Narrow-row Soybean Research Articles

Multi-tactic strategies to manage herbicide-resistant waterhemp (Amaranthus tuberculatus) in corn–soybean rotations of the U.S. Midwest

AbstractField experiments were conducted over 2 yr (2019 to 2020) at two locations in Iowa to evaluate multi-tactic strategies for managing multiple herbicide–resistant (MHR) waterhemp [Amaranthus tuberculatus (Moq.) Sauer] in a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation. The effect of three herbicide programs on A. tuberculatus control was tested in corn (2019). The effects of the prior year’s corn weed control, a cereal rye (Secale cereale L.) cover crop, and soybean row spacing (38-cm vs. 76-cm wide) on A. tuberculatus density, biomass, and seed production were tested in soybean (2020). A herbicide program used in corn with two sites of action provided only 35% control of MHR A. tuberculatus compared with ≥97% control by a herbicide program with three sites of action. In soybean, adequate control of A. tuberculatus (≥90%) in the prior year’s corn crop and use of a cover crop or narrow rows reduced A. tuberculatus density by more than 60% at 3 and 9 wk after planting (WAP) compared with inadequate control (30%) in the prior year’s corn and no cover crop. Cover crop and narrow-row soybean reduced A. tuberculatus density by 44% at 3 WAP compared with no cover crop and wide-row soybean. Inclusion of a single control tactic, adequate control (≥90%) with multiple herbicides in the prior year’s corn, use of a cover crop, or narrow-row soybean reduced A. tuberculatus biomass and seed production at soybean harvest by at least 24% compared with inadequate control (30%) in the prior year’s corn, no cover crop, and wide-row soybean. The combination of all three control tactics reduced A. tuberculatus biomass and seed production at soybean harvest by at least 80%. In conclusion, diverse control tactics targeting A. tuberculatus at multiple life-cycle stages can make substantial contributions to the management of MHR populations.

Growth and development of multiple waterhemp (Amaranthus tuberculatus) cohorts in corn and soybeans

Waterhemp is a troublesome weed species in cropping systems throughout Wisconsin and much of the US Midwest. Coupled with extended emergence window, rapid growth, high genetic diversity, and herbicide resistance to multiple sites of action, waterhemp represents a major challenge for chemical-based weed control. The objective of this experiment was to evaluate the impact of soybean and corn canopy on growth and development of multiple waterhemp cohorts in 2019 and 2020. Treatments consisted of narrow- (38 cm row spacing) and wide-row (76 cm row spacing) soybeans, corn (76 cm row spacing), and fallow (no crop), with 6 waterhemp transplant timings (cohorts) in 2019 and 2020, respectively. Waterhemp seedlings grown in the greenhouse were transplanted at the 2-3 true leaves growth stage to the field at 10-day increments throughout the typical waterhemp emergence season for Wisconsin (June-July). Waterhemp plants were measured for height and harvested for biomass when they reached the flowering stage. The number of days from the first transplant timing to result in 50% and 90% biomass and height reduction (ED50 and ED90) of subsequent waterhemp cohorts were estimated and used as indicators of weed suppression ability by the respective crop. Narrow-row soybeans required fewer days to suppress 50% growth of new waterhemp cohorts followed by corn, wide row soybeans, and fallow, respectively (4, 6, 9, and 14 d for 50% biomass reduction, and 10, 14, 18, and 42 d for 50% height reduction, respectively). Similarly, narrow-row soybeans required fewer days to suppress 90% biomass of waterhemp, followed by corn, wide-row soybeans, and fallow (11, 15, 18, and 78 d, respectively). Conversely, narrow and wide-row soybeans required fewer days to suppress 90% height of new waterhemp cohorts followed by corn and fallow (20, 26, 43, and 85 d, respectively). Rapid soybean canopy closure (e.g., planting soybeans earlier and in narrow row-spacing) combined with other integrated weed management practices have the potential to reduce reliance on POST herbicide application and mitigate herbicide resistance evolution. In corn, mid- to late-season monitoring and management of later emerging waterhemp cohorts is recommended due to longer time required to reach 90% waterhemp growth suppression.

Narrow-row soybean and a cereal rye cover crop suppress glyphosate-resistant horseweed (Conyza canadensis)

AbstractAlternative strategies are needed for management of glyphosate-resistant (GR) horseweed in soybean. Integrating a cereal rye cover crop with soybean planted in narrow rows may improve control and reduce herbicide selection pressure for herbicide-resistant horseweed biotypes. Four site-years of experiments were conducted in Michigan to determine whether fall-planted cereal rye terminated with glyphosate 1 wk prior to (early termination) or 1 wk after (planting green) planting in combination with narrow-row soybean improved GR horseweed management. At postemergence (POST) herbicide application, horseweed biomass was reduced by 71% to 90% when soybean was planted into cereal rye, regardless of termination time, compared with no cover across all row widths. Planting green or narrow-row soybean suppressed horseweed through soybean harvest. When glyphosate was applied POST (noneffective), horseweed biomass was 36% to 46% lower when planting green compared with early terminated cereal rye and no cover. Similarly, planting soybean in 19- and 38-cm rows reduced horseweed biomass by 48% and 28%, respectively, compared with 76-cm rows. Cereal rye did not affect soybean yield pooled over 3 of 4 site-years; however, narrow row soybean yielded 11% to 18% higher than 76-cm rows. Soybean yield was 11% higher when an effective POST herbicide was applied. In conclusion, fall-seeded cereal rye or narrow-row soybean suppressed horseweed compared with no cover and 76-cm rows; however, the effects of early termination did not last throughout the growing season in most cases. Delaying cover crop termination by planting green reduced horseweed biomass and density through soybean harvest, but reduced yield in 1 site-year due to an increased incidence of white mold. These cultural practices have a positive influence on suppressing horseweed that should be part of an overall horseweed management strategy; however, the use of an effective POST herbicide is still needed for complete season-long horseweed management.

Influence of integrated agronomic and weed management practices on soybean canopy development and yield

AbstractThe role of weed suppression by the cultivated crop is often overlooked in annual row cropping systems. Agronomic practices such as planting time, row spacing, tillage and herbicide selection may influence the time of crop canopy closure. The objective of this research was to evaluate the influence of the aforementioned agronomic practices and their interaction with the adoption of an effective preemergence (PRE) soil residual herbicide program on soybean canopy closure and yield. A field experiment was conducted in 2019 and 2020 in Arlington, WI, as a 2×2×2×2 factorial in a randomized complete block design, including early (late April) and standard (late May) planting time, narrow (38 cm) and wide (76 cm) row spacing, conventional tillage and no-till, and soil-applied PRE herbicide (yes and no; flumioxazin 150 g ai ha−1 + metribuzin 449 g ai ha−1 + pyroxasulfone 190 g ai ha−1). All plots were maintained weed-free throughout the growing season. In both years, early planted soybeans reached 90% green canopy cover (T90) before (7 to 9 d difference) and yielded more (188 to 902 kg ha−1 difference) than the standard planted soybeans. Narrow-row soybeans reached T90 earlier than wide-row soybeans (4 to 7 d difference), but yield was similar between row spacing treatments. Conventional tillage resulted in a higher yield compared to a no-till system (377 kg ha−1 difference). The PRE herbicide slightly delayed T90 (4 d or less) but had no impact on yield. All practices investigated herein influenced the time of soybean canopy closure but only planting time and tillage impacted yield. Planting soybeans earlier and reducing their row spacing expedites the time to canopy closure. The potential delay in canopy development and yield loss if soybeans are allowed to compete with weeds early in the season would likely outweigh the slight delay in canopy development by an effective PRE herbicide.

Herbicide coverage in narrow row soybean as influenced by spray nozzle design and carrier volume

The use of reduced drift nozzles that produce larger droplet sizes that are less prone to drift will likely be required for use of future postemergence herbicide applications in soybean in the USA. Experiments to evaluate the effect of reduced drift spray nozzles on spray solution coverage were conducted in the field in Indiana. Air induction extended range (AIXR) and turbo TeeJet air induction (TTI) nozzles that produce extremely coarse to ultra coarse droplets were compared to extended range (XR) and Turbo TeeJet (TT) nozzle that produced fine to coarse droplets. Each nozzle was evaluated for spray coverage at 94 and 140 l ha−1 spray volumes using water sensitive cards. Precipitation varied between site years and resulted in differences in soybean canopy development and spray solution coverage. Coverage was greater at the top of the canopy than at the bottom of the soybean canopy as expected. An interaction occurred at the top and middle of the canopy in which the AIXR and TTI nozzles had similar coverage between the two spray volumes, whereas the XR and TT nozzles had greater coverage at 140 l ha−1 carrier volume than 94 l ha−1 carrier spray volume. Spray solution coverage at the bottom of the canopy, where target weeds would be, was similar between all nozzle types. Coverage was greater at the bottom of the canopy at 140 l ha−1 spray volume than with the 94 l ha−1 carrier volume. Spray volume has a greater influence on coverage than spray nozzle type and AIXR and TTI nozzles are less prone to coverage differences due to spray volume than the XR and TT nozzles.

ダイズ狭畦栽培における土壌処理剤の残効期間の把握とグルホシネートの畦間・株間処理による効率的・効果的雑草防除

瀬戸内地域の条間30 cmダイズ狭畦栽培において効率的・効果的な除草体系を確立するため，メヒシバ，ホナガイヌビユ等の一般雑草が優占する灰色低地土圃場で土壌処理剤の残効期間の長短による生育期茎葉処理剤の要不要の検討ならびに慣行除草体系と新規除草体系の除草効果を比較した。慣行の除草体系は播種後のジメテナミド・リニュロン乳剤の土壌処理とダイズ3葉期のベンタゾン液剤およびイネ科雑草対象茎葉処理剤の混用処理からなり，新規除草体系は播種後の前記土壌処理とダイズ5葉期または7葉期の非選択性茎葉処理剤グルホシネート液剤の畦間・株間処理からなる。作付体系としてダイズ単作およびムギ－ダイズ二毛作を設定した。

Palmer Amaranth (Amaranthus palmeri) Control in Soybean with Glyphosate and Conventional Herbicide Systems

Glyphosate typically controls Palmer amaranth very well. However, glyphosate-resistant (GR) biotypes of this weed are present in several southern states, requiring the development of effective alternatives to glyphosate-only management strategies. Field experiments were conducted in seven North Carolina environments to evaluate control of glyphosate-susceptible (GS) and GR Palmer amaranth in narrow-row soybean by glyphosate and conventional herbicide systems. Conventional systems included either pendimethalin orS-metolachlor applied PRE alone or mixed with flumioxazin, fomesafen, or metribuzin plus chlorimuron followed by fomesafen or no herbicide POST.S-metolachlor was more effective at controlling GR and GS Palmer amaranth than pendimethalin; flumioxazin and fomesafen were generally more effective than metribuzin plus chlorimuron. Fomesafen applied POST following PRE herbicides increased Palmer amaranth control and soybean yield compared with PRE-only herbicide systems. Glyphosate alone applied once POST controlled GS Palmer amaranth 97% late in the season. Glyphosate was more effective than fomesafen plus clethodim applied POST. Control of GS Palmer amaranth when treated with pendimethalin orS-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE followed by fomesafen POST was equivalent to control achieved by glyphosate applied once POST. In fields with GR Palmer amaranth, greater than 80% late-season control was obtained only with systems of pendimethalin orS-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE followed by fomesafen POST. Systems of pendimethalin orS-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE without fomesafen POST controlled GR Palmer amaranth less than 30% late in the season. Systems of pendimethalin orS-metolachlor PRE followed by fomesafen POST controlled GR Palmer amaranth less than 60% late in the season.

Biological Control of Hemp sesbania (Sesbania exaltata) and Sicklepod (Senna obtusifolia) in Soybean with Anthracnose Pathogen Mixtures

In greenhouse and field experiments conducted over 3 yr, tank mixtures of spores of the fungiColletotrichum truncatumandColletotrichum gloeosporioides, formulated in unrefined corn oil and Silwet L-77 surfactant, were evaluated as a mycoherbicide mixture for simultaneous control of hemp sesbania and sicklepod, respectively. In greenhouse tests, 100% mortality and dry weight reduction of hemp sesbania occurred 6 d after treatment (DAT), whereas 15 d were required to achieve 100% mortality and dry weight reduction of sicklepod. In field experiments conducted in narrow-row (51-cm) soybean test plots, a single application of the fungal mixture formulated as described controlled hemp sesbania and sicklepod 94% and 88%, respectively, 28 DAT. Neither fungus, applied as aqueous conidial suspensions, provided control of their respective weed hosts. These results indicate that tank mixtures of these anthracnose-forming pathogens can effectively control hemp sesbania and sicklepod with a single application.

Effect of Row Spacing and Seeding Rate on Soybean Yield

Soybean [Glycine max (L.) Merr.] yield response to narrow row spacing has been consistently positive in the upper Midwest and new split‐row planters have made narrow row soybean production feasible, yet adoption has been slow in Iowa. Wide (76‐cm) and narrow (38‐cm) row spacing and four seeding rates (185,000; 309,000; 432 000; and 556,000 seeds ha−1) were evaluated at three locations during 2004, 2005, and 2006 to determine seed yield in wide and narrow row spacing and four seeding rates and evaluate economic advantages associated with changes in row spacing. Soybean planted in 38‐cm row spacing yielded 248 kg ha−1 greater than soybean planted in 76‐cm rows after adjustment for differences in final plant populations. Maximum yield at all locations was attained at a final harvest population of 462,200 plants ha−1 but >95% of the maximum yield was achieved with final populations as low as 258 600 plants ha−1. Increased production costs associated with greater seeding rates removed the yield benefit from greater harvest plant populations. Farm size of 144 ha with at least 50% of the land base dedicated to soybean production would benefit from conversion from wide to narrow rows. To break even on the investment in a split‐row planter a yield increase of 124 kg ha−1 was necessary for farms with 30% of 288 ha dedicated to soybean production. These data indicate that yield and economic benefits are sufficient to support the production of soybean in narrow rows and at seeding rates below current seeding rate recommendations.

Influence of Soybean Maturity Group and Row Width on Bean Leaf Beetle (Coleoptera: Chrysomelidae) and Bean Pod Mottle Disease in an Early Season Production System

The influence of narrow and wide-row soybeans on infestations of bean leaf beetle (BLB), Cerotoma trifurcata (Forster) (Coleoptera: Chrysomelidae) adults, a vector of bean pod mottle virus (BPMV), and associated incidence of bean pod mottle (BPM) disease were investigated in maturity groups IV and V soybeans in Mississippi. Maturity group IV soybeans had greater cumulative BLB numbers and greater incidence of BPM than maturity group V soybeans in 2000, but not in 2001. Row width was not shown to affect beetle numbers in either study year, but a greater incidence of BPM occurred in narrow row soybeans in 2001 and Maturity Group IV soybeans in 2000. There was no significant correlation between numbers of BLB adults and soybean plants infected with BPM virus when data was analyzed within sample dates or by seasonal totals. Greater yields were obtained in maturity group V soybeans than in maturity group IV soybeans in 2000, but not 2001, whereas row width had no significant effect on yield in either 2000 or 2001. The results presented herein suggest that further investigations of soybean row spacing in relation to BLB and BPM disease should consider large experimental plots to minimize beetle dispersal and spread of BPM disease.

Wheel Traffic to Narrow-Row Reproductive-Stage Soybean Lowers Yield

Soybean producers need to know the loss associated with sprayer tire damage to fully determine the cost and benefit of reproductive-stage pest management strategies. Experiments were conducted to determine the yield loss resulting from wheel traffic applied at the R4-stage to full-season and double-crop soybean planted in three row spacings. Wheel traffic reduced yield when planted in 7.5- and 15-inch row spacing. Traffic did not reduce yield of soybean planted in wider rows, but non-irrigated soybean planted in 36-inch row spacing yielded equal to or less than trafficked soybean planted in narrow rows. Drilled soybean tended to compensate for damaged rows better than 15-inch soybean, but only when environmental conditions were conducive for compensation. Water stress during the time of traffic treatment reduced soybean's ability to compensate for damaged rows. When prorated to 45- to 120-ft booms, yield loss ranged from 6.4 to 1.0%. These data provide information that can be used to refine reproductive-stage pest management decisions in soybean.

Determining the Influence of Spray Quality, Nozzle Type, Spray Volume, and Air-Assisted Application Strategies on Deposition of Pesticides in Soybean Canopy

Many insect and disease problems occur in specific plant canopy locations and cannot be managed with systemic pesticides. Diseases such as Asian soybean rust (ASR) (Phakopsora pachyrhizi) infect plant material deep inside a canopy that is difficult to target. Field studies were established in north central Ohio to determine the effect of different application strategies on targeting of foliar pesticide in narrow-row (18 cm) soybean plantings. Several different application factors were tested, including spray quality, nozzle type, air-assistance, and spray volume. In 2005, the spray mix included a fungicide used for ARS management. In 2006, in addition to the fungicide, an insecticide for soybean aphid management was included. Plant samples were removed from each test plot, and stems and leaves from the bottom third and middle third of the plant were separated for individual analysis. Overall, there was significantly less active ingredient found in the lower third of the canopies than the middle third, and significantly less pesticide residue was found on stems than leaves from the same canopy location. Significantly more fungicide residue was found on lower leaves treated by the medium-quality XR8004 flat-fan nozzle in 2005 than the coarse-quality XR8005 flat-fan nozzle. There were no differences in fungicide residue found on middle canopy leaves between the fine, medium, and coarse quality flat-fan nozzles. The twin-fan pattern nozzles (Turbo Duo and TwinJet) produced the lowest amounts of fungicide residue on the lower leaves in 2005. The mechanical canopy opener produced significantly higher fungicide residues on middle canopy leaves than all other treatments. The Jacto air-assist sprayer using JA3 hollow-cone nozzles produced the highest fungicide residues on lower canopy leaves in 2005. There were some statistical differences between the amounts of fungicide and insecticide residue found on plant tissue in 2006 because of the high amount of variability in the sample data. Overall in 2006, the higher volume XR8004 treatment (187 L ha-1) and the twin-fan TTJ60-11003 treatment at 145 L ha-1 performed similar to the Jacto sprayer making applications at 145 L ha-1 using either flat-fan or hollow-cone nozzles. In general, higher volume applications produced higher amounts of fungicide and insecticide residue on leaves from the middle of the canopy for conventional flat-fan and air-assist applications. Spray volume had less affect on residues measured on leaves from the lower canopy area. Across two years of different canopies at the same spray volume (145 L ha-1), the Jacto sprayer using JA3 hollow-cone nozzles produced more fungicide residue on middle canopy stems and lower canopy leaves than the medium-quality XR8004 flat-fan nozzle.

Reduced Surface Runoff Losses of Metolachlor in Narrow‐Row Compared to Wide‐Row Soybean

Cultural management practices that reduce the off-site transport of herbicides applied to row crops are needed to protect surface water quality. A soybean [Glycine max (L.) Merr.] field study was conducted near Stoneville, MS on Sharkey clay to evaluate row spacing (50 cm vs. 100 cm) effects on metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(methoxy-1-methylethyl) acetamide] transport. One day after the foliar application of metolachlor to 2.03 m wide by 2.43 m long plots, 60 mm h(-1) of simulated rainfall was applied until 25 min of runoff was generated per plot. The calculated mass of metolachlor intercepted by the soybean foliage was greater in narrow-row than wide-row soybean, 0.39 kg ha(-1) vs. 0.23 kg ha(-1), respectively. Field and laboratory studies indicated that less than 2% of the metolachlor intercepted by the soybean foliage was available for foliar wash-off 1 d after application. Antecedent soil water content at the start of the simulations was lower in narrow-row soybean. In turn, there was a 1.7-fold greater time to runoff on narrow-row plots. The greater time to runoff likely contributed to lower metolachlor concentration in runoff from narrow-row plots. Cumulative metolachlor losses were significantly greater in wide-row than narrow-row soybean, 3.7% vs. 2.2%, respectively. Findings indicate that narrow-row planting systems may reduce metolachlor runoff following a post-emergence application.

Radiation balance and evaporation partitioning in a narrow-row soybean canopy

Seeding rate and row spacing of agricultural crops are managed to maximize yield but also have significant implications for energy partitioning and canopy microclimate. The objective of this study was to measure radiation budget components in a narrow-row soybean [ Glycine max (L.) Merr.] canopy, determine soil water evaporation ( E) as the difference between measured evapotranspiration (ET) and plant transpiration ( T), and compare measured E and T with estimates from an energy balance approach. The study was completed in a production soybean field near Ames, IA, with 350,000 plants ha −1 in 0.38 m wide rows. The field was planted on 8 May 2004 and continuous measurements were made from 8 June to 27 September. Shortwave and longwave radiation components were measured above the canopy with hemispherical radiometers. Net all-wave and incoming and reflected shortwave radiation were measured beneath the canopy with line radiometers and downwelling longwave radiation was calculated from a radiation balance. ET was measured with an eddy covariance system and T was measured with sapflow stem gauges. On sunny days under full canopy conditions, nearly 90% of the shortwave and net radiation was attenuated by the canopy with over 80% of the available energy utilized by ET. E accounted for 8–12% of the ET under full canopy conditions although these percentages may be overestimates due to evaporation of dew present on the canopy. With such a large proportion of the available energy consumed by ET, fluxes of sensible heat ( H) were very low and vertical temperature gradients across the soil–canopy–atmosphere interface were only 2–3 °C. A Priestley–Taylor energy balance approach used to estimate E and T tended to underestimate E and overestimate T. The E underestimate may be due in part to E including evaporation of dew. Conversely, T was overestimated by 25 and 14% on sunny days under full canopy conditions even with direct measurement of R n interception by the canopy. Use of shortwave extinction coefficients to estimate R n interception failed to improve T estimates significantly. The high-population, narrow-row planting strategy resulted in a dense canopy that, under full canopy conditions, resulted in very little light penetration or E.

Utility of Residual Herbicides in No-till Double-Crop Glyphosate-Resistant Soybean Production

Field studies were conducted from 2002 to 2004 to evaluate weed control and yield of wheat and glyphosate-resistant soybean grown in rotation using narrow-row residual herbicide systems. The experiment was a randomized complete block design with a split-plot arrangement. Wheat herbicide treatments were the main effect with subplots of soybean herbicide treatments of pendimethalin preemergence (PRE), imazethapyr PRE, clomazone PRE, and glyphosate early postemergence (EPOST) applied alone or in sequence with glyphosate postemergence (POST), and a nontreated control. Wheat herbicide treatments did not affect soybean weed control. Control of tall morningglory increased when a PRE residual herbicide was included with POST applications of glyphosate compared to sequential applications of glyphosate alone, but control was inconsistent across locations. Sicklepod, large crabgrass, Florida beggarweed, and wild poinsettia control was 82, 88, 91, and 89%, respectively, or greater when residual herbicides were followed by a sequential application of glyphosate. In contrast sicklepod, large crabgrass, Florida beggarweed, and wild poinsettia control was 81, 80, 83, and 88%, respectively, or less when residual herbicides were used alone. By mid-season, narrow-row soybean had complete canopy closure and interference from weeds was minimal. Soybean yield was increased up to 380 kg/ha when a residual herbicide was included with a glyphosate application.

Early-season insect defoliation influences the critical time for weed removal in soybean

To develop more effective pest-management strategies, it is essential to understand how different pests interact with each other and the crop. Field studies were conducted in 2003 and 2004 at two Nebraska locations to determine the effects of early-season crop defoliation on the critical time for weed removal (CTWR) in narrow-row soybean. Three soybean defoliation levels were selected to simulate 0, 30, and 60% leaf tissue removal by the bean leaf beetle. Weeds were allowed to compete with the crop until V2, V4, V6, R3, and R5 growth stages. There were also season-long weedy and weed-free treatments. Results indicated that the CTWR in soybean occurred earlier as defoliation levels increased from 0 to 60%. The CTWR occurred at V3, V2, and V1 growth stage for 0, 30, and 60% defoliation levels, respectively. Overall, 60% defoliation resulted in earlier CTWR by at least 14 d. Yield losses from defoliation and weed interference were primarily associated with a reduction in number of pods per plant−1.

A Review of the Effects of Row Spacing on Weed Management in Corn and Soybean

The adoption of narrow row spacings (rows less than 30 inches in width) in soybeans and corn has primarily been driven by the potential for higher yields in narrow- compared to wide-row production systems. In addition to the potential yield advantages, these systems can have a significant impact on the incidence of weeds within a given agroecosystem and on the approach that producers might take for weed management. A review of row spacing experiments included in this review revealed that narrow row spacings have provided significant reductions in the late-season density of weeds in the majority of soybean experiments but only rarely in corn. This research also indicates that narrow row soybeans are more likely to impact the critical time of weed removal in soybean than in corn. The objective of this article is to provide a brief review of the literature related to the effects of row spacing on late-season weed prevalence and management in corn and soybean and to discuss the implications of these results to current production systems.

Late-Season Common Waterhemp (Amaranthus rudis) Interference in Narrow- and Wide-Row Soybean

Field studies were conducted in 2000, 2001, and 2002 at Urbana, IL, to examine the interference potential of common waterhemp that emerged at soybean growth stages VE, V2-V3, V4-V5, R1-R2, and R3-R4 in 19- and 76-cm row soybean. Soybean row width and common waterhemp emergence timing significantly influenced common waterhemp density, biomass, seed production, mortality, and soybean yield loss. Common waterhemp density declined as emergence timings were at later soybean growth stages. This decline happened at earlier growth stages in narrow-row soybean. Significant reductions in common waterhemp biomass and seed production occurred at the V2-V3 and V4-V5 emergence timings for the narrow- and wide-row soybean, respectively. Common waterhemp seed production was more than 23,000 seeds per plant at the VE emergence timing for both soybean row widths. Survival of common waterhemp that emerged after the V4-V5 soybean growth stage was less than 20% in both row widths. Common waterhemp interference reduced soybean seed yield at the VE, V2-V3, and the V4-V5 emergence timings. Row width affected the magnitude of yield reductions at these interference timings, with reductions being less in narrow-row soybean. This research suggests that control measures need to be implemented to common waterhemp plants that emerge before V4-V5 soybean to protect soybean yield and reduce common waterhemp seed production.

Comparison of weed control strategies in glyphosateresistant soybean [Glycine max (L.) Merr.] in Atlantic Canada

Glyphosate-resistant soybean cultivars with low corn heat unit requirements have only recently become available in the Atlantic region of Canada. In a field experiment near Charlottetown, Prince Edward Island, in 1998 and 1999, we evaluated the effect of time of application of glyphosate and application of selected herbicides, flumetsulam, metolachlor, imazethapyr, metribuzin, and bentazon on weed control and tolerance and yield of three glyphosate-resistant soybean cultivars having different maturity ratings. Glyphosate was applied at 0.90 kg ai ha-1 at the first, second or third trifoliate leaf stage, and sequentially at the first and third or second and third trifoliate stages. The sequential treatments of glyphosate resulted in 89 to 100% weed control, and yields achieved with glyphosate applied at the second or third trifoliate stages did not usually differ from sequential treatments. The other selected herbicides did not injure the cultivars and within the group, all treatments resulted in comparable yields. Species that emerge in a single early-season flush were controlled at 6 wk after treatment with glyphosate applied as a single application, but not on a species such as wild radish that have later flushes or emerge throughout the season. Glyphosate applied as a single application at the late second to third trifoliate stage can provide optimum weed control and crop yield with lower herbicide cost than an alternative herbicide treatment. Key words: Herbicide timing, narrow-row soybean, soybean cultivars, 2601R, 2701R, S14-M7

Effect of soybean spatial arrangement and glyphosate dose on Anoda cristata demography

The annual weed Anoda cristata is recognised as a problem of increasing importance in soybean crops and can significantly reduce crop yields. The effect of soybean density, row spacings and glyphosate doses in reduced and no-tillage systems on demography and phenology of A. cristata was examined.The seedbank of A. cristata did not differ between tillage systems and was initially similar for all treatments. In the fourth year, it was higher in plots without glyphosate application than in plots treated with the recommended and half the recommended glyphosate dose. The highest seedbank was observed in wide-row soybean without glyphosate. Total emergence was not affected by the planting pattern with any glyphosate dose, but emergence was higher in the untreated plots than in those sprayed with the recommended glyphosate dose. Total weed mortality without glyphosate was independent of the planting pattern in 2001 but was lower in wide-row soybean in 2002. The application of half of the recommended dose killed almost all A. cristata plants in narrow-row soybean. Mortality was 100% with the recommended glyphosate dose in all years and in all planting patterns.Without herbicides, biomass and seed production per unit area was lower in narrow-row compared to wide-row soybean but no differences in biomass production of the weed were observed between narrow spacings with high and low densities. In all planting patterns, weed biomass and seed production decreased progressively from the first to the third weed emergence. The use of half of the recommended glyphosate dose relative to treatments without glyphosate application reduced biomass and seed production in all planting patterns, but this reduction was greatest in narrow-row soybean. Without glyphosate, in both the row spacings, flowering and seed production occurred earlier for the first relative to the second weed emergence. With the application of half of the recommended dose, flowering occurred only in the first weed emergence and both flowering and seed production were observed first in wide-row relative to narrow-row soybean. The recommended glyphosate dose controlled both the first and the second emergence, but the third emergence was not affected and remained in the vegetative stage till crop harvest. Without glyphosate and using half of the recommended glyphosate dose, reducing soybean row spacing is a more important factor than increasing crop density in reducing A. cristata populations.