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Agronomy & Horticulture -- Faculty Publications Agronomy and Horticulture Department
1995
Velvetleaf (Abutilon theophrasti) Recruitment, Survival,Seed Production, and Interference in Soybean (Glycine max)
John L. Lindquist University of Nebraska-Lincoln, [email protected]
Bruce Maxwell Montana State University
Douglas Buhler University of Minnesota
Jeffrey Gunsolus University of Minnesota
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Lindquist, John L.; Maxwell, Bruce; Buhler, Douglas; and Gunsolus, Jeffrey, "Velvetleaf (Abutilon theophrasti) Recruitment, Survival,Seed Production, and Interference in Soybean (Glycine max)" (1995). Agronomy & Horticulture -- Faculty Publications. 619. https://digitalcommons.unl.edu/agronomyfacpub/619
This Article is brought to you for free and open access by the Agronomy and Horticulture Department at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Agronomy & Horticulture -- Faculty Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Weed Science, 1995. Volume43:226-232
Velvetleaf(Abutilon theophrasti) Recruitment, Survival, Seed Production,and Interference in Soybean(Glycine max)'
JOHNL. LINDQUIST,BRUCE D. MAXWELL,DOUGLAS D. BUHLER, and JEFFREYL. GUNSOLUS2
Abstract.Field studies were conducted at Rosemount,MN, in However,these strategiesrequire a detailedquantitative under- 1992 and 1993to quantifythe demographicprocesses regu- standing of the factors regulating the growth and population lating the populationdynamics of velvetleafin soybeanas dynamicsof weeds. part of a corn-soybeanrotation. A consistent6.8 ? 0.5% of The use of economic thresholdsto determinewhen a herbi- the total velvetleafseedbank emerged each year. Less than cide application is warrantedhas gained attention (28) as a 21% of all velvetleafseedlings survived each year in mixture potential method of reducing herbicide use while minimizing with soybean,due in part to Verticiliumspp wilt infection. economic loss. However,single-year economic thresholds do not The probabilityof seedling survival varied across time of adequatelyaccount for the impact of weed seed productionon emergence.Velvetleaf seed productionin the absenceof crop weed populations in subsequent years (29). Effective use of competitionwas 125 and 227 seedsplant-' in 1992and 1993, economic threshold theory in weed managementcan only be respectively.Velvetleaf plants that emergedearly produced achieved when an accurate,quantitative understanding of the greater numbers of seed than later emerging plants. factors that regulateweed populationdynamics across growing Velvetleafsurvival and seed productionwere reducedup to seasons is obtained. 82% in the presence of crop competition.Soybean yield The populationdynamics of an annualweed areregulated by varied across soybeandensities in both years, but was not five demographicprocesses: seedling recruitmentand survival, reducedacross velvetleafdensities. Nomenclature: Velvet- seed production,dispersal, and seed survival in soil (12, 23). A leaf,Abutilon theophrastiMedicus. #3 ABUTH;soybean, Gly- schematicdiagram of the populationdynamics of an annualweed cine max (L.) Merr.'Evans'; corn, Zea maysL. is shown in Figure 1. The boxes in Figure 1 represent state Additionalindex words. Weed demography, emergence, weed variablesand can be measuredin the field. The valve symbols seedlingmortality, modeling, Verticillium, ABUTH. representthe five demographicprocesses, each of which may be influencedby a numberof factorsincluding competition, preda- tion (by herbivoreor pathogen),and migration.These processes INTRODUCTION can be representedby biologically realisticequations that quan- Crop yield loss due to weed interferenceis the economic tify the rate of transitionfrom one state to another. foundationof weed science (33). Herbicideshave largely been Several studies have been conducted to quantify the demo- developed to eliminate yield loss due to weed interference. graphicsof annualweeds in cereal crops (8, 11, 12, 13, 17, 42). Despite heavy use of herbicides in recent decades, crop losses from weed interferencein agriculturecontinue (13) due to ge- netic diversity, high seed production, and morphological and physiological adaptabilityof weedy species. While herbicides ------P may be highly effective at reducing weed populations, their continued use is often offset by increased frequency of more Crop[4 IAdult Weeds Produced Seeds tolerantweed species (14) or by the developmentof herbicide resistance (16, 25). Restrictionsdue to governmentregulation S81 and public pressuremay severely reduce the available chemical -----> weed controloptions in the nearfuture (1, 6, 16, 24, 34, 44). Thus there is growing need for the development of cost effective, Seedlings D environmentallysafe, alternativeweed managementstrategies. Em
'Received for publicationMay 18, 1994 and in revised form November 15, Seedbank 1994. ContributionNo. 21,186 from the Minnesota Agric. Exp. Stn., St. Paul, MN 55108. 2FormerGrad. Res. Asst., Dep. Agron.Plant Gen., Univ. Minnesota,St. Paul, l S8bDA MN 55108; Asst. Prof., Plant,Soil and Environ.Sci. Dep., MontanaState Univ., Bozeman, MT 59717; Res. Agron., U.S. Dep. Agric., Agr. Res. Serv., National Soil TilthLab., Ames, IA, 50011; andAssoc. Prof.,Dep. Agron.Plant Gen., Univ. Figure 1. A schematicdiagram of the populationdynamics of an annualweed. Minnesota,St. Paul, MN 55108. Currentaddress of first author:Dept. Agron., Boxes and valves representmeasurable state variablesand demographicprocess Univ. Nebraska,Lincoln, NE 68583. variables, respectively. Em = recruitment,S,1 = seedling survival, P = seed 3Lettersfollowing this symbol are a WSSA-approvedcomputer code from production,D = dispersal,and Ssb = seed survivalin the seedbank.Dashed arrows Composite List of Weeds, Revised 1989. Available from WSSA, 1508 West indicatethat the cropmay influencethe weed populationat these stages.Redrawn UniversityAve., Champaign,IL 61820-3133. from Kropffand Lotz (23). 226 WEED SCIENCE
However, few data are availableto quantifythese demographic showed no symptomsof wilt andwere not tested for Verticillium parametersfor most weed species importantin corn and soybean infection. in the U.S. corn belt. Seedbank estimates. Velvetleaf seedbank estimates were ob- Velvetleafis a majorweed in corn and soybean(41), infesting tained on May 5, May 20, and August 20, 1992, and on May 19 morethan 9 million ha of soybeanwith an annualcontrol cost of and August 18, 1993, by removing 16 (1.9 cm diam.) soil cores $225 million (40). Bauer and Mortensen(3) developed a model to a depth of 20 cm in each plot. The 16 samplesfrom each plot to calculate the economic optimum thresholdfor velvetleaf in were pooled, bagged, and stored in a dryeruntil they could be soybean based upon the population dynamics of velvetleaf. wet sieved with a fine mesh screen and the velvetleaf seeds These authorsindicated that the primaryweakness of theirmodel separatedby handand counted (22). Viabilityof recoveredseeds was the limitedbiological dataavailable for accurateparameteri- was virtually 100%, determinedby applying a tetrazoliumtest zation. to a subsampleof 300 seeds. In this research,five demographicparameters regulating the Velvetleaf seedling emergence and survival. Immediatelyafter populationdynamics of velvetleaf in soybean as part of a corn- planting,permanent subplots were establishedwithin each plot soybeanrotation were quantifiedusing data obtainedfrom field using threerandomly placed wire rings (1.0 m2total area). Within experimentsconducted over two growing seasons. Additionally, subplots,velvetleaf seedling emergencewas measuredby regu- soybean yield as a function of soybean and velvetleaf density larly counting and markingnewly emerged seedlings with col- was quantified. ored wire. A differentcolor of wire was used at each sampling date (ca. weekly). Velvetleafplants markedwith the same color wire belonged in the same cohort.Seedling mortalitywas meas- MATERIALSAND METHODS uredby countingand removing wires thatno longer markedlive seedlings at each samplingdate. Seedling survivalas a function General procedures. Experimentswere conductedwithin adja- of time of emergencewas interpolatedfrom seedling mortality. cent fields at the Universityof MinnesotaAgricultural Research In 1992, the firstcohort of velvetleaf seedlingswas very large. Stationnear Rosemount, MN, in 1992 and 1993 on a Waukegan To ensurethat velvetleaf populationswere not excessive, benta- silt loam soil (fine-silty over sandy, mixed, mesic Typic zon [3-(1 Hapludoll)with 6.5% organicmatter. In 1991 and 1992, 72 (25 -methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one m2)plots were seeded (by Bussler,7) with corn and velvetleaf at 2,2-dioxide] was appliedPOST at 1.1 kg ha-1to controlthe first a range of densities using a randomizedcomplete block design flush only. Mortality due to the treatmentwas determinedas with threereplicates. These experimentsserved as templatesfor describedabove. Mortalityof velvetleaf seedlingsemerging after our experimentsin 1992 and 1993, when plots were relocated. the 1992 bentazontreatment was high due to wilt. Therefore,no Fields were disked twice in the autumnand twice for spring POST velvetleaf control was practicedin 1993. seedbed preparation.Potassium was applied (60 kg ha-') to the Velvetleaf seed production. Within each subplot, velvetleaf field in 1992, and no fertilizerwas applied in 1993, as recom- seed capsules were removed as they matured. In 1992, the mendedby the Universityof MinnesotaSoil TestingLaboratory. capsules per subplot were pooled and the numberof lobes per Soybean (cv. Evans) row spacing treatmentsof no crop, 24, 36, capsule counted. Total seed productionper subplot was calcu- and72 cm betweenrows were seeded on May 18, 1992, andMay lated as the productof the total numberof lobes per subplotand 13, 1993. Soybean sowing density within a row was held con- the estimate of 2.95 seeds per lobe (obtainedby counting the stant across treatmentsat 33 plants m-1 row, so spacing and numberof lobes and seeds per capsule in a representativesub- populationdensity were confounded.Three plots per block (one sample of 20 capsules).In 1993, seed productionwas quantified plot for each row spacingtreatment) were randomlyselected and for each cohort by removing capsules separately among the hand weeded for weed-free controls.Three plots per block were differentseedling cohortswithin each subplot. also randomlyselected andhand weeded for crop-freevelvetleaf Soybean yield. Soybeanyield was measuredon October2,1992, monoculturecontrols. In all plots, grass weeds were controlled and October 11, 1993, by cutting plants at the soil surface in with metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2- subplotsand threshingin the field. The resultinggrain was then methoxy-l-methylethyl)acetamide] applied preplant incorpo- bagged, dried at 60 C for 7 d, cleaned, and weighed. ratedat 2.2 kg ha-1,and otherweeds were removedas neededby Statistical analysis. The objective of this researchwas to quan- hand. No precipitationwas measuredwithin five days prior to tify demographicprocesses via the relationshipsbetween meas- plantingin 1992, so approximately1.2 cm of waterwas applied ured state variables. Thus, wherever possible, biologically throughsprinkler irrigation 2 d afterplanting to obtainuniform realistic equationswere used to characterizerelationships, and crop and weed emergence.Subsequent irrigation was not neces- parameterestimates were obtained using SAS (36) NLIN and sary in 1992 or 1993. REG procedures.Data were pooled across years if parameter Wilt symptoms were observed in the velvetleaf populations estimateswere not significantlydifferent (evaluated by compar- in both years of this study.Randomly selected velvetleaf plant ing 95% confidence intervals).Where a nonlinearequation was samples were submittedto the University of Minnesota Plant fit to the data,an approximater2 value was obtainedby subtract- Disease Clinic in 1992, and Verticillium(species unknown)was ing the ratio of the residual sums of squares (RSS) to the isolated throughculture of stem and root tissue. The soybeans correctedtotal sums of squares (CTSS) from 1 (i.e., r2 = 1 -
Volume43, Issue 2 (April-June) 1995 227 LINDQUISTET AL.: VELVETLEAFIN SOYBEAN
Table1. Cumulativevelvetleaf seedling emergencein relationto days afterplanting (DAP) and growing degree days (GDD) accumulatedbeginning May 1 in 1992 and 1993.
1992 1993
DAP GDD Emergencea DAP GDD Emergenceb
C % C %
0 144 0.0 0 84 0.0 8 203 48.1 ? 7.9 7 116 1.5 ? 1.8 18 289 67.9 ? 7.7 19 173 36.0 ? 10.3 28 392 78.3 ? 6.6 26 223 52.4 ? 11.8 36 451 97.8 ? 2.4 33 293 67.4 ? 12.5 50 568 98.8 ? 1.9 40 351 90.7 ? 5.5 57 637 99.2 ? 1.6 46 407 95.0 ? 4.5 64 697 99.3 ? 1.5 54 488 98.7 ? 1.7 71 753 99.4? 1.3 68 635 99.4? 1.3 78 809 99.5 ? 1.2 85 808 99.8 ? 0.7 86 896 99.7 ? 0.5 110 1107 100 ? 0.0 100 1012 100 ? 0.0
aMean? one standarddeviation (n = 69). bMean? one standarddeviation (n = 72).
RSS/CTSS). Wherepairwise comparisons of means were made, error,to the observedvariation in emergenceand its relationship SAS GLM procedurewas used because sample size differed with seedbankcounts. among treatments. The relationshipsin Table 1 and Figure2 providean estimate of velvetleaf seedling recruitment(Em in Figure 1). Robertsand Dawkins (35) suggest that the rate of recruitmentmay be con- RESULTSAND DISCUSSION stantunder a consistentcultivation regime. Thereforethese data may be used to estimate velvetleaf seedling populationdensity Recruitment. Velvetleaf seedling emergence began immedi- based on known seedbankpopulation densities. ately after planting in 1992, but was delayed by approximately Seedling survival. Wilted leaves, a characteristicsymptom of 10 d in 1993 (Table 1). Variableseed germinationinduced by Verticilliuminfection (37), were observedon nearlyall velvetleaf environmentalconditions may explain the differencein cumula- seedlings in both years, despite wet soil conditions. Often the tive emergence in 1992 and 1993. Hard-seedednesshas been entire plant wilted and died, thus Verticilliumwas an important suggested as a mechanism for velvetleaf seed dormancy,and factor in velvetleaf seedling survival. Percent survival of seasonal regulation of dormancy may result from alternate velvetleaf seedling cohorts in velvetleaf monocultureplots or in freeze-thawing and wetting-drying cycles (41). Emergence might, therefore,be expected to respondto moisture and accu- mulatedheat units (2). Using irrigationto ensureuniform emer- gence of the soybeans in 1992 also may be responsiblefor the c'J Ei 600 - earliervelvetleaf emergencein 1992 comparedto 1993. Differ- cri ences in cumulativeemergence patterns across years were not as ? 1 992 0 = 500 0 1993 0 great when related to growing degree days (GDD, base 10 C, maximum30 C) accumulatedfrom May 1 ratherthan days after 0 00 400 0o0 planting(DAP, Table 1). 0 00 00 Totalseedling emergenceas a functionof the springseedbank a) 300 - 0 o 0o0 0 estimatesin 1992 and 1993 are shown in Figure2. While Harper 000 (19) suggests that density dependentemergence is possible due CD) 0 to the frequencyof 'safe sites' being limited,such a responsewas E 1 ? o D YY=37.4+ 0.068(X) not observed. A linear regression was thereforefit to the data. a) 0 0 0 r2=0.62 Parameterestimates were not significantly different between ~~~0 years so datawere pooled for the analysis shown in Figure2. The o 0 1000 2000 3000 4000 5000 6000 7000 slope parameterof 0.068 ? 0.005 indicatesthat a consistent6.8 ? 0.5%of the velvetleaf seedbankemerged each year.This value Springseedbank (seeds m-2) is about 2% higher than the values reportedfor velvetleaf by Pacala and Silander(31). Seed predation,micro-environmental Figure 2. Total cumulative velvetleaf emergence as a function of the spring seedbank estimate in 1992 and 1993. The slope provides an estimate of the variability,and genetic differences in seed dormancywere not proportionof the seedbankthat emerges each year. The root mean squareerror measured but may contribute,along with seedbank sampling (RMSE) of the regressionis 84 (n = 139).
228 Volume43, Issue 2 (April-June) 1995 WEED SCIENCE
Table 2. Percent velvetleaf seedling survival by time of emergence (DAP) in Table3. Velvetleafseed productionacross years in monoculture(V, n = 19) and monoculture(V, n = 9) and in mixturewith soybean (V + S, n = 63). in mixturewith soybean (V + S, n = 93).
1992 1993 Year V V+S
DAP V V+S DAP V V+S seeds plant-l LSDa
1992 124.65 40.37 69.79 1993 226.78 48.93 74.28 8 12.03 12.03a 7 25.92 20.74 18 2.89 4.03 19 57.97 14.64* LSDb 91.00 42.23 28 3.35 1.62 26 52.48 10.31* 36 11.38 2.44* 33 61.32 10.12* aTukey'sLeast SignificantDifference (at p < 0.05) for comparisonswithin a 50 5.40 1.67 40 52.24 7.09* row. 57 3.70 1.67 46 33.79 4.67* bTukey'sLeast SignificantDifference (at p < 0.05) for comparisonswithin a 64 22.22 0.00* 54 57.26 2.84* column. 71 5.56 3.33 68 14.81 3.31 78 6.94 1.67 85 14.81 3.70 86 30.99 0.00* 100 18.52 1.67* but was not influencedby velvetleaf densityor year.A consistent relationship(e.g., hyperbolic)between velvetleaf seed produc- LSDC 21.19 3.74 LSD 30.41 7.04 tion and soybeandensity was not found. Soybeandensities were aPercentof velvetleaf seedlings survivingthe bentazontreatment. combined,and a pairwisecomparison of velvetleaf seed produc- bAn asterisk indicates values within a row for each year are significantly tion across years in monocultureand in mixture with the crop differentat the p < 0.05 probabilitylevel. was made (Table3). cTukey'sLeast Significant Difference (at p < 0.05) value for comparisons Data presentedin Table 3 representthe seed productivity(P within a column. in Figure 1) of a Verticillium-infectedvelvetleaf populationand may be used to estimatethe numberof seeds produced(the seed rain)by a known populationof matureinfected velvetleaf plants. mixture with soybean are shown in Table 2. In both years, By comparison,Munger et al. (27) observedhigh velvetleaf seed velvetleaf seedling survival was reduced in the presence of productionin one yearof theirstudy (770 seeds plant-1in mixture soybean, greater decreases occurring with later cohorts. The with soybean),and low values in the otheryear (17 seeds plant-' reductionin velvetleaf survivalin mixturewith soybeanmay be in mixture).They attributedthe low seed productionto interspe- due to competition for light because complete canopy closure cific competitionfor water.Zanin and Sattin(45) observedhigh occurredwithin 40 to 50 d after planting (DAP) in the mixed velvetleaf seed production(3379 and 4520 seeds plant-) when stand plots. Survival in velvetleaf monocultureplots was inde- grown in mixturewith maize in Italy. pendentof time of cohortemergence, perhaps because complete The use of a constant seed productionvalue or relationship canopy closure never occurredin either year, despite very high withina populationdynamics model requiresthe assumptionthat populationdensities in some plots. Velvetleafsurvival in mixture all matureplants are identicalin their ability to produceseed. In with soybean was relatively consistentacross soybean densities real populations,a naturalhierarchy of plant sizes occurs as a and years, whereassurvival in velvetleaf monocultureplots was result of differences in relative emergence time, seed size, mi- about9% higher in 1993. cro-environment,and genetic variation(32, 43). Because plant Results show that soybean may have a substantialnegative size is often correlatedwith seed production(39), it is important influence on velvetleaf survival,though the relative importance to considerthe factorsthat determine which plantsbecome large. of the crop in reducing velvetleaf survival may be confounded If we assumea plantthat is largerelative to its neighborsat some by the influence of Verticillium.The potentialimpact of soybean earlystage of growthwill remainlarge (as did Pacalaand Weiner, on velvetleaf seedling survival in conditions where wilt is not 32), thenit may also be assumedthat the earliestemerging plants presentis worthy of furtherinvestigation. will have an inherentadvantage over lateremerging individuals, Data reported in Table 2 representestimates of velvetleaf and thereforewill be responsiblefor producinggreater numbers seedling survivorship (Ss, in Figure 1), and may be used to of seed. Table 4 shows the 1993 velvetleaf seed productionby estimate the maturevelvetleaf populationdensity from known cohort in monocultureand in mixturewith soybean. Seedlings seedling populationdensities. Because velvetleaf seedling sur- emergingbetween 8 and 33 DAP producedgreater numbers of vival did not show clear density dependenceand varied consid- seed than the earliest or later emerging velvetleaf cohorts. As erably as a function of time of emergence (Table2), maximum with seedling survival, seed production was reduced in the survivorshipin the presence of the crop may provide the most presenceof soybean, thoughthe relativeimportance of the crop conservativeestimate for modelingmature velvetleaf population in reducing velvetleaf seed productionmay be confoundedby densities. the influence of Verticillium. Seed production. Multiple regression analysis was used to Soybean yield. Soybean yield was not reduced by velvetleaf determine the influence of velvetleaf density (Nw), soybean densityin eitheryear (Figure 3a). Otherresearch has documented density (Ne), and year on velvetleaf seed production(results not substantialsoybean yield loss due to velvetleaf competition(4, shown). Seed productionwas reducedby soybean interference, 5, 10, 18, 20, 27, 30). The lack of velvetleaf competitiveness
Volume43, Issue 2 (April-June) 1995 229 LINDQUISTET AL.: VELVETLEAFIN SOYBEAN
Table4. Velvetleaf seed production(plant--) by cohortin monoculture(V, n = 9) Soybeanyield as a functionof soybeandensity was quantified and in mixture with soybean (V + S, n = 31). Data are from 1993 field season only. using a hyperbolicfunction (modified from Cousens, 8). Parame- ter estimateswere not significantlydifferent across years so data DAP V V+S were pooled (Figure 3b). Soybean yield increasedwith increas- seed plant-l ing soybean density (decreasingrow spacing) treatments. Seed dispersaland survivalin soil. Figure4 showsthe Au- 7 0.04 0.01 19 132.09 44.26*a gust seedbank(pre-seed rain) estimate plotted against the spring 26 81.52 15.06* seedbank (postemergence) estimate. The slope of the linear 33 113.10 5.61* regressionprovides an estimate of the proportionof seeds not 40 24.84 0.83* emergingthat survive through the growing season in soil 46 9.40 0.00 (a part 54 6.50 0.00 of Ssb in Figure 1). Slope values were not significantlydifferent 68 0.00 0.00 across years so data were pooled. Results show 70 ? 6% of all seeds that do not emerge survive throughAugust in soil. Fifty LSDb 74.39 29.83 one percent of the residual variation in the August seedbank aAn asteriskindicates values within a row are significantlydifferent at the p estimatewas accountedfor by the linearmodel. < 0.05 probabilitylevel. The spring 1993 seedbank estimate is plotted against the bTukey'sLeast SignificantDifference (at p < 0.05) for comparisonswithin a autumn 1992 seedbank estimate (August seedbank 1992 column. plus seed rain) in Figure 5. The slope estimate of this relationship provides an estimate of the density-independentoverwintering observedin this study is assumedto be the resultof infectionby survivalof both newly dispersedseeds and those alreadyin the Verticillium. seedbank. Results show 29 ? 3% of seeds within soil in the autumnwill survive throughthe next spring. Forty-six percent of the residual variation in spring seedbank estimate was ac- 600 0 1992 (a) countedfor by the model. Muchof the observedvariation in seedbankestimates was the N 500 0 | 1993 0 resultof predation(herbivory and pathogens), micro- and macro- - 400 environment,machinery-induced migration, and the interaction 0 among these factors(19, 21, 26, 38). The relationshipsin Figures 4 and 5 provide a combined uoa O 400 800 estimateof dispersaland seed survivalin soil (D and5sb in Figure 1), and may be used to estimatethe spring seedbankpopulation m o 200 g 0 0 density based on known seed rain and seedbank population density in the previousyear. ? 100 o o , I Verticilliummay have a significantimpact on velvetleaf seed- CIn 1000 1 0 20 30 40 50 ling survival, seed production,and competitive ability.A nega- Maturevelvetleaf density (plants m-2) cm E 7000 | 1992 ? 600- b 0 6 1992 (b) 6000 0 1993 o c 0 500 I 1 993 1 0 0 a a) O cn 5000 0 o ? p E 00 Ooo00 c 4000 - O o o 400 , 0 00Co0 0 :: 2300 - D 3000 - 0 0 az O ~~~~0O 0 0 CD 2000 o0 0 00 0 a CZ5 200 go @ cn o0 oo cm o o CD -5-8 lO00(}? Ooo?o?Da Y = 287 + 0.70(X) cn 0m m? 0-O5? o~ 100 YO=aNc (1 + (a/b)Nc)) 0 ooo 00 0 cn 0 /r2 =0.79, c: 0 1000 2000 3000 4000 5000 6000 7000 0 20 40 60 80 100 120 140 160 Springseedbank (seeds m-2) Soybeandensity (plants m-2)
Figure4. Augustseedbank estimate as a functionof thecorrected spring seedbank Figure 3. Soybean yield as a functionof velvetleaf density (a) and soybean (NC) estimate (measuredspring seedbankless total emergence). The slope provides density (b). For (b), parameterestimates are a = 24.39 andb = 415.1. The RMSE an estimate of seed survival in soil throughthe growing season. The RMSE of of the regressionis 62 (n = 147). the regressionis 1079 (n = 139).
230 Volume43, Issue 2 (April-June) 1995 WEED SCIENCE
c'J 2. Alm, D. M., E. W. Stoller, and L. M. Wax. 1993. An index model for predicting seed germinationand emergence rates. Weed Technol. 7:560- E 569. 6000 3. Bauer, T. A. and D. A. Mortensen. 1992. A comparisonof economic and -r Q) ~~~~~~0 0 0 economic optimum thresholds for two annual weeds in soybeans. Weed c,,) 5000 / Technol. 6:228-235. 0 0 4. Bauer,T. A., D. A. Mortensen,G. A. Wicks,T. A. Hayden,and A. R. Martin. -= 4000 0 00 0 00 1991. Environmentalvariability associated with economic thresholdsfor soybeans. Weed Sci. 39:564-569. 0 0 cm 3000 0 5. Begonia, G. B., R. J. Aldrich, and C. D. Salisbury. 1991. Soybean yield 00 a) 0 0 0 0 0 and yield components as influenced by canopy heights and durationof - 20000 0 0 competitionof velvetleaf (Abutilontheophrasti Medik.). Weed Res. 31:117- 00000 0 CO aOeD
Volume43, Issue 2 (April-June) 1995 231 LINDQUISTET AL: VELVETLEAFIN SOYBEAN
27. Munger,P. H., J. M. Chandler,J. T. Cothren,and F. M. Hons. 1987. Soybean 37. Sickinger, S. M. 1981. The effects of Verticilliumdahliae (Kleb.) on (Glycinemax)-velvetleaf (Abutilonthe op hrasti) interspecific competition. velvetleaf (Abutilontheophrasti) and crops. M.S. Thesis, Univ. of Wiscon- Weed Sci. 35:647-653. sin, Madison. 141 pp. 28. Norris, R. F. 1992. Ecological perspectiveson utility of thresholdsfor weed 38. Silvertown,J. W. 1987. Introductiontoplantpopulationecology. JohnWiley management.Weed Technol. 6:182-183. and Sons, New YorkNY. 29. Norris, R. F. 1992. Case history for weed competition/populationecology: 39. Solbrig, 0. T. 1981. Studies on the populationbiology of the genus Viola. Barnyardgrass(Echinochloa crus-galli) in sugarbeets(Beta vulgaris).Weed II. The effect of plant size on fitness in Violasororia. Evolution 35:1080- Technol. 6:220-227. 1093. 30. Oliver, L. R. 1979. Influence of soybean (Glycine max) planting date on 40. Stoller,E. W., S. K. Harrison,L. M. Wax,E. E. Regnier,and E. D. Nafziger. velvetleaf (Abutilontheophrasti) competition. Weed Sci. 27:183-188. 1987. Weed interferencein soybeans(Glycine max). Rev. Weed Sci. 3:155- 31. Pacala, S. W. and J. A. Silander Jr. 1990. Field tests of neighborhood 181. population dynamic models of two annual weed species. Ecol. Monogr. 41. Warwick,S. I. and L. D. Black. 1988. The biology of Canadianweeds. 90. 60:113-134. Abutilontheophrasti. Can. J. Plant Sci. 68:1069-1085. 32. Pacala, S. W. and J. Weiner. 1991. Effects of competitive asymmetryon a 42. Watkinson,A. R., W. M. Lonsdale,and M. H. Andrew.1989. Modellingthe local density model of plant interference.J. Theor.Biol. 149:165-179. populationdynamics of an annual plant Sorghumintrans in the wet-dry 33. Radosevich, S. R. 1987. Methods to study interactionsamong crops and tropics.J. Ecol. 77:162-181. weeds. Weed Technol. 1:190-198. 43. Weiner, J. 1985. Size hierarchiesin experimentalpopulations of annual 34. Radosevich,S. R. andC. M. Ghersa. 1992. Weeds,crops, andherbicides: A plants. Ecology 66:743-752. modern-day"neckriddle." Weed Technol.6:788-795. 44. Wyse, D. L. 1992. Futureof weed science research.Weed Technol. 6:162- 35. Roberts, H. A. and P. A. Dawkins. 1967. Effects of cultivation on the 165. numbersof viable seeds in soil. Weed Res. 7:290-301. 45. Zanin, G. and M. Sattin. 1988. Threshold level and seed production 36. SAS. 1988. SAS/STATUser's Guide,Release 6.03. SAS Institute,Cary NC, of velvetleaf (Abutilontheophrast Medicus) in Maize. Weed Res. 28:347- 1028 pp. 352.
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