Soybean Row Research Articles

Effect of row spacing on biological control of sicklepod (Senna obtusifolia) with Colletotrichum gloeosporioides

In field experiments conducted over 3 years, the mycoherbicidal fungus Colletotrichum gloeosporioides, formulated either in 20% (v/v) unrefined corn oil and 0.2% Silwet L-77 surfactant or with an invert emulsion, provided season-long control of Senna obtusifolia in narrow (51 cm) rows of soybean. However, in wide (102 cm) rows, one application of either formulation failed to provide season-long control of S. obtusifolia, and two applications were required to achieve season-long weed control. In narrow (51 cm) rows, one application of the fungus either in unrefined corn oil or an invert emulsion controlled S. obtusifolia an average of >90%, and a second application was not required for season long weed control. Soybean yields in wide-row plots treated with two applications of either the fungus/corn oil or fungus/invert emulsion, or with a single application of the fungal treatments in narrow-row soybean plots, were not significantly different from weed-free control plots, or from plots treated with the herbicide chlorimuron. These results suggest that row spacing can affect mycoherbicidal efficacy of this fungus for controlling S. obtusifolia.

Intercropping of field crops in cotton for the management of purple nutsedge (Cyperus rotundus L.)

A 2-year field investigation was carried out during 2003–2004 to determine the effectiveness of intercropping single and double rows of sorghum, soybean and sesame in a cotton crop on the suppression of purple nutsedge (Cyperus rotundus L.). Results revealed that all three intercrops were effective in inhibiting purple nutsedge density (70–96%) and dry matter production (71–97%) during both years of experimentation. Control in the second year was more effective than in the first year. The seed cotton yield was also depressed by the intercrops but its suppression (8–23%) was far less severe than that of purple nutsedge and its loss was compensated by greater total economic returns. Intercropping of sorghum and sesame produced greater than 20% net benefits (up to 60%) in comparison with the control (cotton alone). Soybean intercropping produced comparable net benefits (95–103%). Sesame two rows intercrop treatment appeared the most profitable with net benefit of 51–59% with good purple nutsedge control (73–92% density suppression, 77–95% dry weight suppression) during both years of experimentation.

Effect of Soybean Row Width and Population on Weeds, Crop Yield, and Economic Return

Field studies were conducted in 2004 and 2005 to determine the effect of soybean row width and population on weeds, canopy closure, crop yield, and economic return in glyphosate-resistant soybean. Soybean leaf area index (LAI) was greater in 19- and 38-cm, compared with 76-cm rows from 8 to 12 wk after planting in the low, moderate, and high soybean populations. Canopy closure was delayed by 2 wk in the moderate population in 76-cm rows compared with the high population in 19-cm rows. Fewer weeds emerged in 19-cm, compared with 76-cm rows following glyphosate application, and increasing the soybean population within a row width did not influence late-season weed emergence. Weed biomass in the weedy control was greater in the very low soybean population compared with the high soybean population within each row width; however, weed biomass in the weedy control was similar in the high and moderate soybean populations. Soybean yield in the weed-free and 10-cm glyphosate treatment did not differ, and yield was greater in 19-cm rows planted at moderate or high, compared with low populations. There was no difference in weed-free soybean yield at low, moderate, and high populations within 38- and 76-cm rows. Gross margins were usually greater in 19- and 38-cm, compared with 76-cm rows. The gross margin for soybean planted in 19-cm rows was usually greater at moderate or high soybean populations compared with lower populations. In 76-cm rows, the gross margin was greatest at the low and moderate soybean populations. When rainfall or other factors limited soybean yield, increasing the soybean population from approximately 300,000 plants/ha to 445,000 plants/ha in 19-, 38-, and 76-cm rows did not result in quicker canopy closure, reduced weed emergence, or greater soybean yield and gross margins.Nomenclature: Glyphosate potassium salt, glyphosate-resistant soybean, Glycine max (L.) Merr., ‘AG2701’

Sicklepod (Senna obtusifolia) Survival and Fecundity in Wide- and Narrow-Row Glyphosate-Resistant Soybean

Experiments were conducted to determine the influence of soybean row width and glyphosate application timing on survival, biomass production, and fecundity of three sicklepod cohorts along with soybean seed yield. The first cohort comprised sicklepod plants that emerged from soybean planting through the V3 stage of soybean (two fully developed trifoliate leaves plus the unifoliate leaves; cohort 1). The second cohort comprised plants that emerged between the V3 to V6 stages of soybean (five fully developed trifoliate leaves plus the unifoliate leaves; cohort 2), and the third cohort emerged after the V6 stage through the R2 stage of soybean (full bloom; cohort 3). Glyphosate was applied at V3; V6; V3 and V6; and V3, V6, and R2 in rows 19 and 97 cm wide, and a nontreated control was included for comparison in each row width. Survival of cohort 1 in 2004 in glyphosate-treated plots occurred only after a single glyphosate application at V3 or V6 in wide rows. Narrowing the soybean row width reduced sicklepod survival throughout the growing season, even without glyphosate. Total biomass production from all cohorts averaged over years was 1,602 g m−2 in wide rows compared with 648 g m−2 in narrow rows. Cohort 1 accounted for 70 and 77% of the total sicklepod biomass in wide and narrow rows, respectively. Cohort 2 contributed 29% of the total sicklepod biomass in wide rows and 22% in narrow rows. Cohort 3 produced minimal biomass, contributing no more than 1% of the total sicklepod biomass. Sicklepod emerging after V6 failed to produce seed in 2004, and no sicklepod seed were produced in 2005 by plants emerging after V3. Averaged over years, sicklepod from cohort 1 in nontreated controls produced 3,695 seed m−2 in narrow rows compared with 6,685 seed m−2 in wide rows. Nontreated sicklepod from cohort 2 in 2004 produced 510 seed m−2 in narrow rows compared with 1,640 seed m−2 in wide rows. Soybean yields were similar among all glyphosate applications averaged over years and row widths, ranging from 3,340 to 3,700 kg ha−1 compared with 1,290 kg ha−1 without glyphosate (61 to 65% yield loss).

Temporal and spatial dynamics of earthworm surface casting in a temperate soybean agroecosystem

Earthworm activity is expected to have a beneficial effect on soil structure and plant growth in agroecosystems, but active populations are temporally dynamic and spatially heterogeneous under field conditions. The objective of this research was to evaluate the temporal and spatial variation in surface casting by earthworms at the row-interrow scale in a temperate soybean agroecosystem. Earthworm populations were manipulated in 2.9 m 2 field enclosures by applying carbaryl pesticide to reduce the naturally occurring earthworm population before juveniles and adults of Aporrectodea caliginosa (Sav.) and Lumbricus terrestris L. were added. Surface casting was measured weekly during a 14-week period. There was very little surface casting during the first 6 weeks of the study, possibly due to insufficient food resources and shade at the soil surface. We collected nearly 3400 casts from weeks 7 to 14, and surface casting rates during this period ranged from 0.95 to 2.51 g dry weight m −2 day −1. More casts were deposited within 30 cm of the soybean row than at distances up to 50 cm from the row, indicating that the planted row was a zone with greater earthworm activity than the inter-row. Better understanding of the timing and small-scale variation in earthworm activities will aid our understanding of how earthworms contribute to plant growth.

Row Spacing and Seeding Rate Effects on Eastern Black Nightshade (Solanum Ptycanthum) and Soybean

Reducing seeding rates in 19- or 76-cm row soybean below the optimum rate may reduce soybean competitiveness with weeds, and indirectly increase production costs to the grower. Field studies in 2001 and 2002 evaluated the effect of soybean seeding rate and row spacing on the emergence, growth, and competitiveness of eastern black nightshade (EBN) in soybean. EBN emergence ceased within 45 d after planting (DAP), and was similar across soybean seeding rates and row spacing. EBN control by glyphosate was not affected by soybean population or row spacing. Soybean planted in 19-cm rows was more competitive with EBN, regardless of seeding rate. Increasing the soybean seeding rate in 76-cm rows from 185,000 seeds/ha to 432,000 seeds/ha reduced EBN dry weight threefold at East Lansing and nearly twofold at Clarksville in 2002. There was no increase in EBN density or dry weight in 19-cm row soybean planted at 308,000 seeds/ha compared with 556,000 seeds/ha, whereas a seeding rate of 432,000 seeds/ha in 76-cm row soybean did not suppress EBN dry weight or increase soybean yield in the presence of EBN compared with a seeding rate of 308,000 seeds/ha.

Self‐Seeding Winter Cereal Cover Crops in Soybean

Soil protection and nutrient scavenging benefits of cover crops have been widely reported. Nevertheless, adoption of cover crops in agronomic farming systems is low. Cover crop systems that do not require annual planting may increase adoption. The objectives of this study were to compare self‐seeding and competitiveness of winter wheat (Triticum aestivum L.), triticale (x Triticosecale Wittmack), and rye (Secale cereale L.) using different planting configurations and management options while growing concurrently with soybean [Glycine max (L.) Merr.]. Cover crops were planted with two or four 19‐cm rows between each 76‐cm soybean row. A no‐cover crop check treatment was also included for comparison. Cover crop species and species × management system interactions were not significant for seed production or soybean seed yield. Averaged across management system, cereals produced 10 656 and 4051 seeds m−2 in 2004 and 2005. The two‐row band, no‐chop treatment (2RBNC) produced the most seed (20 347 and 14 511 seeds m−2) in 2004 and 2005, but also lowered soybean yield the greatest (45 and 40%). The four‐row treatment with a late glyphosate band (4RLB) was the least competitive and yielded 3114 and 3717 kg ha−1 compared to 4019 and 4391 kg ha−1 in the check. Wheat had the greatest self‐seeding, averaging about 31% of the original plant density. The four‐row treatment without a glyphosate band (4RNB) could be used in organic production systems, although additional research is needed to develop less competitive self‐seeding systems for conventional production systems.

Performance of cassava intercropped with maize, soybean and cowpea in the forest zone of Ghana<BR><BR>La performance du manioc intercultive avec le maïs, le soja et le niebe dans la zone forestiere du Ghana

Experiments were conducted in the forest zone of Ghana from 1997 to 1999 to assess the performance of two cassava varieties intercropped with maize, soybean and cowpea at different spatial arrangements. The cassava varieties were “Gblemoduade” and “Ankra” and spatial arrangements were (S1) one row cassava, one row maize, one row soybean, two rows cowpea; (S2) one row cassava, one row maize, two rows soybean, three rows cowpea; and (S3) one row cassava, one row maize, three rows soybean, four rows cowpea. Cowpea rows were planted in the minor season into cassava as a succeeding crop to maize and soybean after their harvest in the major season. Intercropping reduced root yield by 22-37% in “Gblemoduade” and 43-51% in “Ankra” compared to sole crops. “Gblemoduade”, on the average, yielded two and half times greater than Ankra under intercrop (24.8 vs 9.1 t ha -1 ) and monocrop (36.9 vs 17.4 t ha -1 ) as a result of larger number of roots, heavier and bigger roots than “Ankra”. Cassava yield decreased as the number of rows of soybean or cowpea increased. Root yields were similar for S2 and S3 (22.3 and 23.2 t ha -1 for “Gblemoduade” and 8.6 and 8.9 t ha -1 for “Ankra”, respectively), but were smaller than yields at S1 (28.8 t ha -1 and 9.9 t ha -1 for “Gblemoduade” and “Ankra”, respectively). “Gblemoduade” was more aggressive (a strong competitor) than “Ankra” in the system while, both cassava varieties increased in aggressivity with decreased number of soybean or cowpea rows in the mixture (i.e. S1=S2 > S3). Among the crops, maize was more competitive to cassava than soybean and cowpea. Thus, the performance of cassava can be improved by planting 1 row each of cassava, maize and soybean followed by 2 rows of cowpea (after maize and soybean harvest) and by growing the improved variety “Gblemoduade”. Keywords : intercropping, cassava, maize, soybean, cowpea Agricultural and Food Science Journal of Ghana Vol. 3 2004: 193-202

Performance of WeedSOFT for Predicting Soybean Yield Loss

Decision support systems (DSSs) have been developed to assist producers and consultants with weed management decisions. WeedSOFT is a DSS currently used in several states in the north-central region of the United States. Accurate estimates of crop yield loss due to weed interference are required for cost-effective weed management recommendations. WeedSOFT uses competitive indices (CIs) to predict crop yield loss under multiple weed species, weed densities, and relative times of weed emergence. Performance of several WeedSOFT versions to predict soybean yield loss from weed competition was evaluated using CI values in WeedSOFT version 9.0 compared to new CI values calculated from weed dry matter, weed volume, and soybean yield loss in two soybean row spacings (19 and 76 cm) and two relative weed emergence times (at soybean emergence and first trifoliate leaf stage). Overall, new CI values improved predictions of soybean yield loss by as high as 63%. It was especially true with using new CI values based on yield loss compared to those based on weed dry matter or weed volume. However, there were inconsistencies in predictions for most weed species, suggesting that additional modifications are needed to further improve soybean yield loss predictions.

Soybean row spacing and weed emergence time influence weed competitiveness and competitive indices

Weed competitiveness can be quantified with the concept of competitive index (CI), a relative scale of weed competitiveness. Field studies were conducted in 2002 and 2003 in northeastern and southeastern Nebraska to evaluate the influence of soybean row spacing and relative weed emergence time on the competitiveness of major weed species in soybean. Ten weed species were seeded in soybean spaced 19 and 76 cm apart at the planting, emergence, and first trifoliate leaf stages of soybean. Total weed dry matter (TDM), weed plant volume, and percent soybean yield loss were arbitrarily selected as a base for determining the CI for each weed species. Soybean yield loss was the least variable parameter used to quantify weed competitiveness and rank their CIs. In general, weeds grown with soybean planted in 19-cm rows produced less TDM, plant volume, and reduced soybean yield less than weed species grown in 76-cm rows. Later-emerging weeds produced less TDM, plant volume, and reduced soybean yield less than the early-emerging ones. In general, broadleaf species were more competitive than grass weed species. Common sunflower was the most competitive weed species in this study.

Qualidade fisiológica de sementes de soja em função da adubação comfósforo, molibdênio e cobalto

Aiming at evaluating the phosphorus, molybdenum and cobalt application effects in the physiological quality of soybean seeds of the subsequent generation, a field experiment was carried out, in distrophic red latosol, in Dourados, State of Mato Grosso do Sul, Brazil. The experimental design was in randomized blocks, with the treatments organized in a 4x4x2 factorial pattern, with four replications. The studied factors were: four phosphorus levels (0, 100, 200, and 300 kg ha1 of P2O5), four molybdenum doses (0, 25, 50, and 100 g ha1) and two cobalt levels (0, and 1.0 g ha1). Each plot was constituted of five 5.0 m length and 0.5 m width soybean rows. In the obtained seeds, the vigor and germination potential were evaluated through the tetrazolium test and the standard test and emergence at field conditions. The phosphate fertilization provided an increase in the vigor and germination potentials, detected through both tests. The molybdenum and cobalt application through the seeds promoted an increment in the germination rate, also determined by both tests.

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.

Influence of soybean row width and velvetleaf emergence time on velvetleaf (Abutilon theophrasti)

Row spacing and the relative time of velvetleaf emergence affects the time of soybean canopy closure relative to velvetleaf, influencing the growth and development of velvetleaf. Field studies were conducted in northeastern Nebraska in 2002 and 2003 to describe velvetleaf growth as influenced by soybean presence or absence (velvetleaf grown with soybean or in monoculture), soybean row spacing (19 and 76 cm), and relative time of velvetleaf emergence. Velvetleaf seed production, leaf area (LA), and total dry matter (TDM) were greater in 76-cm- than in 19-cm-wide soybean rows. LA, TDM, and seed production of velvetleaf were reduced with later emergence times in both monoculture and with soybean. Velvetleaf LA, TDM, and seed production decreased when grown with soybean compared with when grown in monoculture. Practical implications of this study suggest that narrowing crop row spacing and controlling early-emerging velvetleaf in soybean can be an effective part of an integrated weed management strategy.

Comparison of glyphosate formulations with and without sequential herbicides for no-till soybean in narrow rows

Effective control of weeds during early stages of soybean (Glycine max) growth is critical to minimize crop yield reduction. Experiments were conducted to compare weed control and crop yield with two glyphosate formulations (trimethylsulfonium and isopropylamine salts) applied in the fall or spring, either alone or in combination with sequential pre-or postememergence herbicides in soybean cv. ‘Maple Glen’ no-till planted in narrow rows into grain stubble. In six experiments where glyphosate was applied (three in the fall and three in the spring), there was no difference in weed control or in soybean yield between the two glyphosate formulations. Crop yield was improved over glyphosate used alone by addition of metribuzin in all fall experiments and in two of three spring experiments and by addition of linuron in two of three experiments in both fall and spring. An herbicide that controlled annual broadleaf weeds was needed after fall-applied glyphosate in all experiments to achieve maximum soybean yield. Addition of an effective sequential herbicide after spring applied glyphosate improved yields but not to the same extent as noted with the fall applied glyphosate. A pre-emergence residual herbicide, such as metribuzin or linuron, that controls a broad spectrum of weeds is recommended after fall or spring applied glyphosate to maximize soybean yield.

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.

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.

Critical Leaf Potassium Concentrations for Yield and Seed Quality of Conservation‐Till Soybean

Leaf K concentrations needed for optimum soybean [Glycine max (L.) Merr.] production under conservation tillage systems may be different from those in conventional tillage (moldboard plow) because soil properties (such as soil‐test K distribution) and soybean root distribution within the soil profile under conservation tillage systems differ from those in conventional tillage. Little information is available about adequate leaf K concentrations for soybean on conservation‐tilled soils with significant vertical soil‐test K stratification. This study was conducted at three locations in Ontario, Canada from 1998 through 2000 to estimate the critical leaf K concentrations for conservation‐till soybean on K‐stratified soils with low to very high soil‐test K levels and a 5‐ to 7‐yr history of no‐till management. Three K fertilizer placement methods (band placement, surface broadcast, and zero K), two conservation tillage systems (no‐till and fall tandem disk), and two soybean row widths (19 and 38 cm) were used to create a wide spectrum of production environments. For maximum seed yield, the critical leaf K concentration at the initial flowering stage (R1) of development was 24.3 g kg−1 This concentration is greater than the traditional critical leaf K values for soybean that are being used in Ontario and in many U.S. Corn Belt states. Critical leaf K values for the maximum concentrations of K, oil, and isoflavone in seed were 23.3, 24.1, and 23.5 g kg−1, respectively. The extent of vertical soil‐test K stratification seems to be one of the factors contributing to apparently higher critical leaf K concentrations for conservation‐till soybean.

Soil-applied herbicide use in wide- and narrow-row glyphosate-resistant soybean ( Glycine max)

A field study was conducted in 2001 and 2002 at Blackville, SC, to compare the effectiveness of soil-applied herbicide programmes followed by glyphosate with sequential glyphosate applications in 19- and 97-cm row soybean. Treatments included pre-emergence applications of flumetsulam, chlorimuron plus sulfentrazone, metribuzin, or flumioxazin all applied with S-metolachlor with a subsequent application of glyphosate applied 5 wk after soybean planting (WAP). Additional treatments included sequential applications of glyphosate at 3 and 5 WAP and a non-treated control. Weed control was comparable in wide and narrow rows, and soil-applied herbicides were similar to sequential glyphosate applications in either row width. Amaranthus palmeri S. Wats., Digitaria sanguinalis (L.) Scop., and Cyperus rotundus L. control with all herbicides was 98–100%. All herbicide programmes provided at least 92% Ipomoea hederacea var. integriuscula Gray and I. lacunosa L. control. S-metolachlor plus flumetsulam followed by glyphosate provided 99% Senna obtusifolia (L.) Irwin and Barneby control, which was superior to the 88% and 90% obtained with chlorimuron plus sulfentrazone and metribuzin programmes, respectively. Flumioxazin and sulfentrazone caused up to 12% soybean injury. In 2001, soybean yields and gross profit margins were similar for soil-applied programmes followed by glyphosate and sequential glyphosate applications in wide and narrow rows. Conversely, soybean yields and profit margins in 2002 averaged across soil-applied programs were 380 kg/ha and $50/ha superior to sequential glyphosate applications in wide rows, but were similar in narrow rows. This research indicates a highly efficacious soil-applied herbicide program followed by glyphosate provides similar weed control to sequential glyphosate applications in wide- and narrow-row soybean; however, a soil-applied herbicide programme followed by glyphosate may improve yields and returns of wide-row soybean over sequential glyphosate applications alone.

Influence of Early-Season Yield Loss Predictions from WeedSOFT®and Soybean Row Spacing on Weed Seed Production from a Mixed-Weed Community

Seed production from weeds that are missed by herbicide application can affect future weed populations and management decisions. It may be possible to expand the utility of computerized weed management decision aids to include an estimate of weed seed production resulting from selected treatments based on crop yield potential. Field studies were conducted in soybean near Columbia, MO, to determine whether weed control recommendations based on crop yield potential from a computerized weed management decision aid influence weed seed production in two soybean row spacings. At approximately 28 d after planting, weed densities and heights were entered into WeedSOFT®to generate a list of treatments ranked by predicted crop yields. Treatments included: (1) highest predicted crop yield in a glyphosate-resistant system, (2) highest predicted crop yield in a nonglyphosate-resistant system, (3) a 10% yield reduction, (4) a 20% yield reduction, and (5) an untreated control. These treatments were applied to soybean grown in 38- and 76-cm rows. Treatments that provided 90% or higher control of an individual species at 22 d after treatment usually produced less seed than untreated checks. Weed seed production based on early-season herbicide efficacy showed a linear relationship and was relatively predictable (r2≥ 0.52) for the predominant weed species. For less dominant weed species, weed seed production was not strongly correlated (r2≤ 0.27) to early-season herbicide efficacy but apparently influenced by control of other weed species. Narrow row spacing reduced giant foxtail biomass both years but did not reduce common ragweed and ivyleaf morningglory biomass. Narrow rows did not decrease giant foxtail, common ragweed, and ivyleaf morningglory seed production.

Microenvironment of a corn–soybean–oat strip intercrop system

The microenvironment of a corn ( Zea mays L.)–soybean ( Glycine max [L.] Merr.)–oat ( Avena sativa L.) narrow strip intercrop system was studied on four farms in Iowa, USA during 1991–1993 (one or two farms studied each year) with the objective of explaining previously observed patterns of strip-crop yields. Air and soil temperatures, humidity, wind, and photosynthetic photon flux density (PPFD) for each row in a strip were recorded continuously during 4–13-day periods several times during each growing season; short-term measurements of light extinction through the corn and soybean canopies were made late in the 1993 growing season. Soybean biomass in September 1993 was also determined. Early in the growing season, there typically were no significant differences among rows within a strip in any environmental or growth variable. Later in the growing season, daily PPFD received varied more across each strip than any other environmental variable measured; the outer side of edge rows of corn in north–south rows typically received significantly higher daily PPFD (2–38% more) than inner rows in the strip, while the outside of the soybean row furthest from corn received 36–140% greater PPFD than the corn-side of the row next to the corn strip. Soybean height; leaf mass per leaf area; and leaf mass, stem mass, and pod mass per unit land area in September were strongly related to daily PPFD, with all except height increasing with greater PPFD. Total biomass increased by 32% from the row closest to corn to the row furthest from corn, although there was no change in the ratio of leaf:stem:pod with row position. Thus, PPFD varied the most across the strips of any environmental variable, and crop growth was closely related to PPFD received.