Row Spacing, Redroot Pigweed (Amaranthus Retroflexus) Density, and Sugarbeet (Beta Vulgaris) Cultivar Effects on Sugarbeet Developmentl

April-June 2000 Effects on Sugarbeet Development 11

Row Spacing, Redroot Pigweed (Amaranthus retroflexus) Density, and Sugarbeet (Beta vulgaris) Cultivar Effects on Sugarbeet Developmentl

2 Jenny A. Stebbing , Robert G. Wilson\ 4 3 Alex R. Martin , and John A. Smith

2 Entomology Department, University o/Nebraska, Lincoln, NE 68583 3 Panhandle Research and Extension Center, Scottsbluff, NE 69631 4 Agronomy Department, University o/Nebraska, Lincoln, NE 68583

ABSTRACT Field experiments were conducted in 1996 and 1997 near Scottsbluff, Nebraska, to evaluate sugarbeet and red root pigweed yields as affected by sugarbeet row spacing and red root pigweed densities. Row spacings of 46, 56, and 76 cm were compared. In 1996, sugarbeet gained a height ad vantage over redroot pigweed and sugarbeet root and su crose yield was not affected by weed competition, regard less of row spacing. In 1997, redroot pigweed grew at a faster rate due to warmer temperatures and gained a height ad vantage over sugarbeet and shaded the crop. This resulted in a reduction in sugarbeet root yield. Sugarbeet root yield, averaged over all red root pigweed densities, decreased ap proximately 18% and 25% as row spacing increased from 56 to 76 cm and 46 to 76 cm, respectively. Averaged across row spacing, sugarbeet root and top yield were reduced ap proximately 12% from 4000 red root pigweed plants/ha com pared with the weed free control in 1997. At 15000 red root pigweed plants/ha, sugarbeet root and top yields declined approximately 31 %. When averaged over the entire field, 'Monohikari' produced a higher sugarbeet root yield than 'KW2398', but was not as competitive with red root pigweed as 'KW2398'. Additional Key Words: Row width, weed density

IPublished as Journal Series No. 0999-1, Nebraska Agriculture Research Division. 12 Journal of Sugar Beet Research Vol 37, No 2

Weed competition in sugarbeet has been estimated to cause an 8% annual loss of sugarbeet value through reduction in yield and quality (Schweizer 1981). In regions of the Western United States, 10 to 15% of the total annual weed population found in sugarbeet is not controlled by cultivation or herbicides (Schweizer 1980). Low weed densities compet ing with the sugarbeet crop all season can cause a reduction in root yield (Schweizer 1980). Yield loss depends on weed competitiveness, density, and length of time the weeds are allowed to compete (Schweizer and May 1993). Weeds that emerge within 8 weeks after planting or within 4 weeks after the two true-leaf stage of sugarbeet reduced root yield by 26 to 100% (Dawson 1977). Weed biomass decreased 60 to 74% when sugarbeet was weeded once in the two true-leaf stage ofgrowth about 4 weeks after plant ing. Weeds emerging 4 weeks after planting reduced sugarbeet yields by 26% (Wicks and Wilson 1983). In plots that were hand-hoed for 8 weeks after planting, weed biomass was reduced 97% compared with plots that were not hand-hoed (Wicks and Wilson 1983). Approximately 70% of weeds found in sugarbeet crops are broadleaved species (Schweizer and May 1993). Broadleafweeds become most competitive after they begin shading the crop (Wicks and Wilson 1983). Position of leafarea would be as impol1ant as the total area in deciding the competitive outcome between sugarbeet and weed (Legere and Schreiber 1989). Weeds are able to grow two to three times taller than sugarbeet by mid-summer, and as weed density increases, light becomes more limited and sugarbeet root yields decrease (Schweizer and May 1993). The sugar yield of a crop is directly related to the amount of sun light intercepted and converted within plant material to sucrose (Anony mous 1995). Each megajoule of energy from the sun intercepted by the sugarbeet crop is converted into approximately 1 gram of sugar at harvest. The quicker that leaf area is able to develop and completely cover the field area, the higher the yield (Smith et al. 1991). Sugarbeet seeded in April or May often will not have a full canopy until July (Scott and Allen 1978). The highest sugar yield was obtained when the leaf area index at canopy closure ranged from 3.5 to 4.1 (Rover 1994). Effective full-season weed control in sugarbeet should prevent yield suppressing competition and allow harvest of sugarbeet free of any weeds large enough to interfere with harvest or storage (Dawson 1974). Manipulations of row spacing, plant density, and cultivar selection may provide a means ofreducing the impact ofweed interference on crop yield (Malik et al. 1993). April-June 2000 Effects on Sugarbeet Development 13

A study conducted in India (Gill and Verma 1969) showed that row spacing of 40 cm gave the highest yield while that of 50 and 60 cm gave similar yields. In a yield comparison (Yonts and Smith 1997), 56 cm row spacing produced a greater yield of both roots and sugar than 36 or 76 cm rows. Their study showed that 56 cm row width increased sugar ap proximately 0.4 Mglha over both 36 and 76 cm rows. Narrower rows, such as 45 em, are more likely to produce large yields because they help to compensate for poor plant establishment (Anonymous 1995). Sugarbeet has traditionally been grown in 56 cm rows in Nebraska and Wyoming. Growers became interested in using wider row spacing so field equipment could be used for more than one crop with minimal adjustments (Fornstrom and Jackson 1983). Many studies with controlled plant densities demon strated that sugarbeet grown in rows greater than 50 cm produced less than maximum yield (O'Connor 1983). Winner and Merkes (1975) showed some advantages of 40 and 45 cm row spacings compared to 50 and 35 cm row spacings, although the advantages were not statistically significant. Wiklicky (1981) recommended row spacings of 42 to 45 cm to produce a full leaf canopy. Sugarbeet root yield, sugar percentage, and purity were higher for sugar beet planted in 50 cm rows compared with sugarbeet planted in 60 cm rows (O'Connor 1983). Sugarbeet cultivars may differ in competitiveness with weeds. Sugarbeet should be kept weed free until the six true-leaf stage. After the six true-leaf stage, the sugarbeet canopy will aid in the suppression ofweeds and be more competitive with weeds for light and nutrients. A fuIl leaf canopy could be achieved earlier in the growing season by reducing row spacing and selecting cultivars with rapid canopy development. The sugarbeet cultivar may also influence canopy structure because of varying leaf size and shape. This could either increase or decrease the amount of light absorbed. Therefore this experiment was designed to measure the in fluence of the crop canopy of two cultivars on the competitiveness of redroot pigweed (Amaranthus retroflexus L.) that emerged at the six tlUe leaf stage of sugarbeet development.

MATERIALS AND METHODS

Field experiments were conducted in 1996 and 1997 near Scottsbluff, NE. Soil type was a Tripp sandy loam (coarse-silty, mixed mesic Typic Haplustoll) with a pH of7.5 and 7.7 and organic matter of 0.7 and 1.0 %, in 1996 and 1997, respectively. Soil was sampled prior to seedbed preparation and fertilized in proportion to soil test results. All plots were sprinkler irrigated with the same amount of water and water was applied as needed based on water use by the sugarbeet crop. 14 Journal of Sugar Beet Research Vol 37, No 2

The experimental design was a factorial arrangement of treatments in a split-split block design with four replicates. Main plots consisted of three row spacings 46, 56, and 76 cm. Split plots consisted of sugarbeet cultivars 'Monohikari' and 'KW2398', plus a no-sugarbeet control. Split split plots had redroot pigweed densities of2000, 4000, and 15000 plants/ ha plus a weed-free control. Plots were 11.4 m long and four rows wide. The seedbed was prepared in the spring by moldboard plowing followed by roller harrowing. Sugarbeet was planted at 110000 plants/ha on April 16, 1996 and April 28, 1997. Intra-row plant spacings for 46, 56, and 76 cm rows were 11 , 16, and 19 cm respectively. The plot area was irrigated after planting in both years. Phenmedipham plus desmedipham at 0.17 plus 0.17 kg ai/ha, were applied twice POST to control early emerging weeds in 1996. The first application was when the majority of the sugarbeet were at two true leaves (May 14) and the second at the four true-leaf (May 21) stage of growth. Clethodim was applied post at 0.13 kg ai/ha on May 16 and June 12, 1996 for grass control. Phenmedipham plus desmedipham plus quizalofop at 0.17 plus 0.17 plus 0.08 kg ai/ha, was applied postemergence when the majority of the sugarbeet were at two true-leaves (May 16) in 1997. Clethodim was applied post at 0.13 kg ai/ha on June 6, 1997 for grass control. All weeds not controlled by herbicides were removed by hand with as little soil disturbance as possible. Redroot pigweed seeds were planted within the sugarbeet row May 20, 1996 or May 19, 1997. Redroot pigweed was then replanted June 5 or June 4 in 1996 and 1997, respectively, due to low germination rate. To establish the desired redroot pigweed densities, wooden stakes were placed in each row at different intervals, based on each weed density. At each stake, four to six red root pigweed seed were planted 0.6 cm deep. Redroot pigweed seeds were soaked in water for 24 h before planting to increase the emergence rate in 1997. Sugarbeet stand counts were made every 14 d, beginning mid May and ending the first week of July. The number of plants per row in each plot were counted for a distance of 11.4 m. Redroot pigweed density was counted four times in 1996, every 7 d begirrning June 18. In 1997, redroot pigweed density was counted twice, June 20 and June 30. Leaf area index (LAI) measurements were made using Li-Cor's LAI-2000 Plant Canopy Analyzer. Measurements were taken between 0500 and 0730 h to prevent light reflection off the canopy. Use of a 270 degree cap, on the viewing lens, blocked the operator from the instnunents field of vision. Measurements were taken between the second and third row in each plot. Using a level located on the upper side of the wand, the plant canopy analyzer was kept parallel with the soil surface for all measurements. A April-June 2000 Effects on Sugarbeet Development 15

single measurement above the plant canopy was taken, followed by four more measurements below the canopy. Each measurement was taken at an equal interval along a diagonal transect. The first below canopy measure ment was taken next to the crop in row two, the second measurement was 113 the distance to row tlu'ee, the third measurement was taken 2/3 of the way to row tlu'ee, and the last measurement was next to the third row. The above canopy measurement was used as a reference point to calculate LAI by comparing it to four below canopy measurements. This was performed twice in each plot and the average calculated. LAI was measured ev ery 14 d. Sugarbeet leaf height and width measurements were taken from May 29 through July 8,1996 and from June 4 through August 12,1997. Redroot pigweed height and width measurements were taken from June 28 through August 7, 1996 and July 1 through August 12, 1997. These mea surements were taken weekly in 1996 and biweekly in 1997. Height and width measurements for both sugarbeet and redroot pigweed were taken between row two and three on ten randomly selected sugarbeet and five randomly selected red root pigweed plants in each plot. Sugarbeet height was measured from the base of the plant to the uppermost point on the plant. Width ofsugarbeet was recorded from the outermost portion on each side of the sugarbeet. Redroot pigweed height was measured from the base of the plant to the rughest leaf of the plant. Width of redroot pigweed was recorded from the widest point of the plant. Both height and width were taken from standing plants. Sugarbeet was hand harvested on October 4, 1996 and October 2, 1997. Sugarbeet was dug by hand from rows two and three for a distance of 6.1 m in each plot. Sugarbeet was defoliated and the roots and leaves were weighed separately. Roots were then washed, weighed, and measured for percent sucrose as outlined by the Association ofOfficial Agriculture Chem ists (1955). Redroot pigweed were hand harvested on October 4, 1996 and October 2, 1997. Redroot pigweed were hand removed from each plot prior to removal of sugarbeet and the top mass was weighed. Analysis of variance (ANOVA) was performed on LAI, percent sucrose, root yield, and leafyield (PROC GLM, type III). Homogeneity of variance between years was tested with a F-test at the 0.05 level ofsignifi cance. Redroot pigweed, percent sucrose, sugarbeet top, sugarbeet root, and sucrose yield were analyzed separately by year because variances were not found to be homogenous. Row spacing was considered the main effect, cultivar was the split-plot treatment, and weed density was the split-split plot treatment. Treatment means were compared using Fisher's Protected Least Significant Difference (LSD) Test at the 0.05 level of significance. 16 Journal of Sugar Beet Research Vol 37, No 2

RESULTS AND DISCUSSION

Sugarbeet stand was counted on June 4, 1996 and June 5, 1997 (Tables 1 and 2). Average stand counts for 1996 and 1997 were 67100 and 81300 plants/ha, respectively. Experimental plots were planted on a slope, in 1996, and water runoff from overhead sprinkler inigation and rain at the time of plant establisillnent, resulted in some sugarbeet seed being washed from the soil. Sugarbeet were planted April 16, 1996 and reached the two true-leaf stage on May 14, 28 d after planting. In 1997 sugarbeet reached the two true-leaf stage on May 16, requiring only 18 d to reach this growth stage. Research plots in 1997 were established next to a tree row providing protection from wind and blowing sand. Redroot pigweed density (Tables 1 and 2) was assessed June 25, 1996 and June 30, 1997. Densities were similar in 1996 and 1997. Redroot pigweed density was greater in plots where no sugarbeet were planted com pared to plots seeded with KW2398 sugarbeet. Redroot pigweed densities in the absence of sugarbeet were expected to be higher due to the elimina tion of competition from sugarbeet. Cooler temperatures in 1996 may have accounted for some of the variation in redroot pigweed size and yield compared to 1997. The average high temperature from the date sugarbeet was planted until the date redroot pigweed was planted was 20.3 C for 1996. During the same period, the average low temperature was 2.8 C. The average high temperature from the date of sugarbeet planting to the date the redroot pigweed was planted in 1997 was 21.4 C and the average low was 3.1 C for 1997. For the three weeks following redroot pigweed planting, the average high temperature was 20.6 and 22.3 C in 1996 and 1997, respectively. The average low tem perature during this time frame was 8 and 10.5 C for 1996 and 1997, re spectively. Cooler temperatures in 1996 could have delayed redroot pig weed germination and slowed growth, allowing the sugarbeet to create a canopy and gain a competitive advantage that would last throughout the growing season. Redroot pigweed requires relatively high temperatures for germination (> 25 C) (Bewley and Black 1994). Data indicates that redroot pigweed grew at a faster rate in 1997 than in 1996 (Figure 1). On July 15, 1996, redroot pigweed plant heights averaged 9.6 cm while on July 15, 1997 they averaged 21.6 cm. The faster growth rate in 1997 allowed re droot pigweed to gain a height advantage over sugarbeet and shade the crop (Figure 1). In contrast sugarbeet was able to gain a height advantage over redroot pigweed in 1996 (Figure 1). Table 1. Effect of sugarbeet variety, row spacing, and redroot pigweed on sugarbeet root and top yield, sucrose yield, and '"0> redroot pigweed yield at Scottsbluff, NE during the 1996 growing season. ~ '-< § (1) Redroot tv 0 Redroot Sugarbeet Sucrose Sugarbeet pigweed 0 0 Sugarbeet pigweed root yield Sucrose yield top yield yield

plants/ha plantslha t/ha % t/ha tlha t/ha

Cultivar tr1 ~ No sugarbeet 0* 5800 9.1 ~ V> 0 Monohikari 66900 5300 70.0 15.3 10.7 37.2 0.4 ::l [fJ KW2398 67400 4600 57.8 15.9 9.1 37.0 0.3 c {JQ., LSD (0.05) NS 700 6.7 NS 1.0 NS 1.5 cr... (1) ~ t:1 (1) Row spacing -< (1) 0 76cm 67300 5400 62.8 15.9 10.2 37.3 0.1 '"0 S 56cm 63800 5100 65.6 15.2 (1) 9.6 41.1 0.6 ;::. 46cm 70300 5100 63.5 15.7 10.0 32.1 0.4 LSD (0.05) 4800 NS NS NS NS 6.6 0.4

(continued on next page)

-...l Table 1 (continued). Effect of sugarbeet variety, row spacing, and redroot pigweed on sugarbeet root and top yield, 00 sucrose yield, and redroot pigweed yield at Scottsbluff, NE during the 1996 growing season.

Redroot Redroot Sugarbeet Sucrose Sugarbeet pigweed Sugarbeet pigweed root yield Sucrose yield top yield yield

0' plants/ha plants/ha tlha % tJha tJha t/ha <:: 3 ~ Weed density ...,0 (/J No weeds 65100 0 61.2 15.3 9.8 37.2 0.0 <:: 00., Low 67000 2000 63.4 15.7 9.8 37.5 0.1 ... III (1) Medium 67500 4100 63.3 15.5 9.8 36.9 0.8 ~ i'=' (1) High 68800 14800 65.0 15.9 10.3 36.8 0.6 V> .,...(1) LSD (0.05) NS 600 NS NS NS NS 0.3 (") :r

*No sugarbeet was planted

e: w .-..l Z o N Table 2. Effect of sugarbeet variety, row spacing, and redroot pigweed on sugarbeet root and top yield, sucrose yield, and ::> redroot pigweed yield at Scottsbluff, NE during the 1997 growing season. ~., g'-'

Redroot '"IV 0 Redroot Sugarbeet Sucrose Sugarbeet pigweed 0 0 Sugarbeet pigweed root yield Sucrose yield top yield yield

plantsiha plantsiha tlha % tlha tlha tlha

iTJ Cultivar ~ No sugarbeet 0* 5900 12.5 ~ '"0 Monohikari ::> 72000 5600 51.3 14.3 7.3 30.5 7.3 en ::: KW2398 88700 5200 33.2 (JQ 48.4 15.6 7.6 3.2 til LSD (0.05) 3200 500 NS 0.4 NS 2.7 2.8 aa- t;) Row spacing '"<: 0'" 76cm 74000 5500 42.2 15.0 6.3 26.2 5.2 '0 3 56cm 81200 4900 51.4 14.6 7.5 36.8 6.2 aCD 46cm 85800 6100 55.9 15.3 8.5 32.5 4.4 LSD (0.05) 7000 700 4.9 0.5 0.8 3.3 NS

(continued on next page)

'C> N Table 2 (continued). Effect of sugarbeet variety, row spacing, and redroot pigweed on sugarbeet root and top yield, 0 sucrose yield, and redroot pigweed yield at Scottsbluff, NE during the 1997 growing season.

Redroot Redroot Sugarbeet Sucrose Sugarbeet pigweed Sugarbeet pigweed root yield Sucrose yield top yield yield

'-< plantsiha plantsiha tlha % tlha tlha tiha 0 aeo. Weed density ....,0 (/) s:: No weeds 79600 0 57.1 14.7 8.4 37.3 0.0 IJO ...~ Low 79700 2600 53.2 15.0 8.0 34.1 2.4 ttJ (1) (1) Medium 80300 4400 49.8 15.0 7.5 32.0 4.7 - :;0 (1) rn High 81700 15200 39.2 15.1 6.0 24.6 14.0 (1) ~ ...n LSD (0.05) NS 598 5.7 NS 0.9 3.8 4.0 ::r

*No sugarbeet was planted.

~ w .-.1 Z o N April-JlUle 2000 Effects on Sugarbeet Development 21

60 ---- 97 Pigweed 50 ---96 Pigweed ... -- E 40 ~ ...... 97 Sugarbeet E 0> 'Qj 30· - 96 Sugarbeet ,",,,,, /_---- .~-- I ... C 20 . a::til ~""01'''''-:'-'' 10 .

0 19-Jun 29-Jun 9-Jul 19-Jul 29-Jul 8-Aug 18-Aug

Date Figure 1. Sugarbeet and redroot pigweed height at Scottsbluff, NE during the 1996 and 1997 growing seasons.

Effects of Red root Pigweed Varying densities of redroot pigweed did not influence sugarbeet root, top, or sucrose yield in 1996 (Table 1). Possibilities for this lack of response may be explained by the slow growth rate of the redroot pigweed compared to sugarbeet. Wanner air temperatures in 1997 allowed redroot pigweed to grow faster than sugarbeet and obtain a height advantage over the crop (Figme I). During 1997, 4400 p1ants/ha ofredroot pigweed reduced sugarbeet root and top yield approximately 12% compared with the weed free control (Table 2). As red root pigweed density increased to 15200 plants/ha, sugarbeet root and top yields declined approximately 31 % compared with the weed free control. Yield of sugarbeet root was 57.l t/ha in plots where redroot pigweed was not planted. Sugarbeet root yield was similar in weed free plots and plots with 2600 redroot pigweed plants/ha. Sugarbeet sucrose yield was unaffected by redroot pigweed den sities in 1996 (Table 1). The sucrose yield in 1997 decreased as redroot pigweed populations increased, however the percent sucrose was not af fected (Table 2). Sugarbeet root yield declined as redroot pigweed density increased but 2600 plants/ha did not cause a significant yield loss. Yield of sugarbeet tops was not affected by redroot pigweed den sities in 1996 (Table 1). However, in 1997 sugarbeet top yield was greater in the weed free control compared to the yield in plots with redroot pig weed at 4400 and 15200 plants/ha (Table 2). As redroot pigweed density increased from 4400 to 15200 plants/ha, sugarbeet top yield decreased from 32.0 to 24.6 t/ha. 22 Journal of Sugar Beet Research Vol 37, No2

Sugarbeet height and width were not affected by redroot pigweed at any density in 1996 (data not presented). Sugarbeet height in 1997 was also unaffected by the varying densities of red root pigweed (data not pre sented). However, sugarbeet plant width was affected by redroot pigweed densities in 1997. On July 28, the widest sugarbeet plants averaged 26.0 cm, in redroot pigweed densities of0 and 2600 plants/ha while sugarbeet in plots containing 4400 and 15200 redroot pigweed/ha averaged 25.5 cm. (Table 3). The yield of redroot pigweed in 1997 was greatest at 15200 red root pigweed/ha (Table 2). Correlations between sugarbeet root yield and redroot pigweed yield or sugarbeet top yield and redroot pigweed yield were not significant in 1996. However, in 1997 correlations were signifi cant between sugarbeet root yield and redroot pigweed yield, and sugarbeet top yield and redroot pigweed yield with correlation coefficients of -0.42546* and -0.42655* respectively (*significant at the 0.05 level). There fore, as redroot pigweed yield increased, sugarbeet root and top yield de creased. Redroot pigweed height was significantly affected by redroot pig weed density on July 15, 1996 and July 1, 1997 (data not presented). Pig weed plants were taller in 1997 than in 1996. The taller redroot pigweed plants caused more shading of sugar beet thus increasing plant competition for light and consequently reducing sugarbeet yield. By July 15, 1996 and July 1, 1997, redroot pigweed plants were wider in high densities of re droot pigweed than those found in low densities (Table 4). Redroot pig weed densities of2600 plants/ha had the narrowest redroot pigweed plants while the 15200 plants/ha had the widest plants. By August these early season differences were no longer evident. Redroot pigweed plants found in high densities were probably competing with not only sugarbeet but also other redroot pigweed plants for sunlight and other resources. Sugarbeet in low redroot pigweed density plots were able to more actively compete for sunlight limiting the space available for redroot pigweed. Approximately 8.5 weeks elapsed from the time ofsugarbeet plant ing to redroot pigweed emergence in the 1996 season. Wicks and Wilson (1983) reported that weed emergence eight weeks after sugarbeet planting had little effect on sugarbeet yield. Redroot pigweed did not affect sugarbeet yield, in 1996. However in 1997, redroot pigweed emerged on June 14, 1997, approximately 6.5 weeks after sugarbeet planting. Wicks and Wilson (1983) showed that weeds emerging six weeks after sugarbeet planting caused a reduction in sugarbeet yield. Results from 1997 concur with this research. Table 3. Effect of sugarbeet variety, row spacing, and redroot pigweed density on sugarbeet plant width (cm) at Scottsbluff, NE during the :P '0 1996 and 1997 growing seasons. ~ '--< 1996 1997 § Treatment May29 June 5 June 12 June 17 June 24 July 1 July 8 June 4 June 17 June 30 July 15 July 28 Aug 11 (1) N 0 Plant width, cm 0 0 Cultivar Monohikari 2.1 3.1 6.3 10.6 16.1 19.7 22.7 3.8 10.9 20.6 21.5 25 .9 27.3 KW2398 2.0 3.2 5.7 10.3 16.9 20.2 22.2 4.2 11.3 19.8 21.1 25.6 27.8 LSD (0.05) NS NS NS NS 0.4 NS NS NS NS NS NS NS NS trl ~ Row spacing ~ '"0 76cm 2.0 3.1 6.0 10.5 16.4 20.8 24.1 4.0 10.5 20.1 23.2 28.1 29.7 ;:l C/l t:: 56cm 2.2 3.2 6.0 10.6 17.0 20.0 22.2 3.8 10.9 20.1 20.7 25.2 26.5 (JQ .".... 0' 46cm 2.1 3.2 6.0 10.4 16.1 19.1 21.0 4.3 12.0 20.5 20.1 27.0 26.5 (1) LSD (0.05) NS NS NS NS NS 1.3 0.9 NS NS NS 1.3 1.2 1.9 ~ t:J (1) <: ~ 0 Weed density '0 3 No weeds 2.0 3.1 5.7 10.2 16.3 19.8 22.3 4.0 10.6 19.8 21.7 26.0 28.5 (1) S. Low 2.2 3.2 6.0 10.2 13.4 20.1 22.1 4.1 11.5 20.4 21.2 26.0 27.5 Medium 2.1 3.2 6.2 10.7 16.6 19.8 22.6 4.1 11.2 20.5 21.1 25.5 27.9 High 2.1 3.2 6.2 10.8 16.7 20.0 22.7 3.9 11.2 20.2 21.2 25 .5 26.5 LSD (0.05) NS NS NS NS 1.3 NS NS NS NS NS NS 0.5 NS

N w N Table 4. Effect of sugarbeet variety, row spacing, and redroot pigweed density on redroot pigweed plant width (cm) at .j>. Scottsbluff, NE during the 1996 and 1997 growing seasons.

1997 Treatment June 28 July 3 July 15 July 22 July 31 Aug 7 Aug 14 Jul~ 1 Jul ~ 15 Jul~ 28 AuglJ Plant width, cm Cultivar ..., 0 No sugarbeet 3.3 4.5 15.5 27.0 37.5 44.3 46.6 14.4 24.7 47.4 51.2 § Monohikari 2.3 2.6 6.6 10.7 13.2 13.6 13.8 8.5 11.3 27.0 32.2 ~ ....,0 KW2398 1.8 2.6 4.7 11.1 10.1 10.9 11.6 6.8 10.5 20.7 32.4 rn ::: LSD (0.05) 0.6 0.6 3.1 5.1 3.9 6.4 4.0 1.4 2.8 3.3 (JQ 9.9 ...'" to C1> Row spacing SP. ;:d 76cm 2.2 3.0 9.6 19.5 20.4 23.0 23.0 10.2 15.6 32.1 42.8 C1> 56 cm 2.6 3.4 8.9 15.2 23.2 22.7 22.7 10.1 15.6 34.6 38.2 '"...C1> ()'" 46 em 2.6 3.4 8.2 14.1 19.4 23 23.1 9.3 15.3 28.4 34.7 ::T LSD (0.05) NS NS 1.4 8.5 NS NS NS NS NS 4.3 NS

Weed density Low 2.1 2.9 7.6 14.0 18.0 21.5 23.9 8.4 14.3 28.5 37.1 Medium 2.6 3.3 9.4 16.2 22.2 25.4 25.0 9.5 15.6 32.8 38.7 ~ High 2.7 3.5 9.8 18.7 22.7 21.8 23.0 11.8 16.6 33.7 39.9 UJ LSD (0.05) NS NS 0.8 4.2 NS 3.5 NS 0.8 1.1 2.5 NS .-.1 Z 0 N April-June 2000 Effects on Sugarbeet Development 25

Effects of Row Spacing Row spacing influenced sugarbeet root yield in 1997 only (Table I and 2). Lack of influence of row spacing on sugarbeet root yield may have been due to cooler weather in 1996. Theurer and Saunders (1995) also experienced similar environmental changes while conducting experiments on smooth root sugarbeet. Lower stand counts in 1996 (Table I) may have also contributed to the lack ofrow spacing influence on yield. Sugarbeet root yield decreased 18% when row spacing increased from 56 to 76 cm and 25% from 46 to 76 cm in 1997. When sugarbeet is established at a population of75000 plants/ ha, the distance between sugarbeet plants is approximately 17 cm in a 76 cm row spacing, while the distance between sugarbeet plants is 22 cm in a 46 cm row. Therefore when the crop is spaced in 46 cm rows, the distance between plants is greater and intra-plant competition is less. O'Connor (1983) showed that sugar yield and extractable sugar yield at 50000 and 65000 plants/ha were not significantly affected by row width. However, at plant densities of 80000 and 100000 plants/ha, yields of sugar and extract able sugar were significantly increased in 50 cm row spacing when com pared to 60 cm row spacing (O'Connor 1983). He concluded that this might be caused by effects of plant distribution ratio or rectangularity at the higher plant densities. Sugarbeet yield was reduced when planted in 76 cm rows as compared to 56 cm rows in previous research (Fomstrom and Jackson 1983). Jaggard (1979) also showed that sugarbeet yield was reduced when row spacing exceeded 51 cm. Row spacing influenced the sucrose content of the sugarbeet in 1997 only (Table 1 and 2). The sucrose content was greatest for 46 cm rows and least in 56 cm rows in 1997 (Table 2). Fomstrom and Jackson (1983) found that sucrose levels between 56 and 76 cm rows did not vary. Yonts and Smith (1997) showed that sucrose content was similar for 76, 56, and 35 cm. However, in year one of their experiment, data suggested that su crose content was greatest for 35 cm while least in 56 cm rows. The rea sons for the differences in 1996 and 1997 are not known. Sugarbeet top yield in both years was greatest at the 56 cm row spacing (Tables I and 2). The cause for the increase in sugarbeet top yield could be related to an increase in plant competition between sugarbeet at lower row spacings. Sugarbeet leaf height was influenced by row spacing on selected dates in 1996 (data not presented). On June 5, 1996, sugarbeet was taller in 46 cm rows compared to 76 cm rows. By June 17, the 76 cm row spacing had taller plants at 4.6 cm and 46 cm row spacing plants had the shortest sugarbeet plants at 4.2 cm. On July 1, sugarbeet in 76 cm rows was taller than in 46 cm rows. The reversal in height may be related to sugarbeet in 76 26 Journal of Sugar Beet Research Vol 37, No 2 cm rows having to grow more to compete for nutrients and water caused by smaller intra-row spacing, while 46 cm sugarbeet had more intra-row spac ing. Sugarbeet canopy width was greater for 76 cm rows on July 8, 1996 at 24 cm, while 46 cm rows had the narrowest plant canopies at 21.0 cm (Table 3). Because of these differences in July, measurements were ex tended later into the growing season in 1997. Sugarbeet leaf height in 1997 was unaffected by row spacings. In July of 1997, 76 cm row spacing had the widest plants, while 46 and 56 cm rows did not differ (Table 3). This might be due to the fact that sugarbeet in 76 cm row spacings took advan tage of the extra space between the rows, while plants growing in rows spaced 46 or 56 cm had less between row area for plants to utilize. LAI differences in row spacing existed June 19, all of July, and August 8 and September 13 in 1996 (Table 5). LAI was often greater for the 46 cm row spacing than for wider rows. On all dates, except the June date, 46 cm rows had greater LAI than the 76 em row spacing. Data col lected June 18, 1997 indicated 46 cm rows had a greater LAI than 76 cm rows (Table 6). On July 3, leaf area was greater for sugarbeet planted in 46 cm rows.

Effects of Cultivar The sugarbeet cultivar 'Monohikari' had a greater root and su crose yield than 'KW2398' in 1996 (Table I). The root yields of the two were statistically similar in 1997, however Monohikari had a lower su crose content than KW2398, (Table 2). Sugarbeet top yield was greater for KW2398 than Monohikari in 1997 only (Tables 1 and 2). On June 17, Monohikari plants measured 4.7 cm while KW2398 plants measured only 4.0 cm (data not presented) and this continued through July 8. KW2398 plant width was greater than Monohikari in June 1996 (Table 3). Monohikari plant height in 1997 was greater than KW2398 from mid-June through mid-August. Redroot pigweed produced more growth with Monohikari (7.3 t/ha) than with KW2398 (3.2 t/ha) in 1997 only (Tables 1 and 2). The re duced weight of redroot pigweed at harvest in the KW2398 cultivar may have been the result ofthe increased top yield ofKW2398. The correlation between redroot pigweed weight and sugarbeet top yield was significant with a coefficient of-0.42655* (*significant at the 0.05 level). Monohikari allowed redroot pigweed to grow taller (data not presented) and wider than KW2398. These measurements were significant on July 15, 1996 and July 1,1997. Leaf area index (LAI) differed between sugarbeet cultivars on three dates in June and one in July during 1996 (Table 5). K W2398 had a greater LAI than Monohikari on those four dates. Data evaluated in 1997 suggests that KW2398 had a greater LAI than Monohikari on July 3 (Table 6). High Table 5. Leaf area index (LAI) of sugarbeet measured during the 1996 growing season at Scottsbluff, NE. :> ~ May 30_June 13 ~une 19 June 26 JlJly 3 July 17 July 24 Aug 8 Aug 14 Aug 28 Sept 13 Sept 28 § --co N LAI 0 0 Variety 0 Monohikari 0.03 0.09 0.43 0.99 1.20 2.44 2.94 4.57 5.23 4.68 4.86 3.79 KW2398 0.04 0.17 0.52 1.30 1.70 2.75 3.18 4.62 5.10 4.43 4.82 3.76 LSD (0.05) NS 0.06 NS 0.30 0.22 0.21 NS NS NS NS NS NS tTl ~ Row spacing ~ 'J> 0 76 cm 0.04 0.14 0.49 1.02 1.29 2.10 2.63 4.21 4.82 4.52 4.33 3.29 ::l Vl 56 cm 0.03 0.13 0.40 1.12 1.37 2.64 3.13 4.63 5.23 4.65 4.98 4.10 :: oc 46cm 0.04 0.13 0.54 1.29 1.70 3.04 3.42 4.93 5.45 4.50 5.21 3.95 .., cr'" co LSD (0.05) NS NS 0.11 NS 0.32 0.34 0.62 0.60 NS NS 0.26 NS ~ t;I co <: Pigweed density ~ 0 '0 No weeds 0.04 0.12 0.46 1.03 1.37 2.45 2.86 4.60 5.21 4.42 5.07 3.76 3 co Low 0.02 0.11 0.42 1.14 1.42 2.55 3.12 4.59 5.11 4.62 4.66 3.48 a Medium 0.04 0.17 0.54 1.29 1.55 2.73 3.13 4.65 5.11 4.60 4.60 3.72 High 0.04 0.12 0.48 1.11 1.46 2.64 3.12 4.53 5.33 4.59 5.02 4.14 LSD (0.05) NS NS NS NS NS NS NS NS NS NS NS NS

N -....l N Table 6. Leaf area index (LAI) of sugarbeet measured during the 1997 growing season at Scottsbluff, NE. 00

June 18 July 3 July 17 July 31 Aug 12 Sept 2 Oct 1 LAI Cultivar Monohikari 0.54 1.47 3.21 4.10 4.79 5.38 4.35 '-< KW2398 0.75 1.98 3.40 3.89 4.88 5.46 4.44 0., 3 LSD (0.05) NS 0.26 NS NS NS NS NS ~ 0...., .,rJj Row spacing (JQ .., 76cm 0.47 1.17 2.63 3.70 4.85 5.49 4.22 '"co (!) 56 cm 0.61 1.57 3.41 4.04 4.92 5.43 4.55 ~ ~ 46 cm 0.82 2.41 3.89 4.26 4.73 5.34 4.41 (!) ..,'"(!) LSD (0.05) 0.31 0.68 1.18 NS NS NS NS '"() ::r

Pigweed density No weeds 0.62 1.71 3.21 3.97 4.87 5.27 4.34 Low 0.68 1.79 3.36 3.94 4.65 5.25 4.50 Medium 0.61 1.72 3.35 4.02 4.80 5.33 4.40 ~ High 0.61 1.68 3.32 4.06 5.02 5.84 4.35 w :.J LSD (0.05) NS NS NS NS NS 0.38 NS Z 0 N April-June 2000 Effects on Sugarbeet Development 29

taproot to leaf weight ratio has been shown to increase sucrose production in sugarbeet hybrids (Snyder, 1985). Doney and Martens (1994) suggest that any increase ofphotosynthate to the root should result in an increase in sucrose yield because sucrose yield is the product ofroot yield and sucrose concentration. The larger LAI found in KW2398 aided in competition with redroot pigweed. Redroot pigweed yield in KW2398 was 3.2 t/ha while the yield of redroot pigweed in Monohikari was 7.3 t/ha in 1997. Results ofthis research suggest that producers should use a 46 cm row spacing rather than 56 or 76 cm when sugarbeet populations exceeded 80000 plants/ha. This provided the most stable environment for sugarbeet yield. Sugarbeet yield was not always greatest in 46 cm rows but this row spacing was better able to control redroot pigweed in 1997. LAI increased at a faster rate for 46 cm rows in both years. Monohikari produced a higher sugarbeet root yield than KW2398 in 1996, but did not compete with redroot pigweed as well as KW2398 in 1997. Canopy closure was complete earlier in the season for KW2398, reducing the competition by redroot pigweed. The competition between sugarbeet and red root pigweed does not appear to be a significant problem if sugarbeet is planted earlier in the growing season and achieve a height advantage over the weed. Planting sugarbeet earlier in the growing season is beneficial as this allows sugarbeet to have a competitive advantage over late emerging redroot pigweed.

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