied in the same manner to al1 plots and the demand by the corn at this stage was low. There was also no evidence that the rnulch benefited from the applied N. Harvest index was the only parameter measured for corn growth and developrnent that was not afEected by the mulch. It is possible that this attribute is sornewhat resistant to change. Duvick (1984) shows little or no change in HI over 50 years of corn breeding. Yield gains over that time are attributable to other factors including increased tolerance to hi& plant densities, increased kemel size, resistance to insect infestation and stock lodging, but not to an increase in grain yield/plant weight. Even under conditions where water, nutnents, or light are limited HI changes little (Duvick 1984; Tollenaar rf al. 1997). 6.2 Nitrogen Effecb Apart from the obvious effect each year of increased production where there was N added compared to where there was no additional N, there was no yield evidence of any benefit to the corn from N released by decomposing clover fiom year 1 to 2. Akobundu (1982) found corn yields could be maintained over four cropping events (2 years of continuous corn) when there was an associated leguminous living mulch (Psophocarpiîs palutris), even with no added N. Grubinger and Minotti (1990) also reported higher corn leaf N levels fkom plots where a white clover living mulch had been partially rototilled two weeks afler corn emergence. Yieids in those treatments were consistently as high as yieids in the cultivated control. Grubinger and Minotti (1990) estimated roughly 22 kg.N ha-' had been incorporated when the clover was rototilled. However, in both of these studies, pure stands of legurnes were used. Wleit was not possible in this snidy to statistically analyze for any residuai N effect over two years, there did not appear to be any effect. The low percentage of clover (1 1 % and 6 % by weight in 1995 and 1996, respectively) in the plots is probably the main reason. These arnounts of white clover were less than expected derobservations of the plot area in the Autumn of 1994. The arnount of N supplied to the corn from a sward with less than 10% clover would be negligible and masked by the inherent soi1 variability. The reductions in yields due to kost in the second year would probably mask any residud N effect. Even where clover fonned a high percentage of the sward some mechanical disturbance of the sward may be required to maxirnize decomposition and rnineralisation of the N contained in legume root nodules and tissues, since these processes are slower under intact and undisturbed soils (Box et ai., 1980; Needham, 1983). The tilling of a stnp for corn planting rnay serve this purpose for a release of N early in the season, whereas killing the sward with herbicide alone may not have as great an effect. 6.3 Imgation Imgation had no effect on any parameters measured either in treatments with mulch or without mulch. Results from other studies show imgation effects pnmarily in years with moisture stress and little or no efEect on yield when there is sufficient precipitation. Vrabel(1983 as quoted in Grubinger and Minotti, 1990) reported reduced corn yields due to an extended dry period. Adams et al. (1 970) also showed a large effect of irrigation in two of the three years of their study in the South-east of the USA In the year that showed no effect from imgation, there was above average rainfall. In one of the years where there was a significant irrigation effect on corn yield, there was above average rainfall, but both July and Augua had precipitation less than 50% of normal. In the current study both years had greater than average rainfall(18% higher in 1995 and 38% higher in 1996). While August in 1 995 and 1996 was drier than normal (35 and 7 1% lower, respectively), July had higher precipitation than normal (57 and 10 1% higher, respectively). Other studies have reported no irrigation effects when there has been sufficient rainfall. Lake (199 1). in a study of corn cropping in grass living rnulches. reported no improvernent in corn yield corn imgation. Tollenaar et al. ( 1997) compared the response of two corn cultivars (old and new) to various stresses in Southem Ontario. Imgation had no effect on either cultivar for any of the parameters measured. In al1 three years of their study there was sufficient rainfall to maintain high SM levels. There was, however, an increase in weed growth due to imgation. The absence of an imgation effect in both conventional and mulch plots suggests that there was adequate precipitation and SM. Cornpetition between the corn and the mulch for water in this situation would be minimal and would not have cause the delayed corn development and the reduction in yield. 6.4 Soi1 Moisture As a number of authors have noted ( Kurtz et al.. 1952; Jones et d,1968; Hill and Blevins, 1973). SM tends to be generally higher under living mulches and no-till. This effect appears to be due to a combination of a reduction in evaporation, increased infiltration and percolation, and an increase in organic matter content. Although there was no irrigation effect on corn yields in this study, either with rnulch or without mulch, the results of SM measurements in 1996 do bear out the observations of previous studies. Soi1 moisture levels were always higher under mulch than under bare soi1 after the middle of July. From the time of planting to the middle of July the SM levels at al1 measured depths were equal. While there rnay be higher SM levels under mulch. it is not clear whether this has a beneficiai impact upon corn yields. It is possible that the higher levels of water available to the crop mitigates some of the other effeas of cornpetition. However, the higher SM probably resulted in a decrease in soil temperature under the mulch. 6.5 Frost The frost on June 22, 1996 was unexpected but the differential effects it had on mulch and conventional treatments are important. Based upon the frost scores. corn grown in a mulch, no rnatter how the mulch was suppressed, was damaged far more than corn grown in the conventional controls. The developmental stages of the corn between the mulch and bare soil was not different enough to account for the difference in damage. The darnage may be partially attributable to the higher SM levels and lower temperatures under the mulch. Since the amount of long wave (LW) radiation (heat) given off by an object is directly proportional to its temperature, a wann surface will emit more LW radiation than a cool surface. On a night when the air temperature drops to near 0' C, LW radiation emitted by the soil surface may mitigate the low arnbient temperature somewhat. Temperature readings for the site were measured at a height of approximately 1.5 m above the ground. Minimum temperatures measured just above the surface are typically lower than those recorded 1.5 m above the surface (Environment Canada, 1982) and so it is expeaed that the low temperature of - 1.8'C recorded at the site was higher than the temperatures experienced by the newly emerged corn plants. Based on climate data fkom the period 1951 - 1980 (Environment Canada, 1982) there is a 10% chance (1 in 10) of frost occurring later than June 19 in the area near the study site and there is a 25% (1 in 4) chance of a spring fkost later than Iune 1 3. The latest recorded fiost date for the Nova Scotia Agricultural College was July 3. Since most corn would have been planted before these dates in order to maximize the length of the growing season, there is a good chance (at least 1 year in 10 and probably 1 year in 4) that emerging corn will experience night time temperatures below 0' C. 6.6 Straw Mulch The straw mulch in 1996 was marginally successful. While it yielded the same as both HERB and TILLERB (535 1 kg ha-' compared to 6047 and 6987 kg ha-' , respectively: n.s. p>0.05) it was difficult to manage. The straw could only be placed in the corn row after the corn had reached a height of at least 10 cm and even then it was difficult to lay a thick enough layer in the corn row that suppressed the mulch but did not shade the corn plants. It was finally chopped with a lawn mower and blown against the corn row. It was somewhat effective in reducing the mulch growth dunng July and early August. Suppression was no longer evident in October. 6.7 Methodology Improvements The use of two levels of N (O and 1 15 kg N ha-') in the study was designed to test for an effect on the yield of corn of any N released by the incorporated mulch and any N released by decomposing clover tissue. An initial examination of the sward in the fdl of 1994 showed a higher percentage of clover than was later found during the two growing seasons. Since the clover content was so low, it would have been reasonable to use only the recornrnended level of N within the design. Aiternatively, increasing the clover content of the mulch to more than 50% by reseeding may have made a test for residuai N contribution more successfùl. Mowing of the mulch in between rows early in the growing season was done whenever the average sward height reached 12 cm. This method was chosen instead of mowing at regular intervals to account for changes in mulch growth rates. However, mowing of the înulch may have been more effective if the mowing times were based on . corn seedling height. If the mowing ensured the mulch never was taller than the corn, it is probable that mowing would have been required at shoner intervals in the first few weeks dercorn emergence but afler that the interval would have been quite long. In this study. there were times when the mulch was higher than the corn seedlings for at least a week before mowing. This would have reduced the light reaching the corn, and possibly have hnher delayed corn development which was already set back due to the cooler soils under the mulch. Regular measurements of soi1 temperature taken in both the mulch plots and the control plots would have been a worthwhile addition to this study. Without this specific data, the results are only speculative in many cases. It would be important to begin measurements as early as possible in order to determine how soon differences in temperature show up. In addition to these temperature measurernents, SM readings at smaller depth increments might be helpfiil in detemiinhg the extent of differences in SM near the surface between mulched and bare ground plots. The use of straw is problematic as a suppression technique for the mulch. It is not possible to put on a layer thick enough to aid in the suppression until the corn is relatively ta11 (10-1 5 cm) and even then it must be chopped and blown against the corn row to suppress any mulch regrowth from the tilled arip. 6.8 Conclusions Studies of corn grown in living muIches which repon equal or increased yields compared to conventional tillage have been performed primarily in regions with milder springs and hotter surnrners than N.S. (Hartwig, 1988; Grubinger and Minotti, 1990; Fischer and Bumll, 1993). From the results of this study, it is possible to suggest corn production in a living mulch can be successful in N.S. but there is a risk which may be greater than with conventional practices. Yields in some years will be comparable to yields from conventional tillage practices. Herbicide applied in strips is better for mulch suppression than tilled stnps and the combination of the two is most successfùl. While measurements of mulch DM did not show control of the rnulch was greatest where both suppression methods were used, the corn yields were highest in these mulch treatrnents. Irrigation in this study had no effect on corn yields. In both years it is probable that there was adequate precipitation for corn growth but in the surnmer of 1997 precipitation was far below normal and it is possible that cornpetition for rnoisture between the mulch and the corn rnight have reduced the corn yields. There was no noticeable effect of a transfer of N fiom decomposing mulch tissue to the corn. In the Maritimes where corn produaion is nsky even under conventional systems, altering the growing practices and thereby increasing the risk rnay not be practical. There are recommendations that no-till corn not be planted on poorly drained soils due to the delay in emergence and development caused by the lower soil temperatures. This recommendation rnay be more important when recomrnending the use of living mulches. If the presence of the mulch retards the warming of the soil in the spnng and increases the risk of fiost damage to the crop, great care would have to be exercised in choosing suitable sites for corn production. It rnay be possible that cold tolerant corn cultivars rnay be developed which would either respond with faster development under early season cool temperatures, or be tolerant of bnef periods of freezing temperatures (Duncan and Widholm, 1994). The use of living mulches in vegetable and agronornic crop production has been studied extensively in experimental conditions but rarely used by fmers (Hartwig, 1996: personal communication). There is considerable evidence (some contradictory) concerning the use of living mulches and how one rnay attain reasonable yields with them; however, accurate costs for the pnmary crop are difficult to calculate from experimental conditions, especially since labour costs are not included. Living mulches do have potential for agronomic and horticultural crop production but their use for corn production rnay be limited. In cooler regions they rnay be more practical for crops that are more tolerant of cool temperatures. LITERATURE CITED Abdul-Baki, AA., and Teasdale, J.R 1993. A no-tillage tomato production system using hairy vetch and subterranean clover mulches. HortScience. 28: 106- 108. Adams, W.E., Pallas, J.E.Jr., and Dawson, R.N. 1970. Tillage methods for corn-sod systems in Southem Piedmont. Agron. J. 62:646-649. Akobundu, 1.0. 1982. Live mulch crop production in the tropics. World Crops. July/August: 125- 126,144-145. Atlantic Provinces Field Crop Guide, 199 1. Publication No. 100. Atlantic Provinces Agriculturai Se~cesCo-ordinating Comrnittee. Augustin, LM. 1994. Evaluation of living rnulches in corn. Unpublished fourth year project. Nova Scotia Agricultural College, Truro . Barnes, D.L.and Woolley, D.G. 1969. Effect of moisture stress at different stages of growth. 1. Cornparison of a single-eared and a two-eared corn hybrid. Agron. J. 61 :788- 790. Beale, O.W., Nua, G.B., and Peele, T.C. 1955. The effects of mulch tillage on runoff, erosion, soi1 properties, and crop yields. Soil Sci. Soc. Am. Proc. 19:244-247. Benson, G.O. and Pearce, R.B. 1987. Corn perspective and culture. pp 169-23 1. In: Watson, S.A. and Ramstead, P.E. (eds.) Corn, chemistry and technology. AACC St. Paul, Mn. 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Topp, G.C., Davis, J.L., and Annan, A.P. 1980. Electromagnetic determination of soil water content: Measurernents in coaxial transmission lines. Water Resources Research. 2 61574-582. Unger, P.W. 1978. Straw mulch effects on soil temperatures and sorghum germination and growth. Agron. J. 70:858-862. VrabeI, T.E. 1983. Effects of suppressed white clover on sweet corn yield and nitrogen availability in a living mulch cropping system. Ph.D Thesis, Comell Univ. Ithaca, New York. Watson, S.A. 1988. Corn marketing, processing, and utilization. pp. 88 1-940. In: Sprague, G.F. and Dudley, J.W. (ed.) Corn and corn improvement. Amer. Soc. Agronomy. Madison, Wisconsin. Wiles L.J., William, RD., Crabtree, G.D., and Radosevich, S.R. 1989. Analyzing cornpetition between a living mulch and a vegetable crop in an interplantins system. J. Amer. Soc. Hort. Sci. 1 14: 1029-1034. IMAGE EVALUATION TEST TARGET (QA-3) APPLIED 4 IMAGE. lnc - 1653 East Main Street --,-. Rochester. 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