Glyphosate was evaluated at 0.8, 1.3, and 1.7 kg ae/ha applied at the two-leaf, four-leaf, or two- to three-tiller growth stage for red rice control. In addition, red rice seedheads were counted concurrently with soybean harvest at each of three locations to assess treatment effect on seedhead reduction. Field studies were conducted at Starkville, MS, in 1994 and 1995 and Shaw, MS, in 1995. A significant rate response was not observed for red rice control 2 and 4 wk after treatment (WAT) or for seedhead reductions. Glyphosate controlled red rice 88, 91, and 88% 2 WAT when applied to two-leaf, four-leaf, or two- to three-tiller red rice, respectively. Due to subsequent seedling emergence, control from glyphosate applications to two- or four-leaf red rice 4 WAT was 51 and 84%, respectively. Red rice treated at the two- to three-tiller stage was controlled 91% 4 WAT. When compared to the nontreated control, seedheads were reduced 97% by two- to three-tiller applications, compared to 87 and 56% reductions from four- and two-leaf applications, respectively.

Tolerance and yield of ‘H8001 RR’ soybean to single and sequential glyphosate applications made during vegetative and reproductive stages were examined in field trials in 2000 and 2001. In addition, germination and seedling characteristics from harvested seed were assessed. Glyphosate-resistant soybean tolerated single and sequential applications of 1.7 kg ae/ha glyphosate with minimal injury (≤5%), except for an R2 application in 2000 that caused 20% injury. All observed injury was transient and did not adversely affect soybean density, height, seed weight, or seed yield. Germination and seedling growth of harvested seed were similar to those of a nontreated control. These results suggest that glyphosate-resistant soybean can withstand sequential applications of glyphosate during vegetative and reproductive stages without prolonged adverse effects.

Two separate field experiments were conducted over a 2-yr period in Fayetteville, AR, during 2012 and 2013 to (1) evaluate POST herbicide programs utilizing a premixture of dimethylamine (DMA) salt of glyphosate + choline salt of 2,4-D in a soybean line resistant to 2,4-D, glyphosate, and glufosinate and (2) determine efficacy of herbicide programs that begin with PRE residual herbicides followed by POST applications of 2,4-D choline + glyphosate DMA on glyphosate-resistant Palmer amaranth. In the first experiment, POST applications alone that incorporated the use of residual herbicides with the glyphosate + 2,4-D premixture provided 93 to 99% control of Palmer amaranth at the end of the season. In the second experiment, the use of flumioxazin, flumioxazin + chlorimuron methyl, S-metolachlor + fomesafen, or sulfentrazone + chloransulam applied PRE provided 94 to 98% early-season Palmer amaranth control. Early-season control helped maintain a high level of Palmer amaranth control throughout the growing season, in turn resulting in fewer reproductive Palmer amaranth plants present at soybean harvest compared to most other treatments. Although no differences in soybean yield were observed among treated plots, it was evident that herbicide programs should begin with PRE residual herbicides followed by POST applications of glyphosate + 2,4-D mixed with residual herbicides to minimize late-season escapes and reduce the likelihood of contributions to the soil seedbank. Dependent upon management decisions, the best stewardship of this technology will likely rely on the use multiple effective mechanisms of action incorporated into a fully integrated weed management system.

A field study was conducted during 1999, 2000, and 2001 at Stoneville, MS, on a Dundee silty clay loam to determine the impact of a rye cover crop with one or two postemergence (POST) herbicide applications on weed control, yield, and net return in narrow-row glyphosate-resistant, glufosinate-resistant, and conventional soybean systems. Cover crop systems included no–cover crop conventional tillage (CT), no–cover crop no-tillage (NT), and rye NT, all with early POST (EPOST), EPOST followed by late POST (LPOST), and no-herbicide weed management. Weed control and net return among glyphosate-resistant, glufosinate-resistant, and conventional soybean systems were similar. One POST ($111/ha) application of herbicides was more profitable than two POST ($79/ha) applications regardless of soybean cultivar and cover crop system. Rye residue reduced total weed density by 9 and 27% and biomass by 19 and 38% compared with no–cover crop CT and NT, respectively. In the rye cover crop, input costs were higher because of the additional cost of seed, planting, and rye desiccation. The additional cost resulted in a lower net return with the rye cover crop ($29/ha) compared with the no–cover crop CT ($84/ha) or NT ($87/ha) system, even though soybean yield in the rye cover crop system was comparable to that from the no–cover crop CT and NT systems. These results showed that because of additional cost, rye cover crop–based soybean production was less profitable compared with existing no–cover crop–based production systems.

New trait technology incorporating 2,4-dichlorophenoxyacetic acid (2,4-D) resistance in soybean provides an alternative method to control weeds. However, the effect of postemergence treatments of 2,4-D on aryloxyalkanoate dioxygenase-12 (AAD-12) soybean on injury and yield components has not been reported. Our objectives were to characterize the effect of 2,4-D (dimethylamine salt) rates (0, 1,120, and 2,240 g ae ha−1) and soybean growth stage (V5, R2, or V5 followed by R2) on AAD-12 soybean injury and yield components. Less than 3% soybean injury was observed when 2,240 g ha−1 of 2,4-D was applied to R2 soybean, and less than 1% soybean injury was caused by 1,120 g ha−1 of 2,4-D. Seed yield, seed mass, pod number, seed number, seed per pod, reproductive node number, pods per reproductive node, node number, and percent reproductive nodes were not affected by 2,4-D treatments when applied at the V5, R2, or the V5 followed by R2 soybean growth stage. This research demonstrates that soybean transformed with AAD-12 can tolerate foliar applications of 2,4-D at rates up to 2,240 g ha−1 with no effect on soybean grain yield components.

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.

A field experiment was conducted in Boone and Callaway counties in Missouri in 2010 and 2011 to investigate herbicide programs for the management of summer annual grass and broadleaf weeds in soybean resistant to 2,4-D and glufosinate. Results revealed that the addition of 0.56, 0.84, or 1.12 kg ha−1 2,4-D (amine) to either or both POST applications of glufosinate in a two-pass POST herbicide program increased control of common waterhemp compared to two POST applications of glufosinate alone. Similar levels of common cocklebur, giant foxtail, large crabgrass, and barnyardgrass control were achieved with any of the two-pass POST programs that contained 2,4-D compared to two-pass POST programs containing glufosinate alone. Similar control of these species was also achieved with the inclusion of 2,4-D in either the first or second pass of glufosinate. Two-pass programs resulted in the highest levels of weed control (90%). Annual grass and broadleaf weed control was as much as 59% lower when one-pass POST herbicide programs were compared to PRE followed by (fb) POST or two-pass POST programs. However, one-pass POST programs were sufficient to obtain exceptional control of common cocklebur and giant foxtail. Across all site years, soybean yields ranged from 2,680 to 3,100 kg ha−1 for all herbicide treatments, but did not differ statistically. Overall, results from these experiments indicate that compared to glufosinate alone, PRE fb POST or two-pass POST herbicide programs that incorporate 2,4-D amine with glufosinate in 2,4-D–resistant soybean enhance control of common waterhemp, while providing similar levels of control of other summer annual grass and broadleaf weeds.

A field study was conducted during 2000 and 2001 at Stoneville, MS, to determine the effects of isopropylamine, trimethylsulfonium (Tms), diammonium, and aminomethanamide dihydrogen tetraoxosulfate (Adt) salt formulations of glyphosate on weed control, growth, chlorophyll content, nodulation, nitrogen content, and grain yield in glyphosate-resistant soybean and to assess potential glyphosate accumulation in soybean nodules. Glyphosate-Tms and glyphosate-Adt injured soybean, and visible injury ranged from 29 to 38% 2 d after late postemergence (LPOST) application; however, soybean recovered by 14 d. Glyphosate formulations had no effect on chlorophyll content, root and shoot dry weight, or nodule number but reduced nodule biomass by 21 to 28% 14 d LPOST. Glyphosate levels in nodules from treated plants ranged from 39 to 147 ng g−1 (dry weight), and leghemoglobin content was reduced by as much as 10%. Control of five predominant weed species 14 d after LPOST was > 83% with one application and > 96% with two applications regardless of the glyphosate salts used. Soybean yields were generally higher with two applications than with one application regardless of glyphosate formulation. These results indicate that soybean injury and inhibition of nodule development with certain glyphosate formulations can occur, but soybean has the potential to recover from glyphosate stress.

Field studies were conducted during the years 2000 to 2003 at Stoneville, MS, to determine the efficacy of fall deep tillage and glyphosate applications on redvine and trumpetcreeper populations and soybean yield in glyphosate-resistant soybean. Fall deep (≈45 cm) tillage for 1, 2, and 3 yr reduced redvine density by 95, 88, and 97%, respectively, compared with shallow (≈15 cm) tillage, but deep tillage did not reduce trumpetcreeper density. Glyphosate applied preplant reduced trumpetcreeper density (25 to 44%), but not redvine density, compared to that with no glyphosate. Glyphosate early postemergence (EPOST) either alone (45 to 67%) or followed by (fb) late postemergence (LPOST; 59 to 83%) reduced density of trumpetcreeper, but not of redvine, compared to that with no herbicide. However, dry biomass of both vines was reduced with glyphosate EPOST or LPOST compared to that with no herbicide. Soybean yields were higher with deep tillage vs. shallow tillage, glyphosate preplant application vs. no glyphosate, and glyphosate EPOST either alone or fb LPOST vs. no herbicide. Redvine did not reestablish in 2003, which was after skipping fall deep tillage for 1 yr following three consecutive years of deep tillage compared with shallow tillage. It is possible to manage redvine infestations with fall deep tillage and trumpetcreeper infestations with glyphosate preplant and postemergence (POST) in-crop applications. Integration of fall deep tillage and glyphosate POST applications could be an effective strategy to manage combined infestations of these vines in glyphosate-resistant soybean.

New trait technology incorporating 2,4-D resistance in soybean is dependent upon the ability of the plant to metabolize 2,4-D by the aryloxyalkanoate dioxygenase-12 protein (AAD-12). Our objectives were to determine AAD-12 expression during the daytime, throughout the leaf canopy, and before and after 2,4-D treatment for the events DAS-68416-4 and DAS-21606-3. Field experiments were conducted near Wanatah, IN in 2009 and Fowler, IN in 2009, 2010, and 2011. During the daytime, total AAD-12 expression was lowest between 12:30 and 15:30, averaging 161 ng cm−2, as compared to an average of 245 ng cm−2 in the morning and 243 ng cm−2 in the evening. The youngest fully emerged trifoliate in the DAS-68416-4 event had the highest AAD-12 expression, with means ranging from 369 to 390 ng cm−2, while the older leaves maintained a lower level of expression, 171 to 211 ng cm−2. The youngest leaves of event DAS-21606-3 had the highest level of AAD-12 expression (205 to 225 ng cm−2), while the level of AAD-12 was lower in older leaves (71 to 149 ng cm−2). In general, 2,4-D treatments did not reduce AAD-12 expression at 3, 7, 14, and 21 days after treatment; however, in a few instances AAD-12 expression was increased or decreased by 8 to 11% after 2,4-D treatment. Expression of AAD-12 was between 152 to 390 ng cm−2 for DAS-68416-4 and from 71 to 244 ng cm−2 for DAS-21606-3.

Field studies were conducted over 2 yr in Louisiana to evaluate entireleaf morningglory and hemp sesbania control by glyphosate isopropylamine salt applied alone at 420, 560, and 700 g ae/ha and in combination with chlorimuron at 4, 6, 9, and 11 g ai/ha. Comparison treatments included glyphosate alone at 840 g/ha, glyphosate at 1,400 g/ha plus chlorimuron at 13 g/ha, and sethoxydim at 210 g ai/ha plus chlorimuron at 11 g/ha. Entireleaf morningglory and hemp sesbania were controlled no more than 77% with glyphosate rates as high as 840 g/ha. Chlorimuron added to glyphosate increased entireleaf morningglory control in two of the three experiments to 83 to 88% and hemp sesbania control to 86 to 98%. Response to chlorimuron was not rate dependent. In most cases, glyphosate–chlorimuron mixtures did not antagonize barnyardgrass control, and increasing rate of glyphosate did not always increase control. Soybean was injured no more than 6% by chlorimuron at one location, but injury was as great as 30% at another location, which was possibly due to smaller soybean size. Differences in weed control and soybean injury among the glyphosate and glyphosate–chlorimuron combinations were not reflected in soybean yields. Because injury occurred early season, sufficient time remained for soybean to recover and produce acceptable yields. Weed control by sethoxydim plus chlorimuron was less consistent than that by glyphosate plus chlorimuron, and soybean yield in most cases was inferior to the glyphosate treatments.

Three field studies were conducted during 1998 to 2002 at Stoneville, MS, to examine the efficacy of glufosinate and glyphosate on redvine and trumpetcreeper control in glufosinate- and glyphosate-resistant soybean. Glyphosate at 2.52 kg ae/ha applied approximately 3 wk before planting soybean reduced trumpetcreeper density (45 to 52%) but not redvine compared with no glyphosate in both glufosinate- and glyphosate-resistant soybean. However, glyphosate applied preplant reduced biomass of both species in glufosinate-resistant soybean. Glyphosate early postemergence (EPOST) followed by (fb) late postemergence (LPOST) had no effect on redvine density but reduced trumpetcreeper density (70%) compared with the no-herbicide control. There were no differences in densities and biomass of redvine and trumpetcreeper and soybean yield among isopropylamine, diammonium, and aminomethanamide dihydrogen tetraoxosulfate salts of glyphosate. Overall, trumpetcreeper is more susceptible to glyphosate than redvine. Glufosinate EPOST with or without acifluorfen or glufosinate EPOST fb LPOST had no effect on densities of redvine and trumpetcreeper but reduced biomass 45 to 76% and 35 to 58%, respectively, compared with the nontreated control. These results show that glyphosate preplant and POST in-crop applications can reduce trumpetcreeper density but not redvine, and glufosinate POST applications can suppress growth of both species.

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.