Field studies were conducted in South Carolina and Georgia to evaluate weed control and soybean tolerance and yield after nonionic surfactant addition to combinations of chlorimuron plus an adjuvant-containing glyphosate formulation. Treatments included glyphosate alone, at 420 or 840 g ae/ha, or in combination with 6 or 9 g ai/ha chlorimuron and all possible combinations with or without 0.25% (v/v) nonionic surfactant. Other treatments included a weed-free and nontreated check. Chlorimuron plus glyphosate improved entireleaf, smallflower, and tall morningglory control over glyphosate alone, but nonionic surfactant addition did not further improve the control of any species, except tall morningglory. Up to 31% early-season injury was observed with the three-way mixture. Soybean injury was greater, and yields were reduced in one of three trials when nonionic surfactant was added to chlorimuron plus glyphosate combinations. This research indicates that there would be no benefit from the nonionic surfactant addition to this adjuvant-containing glyphosate formulation when combined with chlorimuron.

Two field studies were conducted in 2004 and 2005 to evaluate the response of glyphosate-resistant ‘TV52R42’ and ‘TV52R14’ soybean to glyphosate at 0, 458, 916, 1,375, 1,833, or 2,749 g ae/ha applied at the R4 reproductive growth stage and at R4 followed by the same rates at R6. A single glyphosate application at the R4 stage did not injure soybean or cause reductions in soybean height, nodes per plant, branch pods, main stem pods, or yield, regardless of application rate. Likewise, sequential applications of glyphosate at the R4 followed by R6 growth stages resulted in no adverse effects on growth parameters or yield. Preliminary findings indicate excellent tolerance to glyphosate application at reproductive growth stages beyond the currently labeled R2 growth stage restriction in varieties evaluated. Research findings warrant evaluation of an expanded range of germplasm under a variety of environmental conditions and potential effects on seed quality for establishment of tolerance levels and potential herbicide label amendments.

In fall 2011, cotton and soybean consultants from Arkansas, Louisiana, Mississippi, and Tennessee were surveyed through direct mail and on-farm visits, and rice consultants from Arkansas and Mississippi were surveyed through direct mail to assess the importance and level of implementation of herbicide resistance best management practices (HR-BMPs) for herbicide-resistant weeds. Proper herbicide timing, clean start with no weeds at planting, application of multiple effective herbicide modes of action, use of full labeled herbicide rates, and prevention of crop weed seed production with importance rating of ≥ 4.6 out of 5.0 were perceived as the most important HR-BMPs in all crops. Purchase of certified rice seed was on 90% of scouted hectares. In contrast, least important HR-BMPs as perceived by consultants with importance ratings of ≤ 4.0 in cotton, ≤ 3.7 in rice, and ≤ 3.8 in soybean were cultural practices such as manual removal of weeds; tillage including disking, cultivation, or deep tillage; narrow (≤ 50 cm)-row crops, cover crops, and altered planting dates. Narrow crop rows and cover crops in cotton; altered planting dates in cotton and soybean; and cleaning of farm equipment and manual weeding in rice and soybean is currently employed on ≤ 20% of scouted hectares. Extra costs, time constraints, adverse weather conditions, lack of labor and equipment, profitability, herbicide-related concerns, and complacency were perceived as key obstacles for adoption of most HR-BMPs. With limited adoption of most cultural practices that reduce risks of herbicide-resistant weeds, there are opportunities to educate growers concerning the proactive need and long-term benefits of adopting HR-BMPs to ensure sustainable weed management and profitable crop production.

In November 2004, a 29-question survey was mailed to Delaware soybean growers to determine grower perceptions of glyphosate-resistant (GR) horseweed and if glyphosate applications, GR soybean usage, and management practices had been altered in lieu of the presence of resistance. A total of 213 valid responses were received. Ninety-eight percent of respondents reported planting GR soybean at some point in the last 5 yr, with 90% reporting having planted GR soybean 3 or more years. The presence of GR horseweed on-farm was reported by 38% of the respondents and 95% of those growers with GR horseweed on-farm reported implementing one or more changes in GR soybean management. The most frequent change (66% of growers) due to resistant horseweed was the application of another herbicide with a different mode of action before planting. Forty-eight percent of growers with resistance on-farm reported a $5 to $17/ha increase to manage for GR horseweed, with 28% reporting a greater than $17/ha increase. Regardless of experience with GR horseweed, approximately 80% responded that it was worthwhile to incur additional costs now to preserve glyphosate for future use. Soybean grower reliance on glyphosate has not decreased in light of GR horseweed in Delaware. Misconceptions of timing for the selection of GR horseweed biotypes and the future availability of new herbicides with different modes of action exist within the farming community.

Field trials were conducted in Georgia and South Carolina in 2000 and 2001 to evaluate the effects of glyphosate, diflubenzuron, and boron combinations on crop injury and soybean yield. No interactions among glyphosate, diflubenzuron, and boron were observed. Both glyphosate and boron caused significant foliar injury (chlorosis and leaf burn) ranging from 7 to 10% in one of three locations. Diflubenzuron did not cause significant soybean injury at any location. Soybean injury symptoms observed in these experiments were transient and did not negatively affect yield.

Row spacing affects the time of canopy closure, thus influencing the growth and development of both crop and weeds. Field studies were conducted in 1999, 2000, and 2001 at Mead, NE, and 2000 and 2001 at Concord in eastern Nebraska to determine the effects of three row spacings (19, 38, and 76 cm) on the critical time for weed removal (CTWR) in dryland soybean. A three-parameter logistic equation was fit to data relating relative crop yield to increasing duration of weed presence. In general, earliest CTWR occurred in the 76-cm rows, and coincided with the first trifoliate stage of soybean. Latest CTWR occurred in the 19-cm rows and coincided with the third trifoliate. The CTWR in 38-cm rows occurred at the second trifoliate. Practical implications are that planting soybean in wide rows reduces early-season crop tolerance to weeds requiring earlier weed management programs than in narrower rows.

Corn and soybean growers across Indiana were surveyed to assess their perceptions about the importance of preplant and POST weed control timing, focusing mainly on soybean production. Despite studies demonstrating the importance of planting into a clean field, almost a third of Indiana growers do not think it is important to plant into a weed-free seedbed and 74% do not use residual herbicides in glyphosate-resistant soybean production systems. Growers who farmed less than 200 ha were more likely to overestimate the ability of soybean to tolerate weed interference than growers who farmed more hectares. Growers who manage smaller farms were also more likely to use a one-pass weed control program than larger growers. This suggests that yield losses to weed interference may be greater for smaller farms than for larger farms. Weed size and density were the most common criteria used by growers to decide when to apply herbicides. This suggests that field scouting plays an important role in the decision-making process of growers. However, a substantial proportion of growers apply POST herbicides to large common lambsquarters and giant ragweed in an attempt to minimize the number of trips across the field for weed control. Delayed control of these species likely contributes to reduced crop yields, higher application rates, and to the survival of treated plants. Opportunities to improve control and increase yields through more optimal herbicide use appear possible for Indiana corn and soybean growers.

The effects of ammonium sulfate and pelargonic acid on weed control with glufosinate and glyphosate and safety to glufosinate-resistant and glyphosate-resistant soybean were investigated in the greenhouse and field. Annual and perennial weeds varied in their sensitivity to the herbicides. Based on fresh weight reduction 10 d after treatment (DAT), common milkweed was more tolerant to glufosinate, and horsenettle was more tolerant to glyphosate. Giant foxtail was highly sensitive to both herbicides. The activity of glufosinate on common milkweed and of glyphosate on horsenettle was enhanced with the addition of 5% (wt/v) ammonium sulfate. The addition of pelargonic acid at 3% (v/v) did not enhance the activity of glufosinate or glyphosate on any weed, and it antagonized common lambsquarters and giant foxtail control with glufosinate and with glyphosate. Glyphosate was more effective than glufosinate in suppressing the regrowth of the perennial weeds horsenettle and common milkweed, but addition of ammonium sulfate and pelargonic acid was not beneficial with either herbicide. Under field conditions, the addition of ammonium sulfate or pelargonic acid to glufosinate or glyphosate did not improve efficacy on annual weeds. The addition of pelargonic acid improved yellow nutsedge control with glufosinate, but only at 6 DAT. Glufosinate and glyphosate applied alone or in combination with ammonium sulfate were safe to transgenic soybeans resistant to the respective herbicide. The addition of pelargonic acid to glufosinate or glyphosate in the greenhouse caused a rate-dependent reduction in soybean fresh weight. In the field, slight soybean injury with the addition of pelargonic acid was evident 6 DAT, but not 23 DAT. Addition of ammonium sulfate can increase the efficacy of glufosinate and glyphosate on perennial weeds without negatively affecting soybean yield.

Studies were conducted with one glyphosate-resistant and three conventional soybean cultivars to determine the effects of reducing row spacing from 76 to 38 and 19 cm, in combination with increased soybean population and herbicide applications (single and sequential), on sicklepod control and soybean yield. ‘Hutcheson’ was the only cultivar, and only under adverse conditions (1998), for which the 19-cm rows provided greater sicklepod control than the 38-cm rows. It was only under optimum conditions (1997), and only with conventional cultivars and sequential herbicide applications, that the 19-cm–row yield of 3,350 kg/ha was 21% greater than the 38-cm–row yield in medium population (455,375 plants/ha), and 21 and 64% greater than the 19- and 76-cm–row yield in low populations (241,000 plants/ha), respectively. In 1998, the same treatment showed similar late-season sicklepod control (> 80%) and a yield of 1,890 kg/ha, with no difference between the 19- and the 38-cm rows; but the yields were 15 and 24% greater than the yield of the narrow (19 and 38 cm) and that of the 76-cm row in low populations, respectively. Low populations with sequential applications in narrow rows and in the 76-cm row showed similar late-season sicklepod control (59 to 70%) and yield in 1998; but in 1997, the narrow rows showed 18 and 32% greater late-season control and yield than the 76-cm row, respectively. In 1997 and 1998, for ‘Hartz 5088RR’ (glyphosate resistant) with medium population and narrow rows, the single and sequential applications provided similar late-season sicklepod control (> 80%) and yield, and at least 10 and 24% greater yield than the narrow and the 76-cm rows in low populations, respectively.

Field experiments were conducted from 1999 to 2001 to evaluate preemergence (PRE) activity of cloransulam on broadleaf weed species and to determine the effectiveness of cloransulam as a PRE herbicide in glyphosate-resistant soybean weed management systems. Cloransulam PRE controlled prickly sida, velvetleaf, and morningglory species even at reduced rates (recommended rate 36 g ai/ha) but only suppressed growth of Palmer amaranth, hemp sesbania, and sicklepod. Cloransulam applied PRE provided initial control or suppression of most weeds, but late-season control declined appreciably. Adding metribuzin to cloransulam PRE generally improved control of hemp sesbania, Palmer amaranth, annual grasses, and morningglory species, leading to soybean yield increases. Control of weeds was greater on a silt loam soil compared with a silty clay soil. Delayed herbicide activation by rainfall or irrigation reduced control of hemp sesbania and prickly sida and affected efficacy more than soil texture. Single postemergence (POST) applications of glyphosate or fomesafen plus fluazifop-P provided 90% or less control of most weed species. When glyphosate POST or fomesafen plus fluazifop-P POST followed PRE applications of cloransulam or cloransulam plus metribuzin PRE, control of all weeds was generally greater than 85%. The highest soybean yields were recorded from treatments that contained sequential PRE followed by (fb) POST herbicide applications. Composition of weed flora determined the effect of herbicide program on soybean seed yield. No yield benefit was gained from the sequential program when the dominant species was Palmer amaranth, which was controlled by glyphosate. When hemp sesbania was the dominant species, PRE herbicides fb glyphosate POST increased yield compared with total POST glyphosate.

Application of glyphosate in combination with planting soybeans in narrow rows is an effective practice for management of weeds in glyphosate-resistant soybean. Farmers in Michigan reported higher levels of Sclerotinia stem rot (caused by Sclerotinia sclerotiorum) in fields of glyphosate-resistant soybean. Studies were conducted to determine if glyphosate or shading reduced the defense response of glyphosate-resistant soybean to S. sclerotiorum. Glyphosate caused shikimate accumulation in glyphosate-susceptible cultivar GL2415 but not in glyphosate-resistant cultivar GL2600RR. Ethylacetate extracts containing the plant defense compound glyceollin inhibited S. sclerotiorum hyphae in a rate-dependent manner. Glyphosate had no effect on either baseline or induced levels of glyceollin in glyphosate-resistant soybean, indicating that glyphosate did not impair plant defense responses to S. sclerotiorum. Shade levels of 60 and 90% in the greenhouse did not inhibit the induction of glyceollin synthesis. Glyphosate herbicide and shading did not affect the glyphosate-resistant soybean defense response to S. sclerotiorum.

Soybean consultants from Arkansas, Louisiana, Mississippi, and Tennessee were surveyed by direct mail and by on-farm visits in fall 2011 to assess weed management practices and the prevalence of weed species in midsouth U.S. soybean. These consultants represented 15, 21, 5, and 10% of total soybean planted in Arkansas, Louisiana, Mississippi, and Tennessee, respectively, in 2011. Collectively, 93% of the total scouted area in these four states was planted with glyphosate-resistant (RR) soybean. The adoption of glufosinate-resistant (LL) soybean was greatest in Arkansas (12%), followed by Tennessee (4%), Mississippi (2%), and Louisiana (< 1%). Only 17% of the RR soybean was treated solely with glyphosate, compared with 35% of LL soybean treated solely with glufosinate. Across four states, average cost of herbicides in RR and LL soybean systems was US$78 and US$91 ha−1, respectively. Collectively across states, total scouted area under conventional tillage was 42%, stale seedbed was 37%, and no-tillage was 21%. Palmer amaranth and morningglories were the most problematic weeds in all four states. Additionally, barnyardgrass and horseweed were the third most problematic weeds of Arkansas and Tennessee, respectively, and Italian ryegrass was the third most problematic weed in Louisiana and Mississippi. Glyphosate-resistant Palmer amaranth infested fewer fields in Louisiana (16% of fields) than it did in the remaining three states (54% collectively). Average Palmer amaranth hand-weeding costs in the midsouth was US$59 ha−1. Three-fourths of the midsouth consultants stipulated the need for continued research and education focused on management of glyphosate-resistant and glyphosate-tolerant weed species.

In recent years, glyphosate-resistant (GR) giant ragweed has become an increasingly problematic weed in soybean in Missouri and throughout many areas of the Midwest. Field trials were conducted in 2010 and 2011 to determine the influence of various application timings and glyphosate tank-mix combinations on the survival of GR giant ragweed in soybean. Glyphosate was applied alone and in combination with selected tank-mix combinations early POST (EPOST) to 10-, 20-, or 30-cm GR giant ragweed. Treatments received late POST (LPOST) applications approximately 3 wk after EPOST applications, once GR giant ragweed resumed growth. GR giant ragweed survival 2 wk after EPOST applications (2 WAEPOST) ranged from 60 to 100%. Glyphosate plus fomesafen applications to 10-cm plants resulted in 60% survival of GR giant ragweed, which was the lowest survival observed across all treatments and application timings. However, GR giant ragweed survival ranged from 37 to 98% 4 wk after LPOST applications (4 WALPOST). Glyphosate plus fomesafen applications to 10-cm plants resulted in 37% survival of GR giant ragweed 4 WALPOST, which was the lowest survival observed across all treatments and application timings. Few differences in soybean yields were observed between herbicide treatments within application timings in either year, and all herbicide treatments resulted in yields higher than the nontreated control. In 2011, soybean yields were reduced following glyphosate tank-mix combinations to larger plants. Overall, higher soybean injury and GR giant ragweed survival was observed in response to glyphosate tank-mix combinations made to taller plants. Results from this research indicate that additional methods of GR giant ragweed control, other than POST-only glyphosate tank-mix combinations, will be needed for effective management of GR giant ragweed in GR soybean.

Studies were conducted in 2005 and 2006 at three Iowa locations to determine the effect of soybean seeding rate and glyphosate application timing on weed management and grain yields in glyphosate-resistant soybean. End-of-season weed populations were affected by soybean seeding rate at only one location, with higher weed densities present in the lowest seeding rate when glyphosate was applied at the V2 soybean growth stage. Although weed populations were not consistently affected by soybean population, weed biomass present at soybean harvest was inversely related to soybean population. At the location with the highest weed populations, no single glyphosate application provided yields equivalent to the weed-free control. At the other locations, glyphosate application timing did not affect soybean yield. Lower soybean yields occurred with 240,000 seed/ha compared with 420,000 seed/ha at all locations and with 300,000 seed/ha at two locations.

Soybean producers of South Carolina were surveyed in 2000 through mail and on-farm visits to determine which production practices limit seed yields the most. Production systems have direct implications on weed management practices, difficulties, and future problems. When asked to describe two extension and research focus areas that would improve current soybean production, the number one response was improved weed control strategies. Fifty-seven percent of the respondents indicated that weeds were the pest that most limited soybean yield. Sicklepod, Palmer amaranth, morningglory species, common cocklebur, yellow nutsedge, and Texas panicum were considered the most problematic weeds. More than half the producers (68%) used glyphosate-resistant soybean, with one-half of these farmers producing soybean in row widths > 51 cm. Of the farmers who grew conventional soybean, 10% used narrow rows (≤ 51 cm), whereas 22% used wide rows (> 51 cm). Only 27% of the farmers who planted glyphosate-resistant soybean used a soil-applied herbicide. Forty-three percent of farmers based the need for a postemergence herbicide on weed size, 21% used no repeatable method, 21% relied on days after soybean emergence, 11% depended on soybean size or groundcover, and only 4% used a combination of weed density and size. Thirty-five percent of the farmers did not rotate soybean with another crop. In fields where soybean parasitic nematodes reduced soybean growth or caused plant death, late-season weed infestations were often exacerbated, especially in the absence of nonresidual herbicides. Specific reasons for difficulty in managing weeds in soybean may include a diverse weed spectrum, which is difficult to control with a single herbicide, the use of wide-row soybean, the lack of crop rotation, the prevalence of nematodes that directly affect crop development and, in turn, weed management, the lack of an appropriate criterion for timing postemergence herbicide applications, and the greater attention paid to production of more valuable crops. Educational efforts addressing weed management problems and association of such problems with other production practices are needed if weed management is to be improved and weed-induced yield losses minimized.