Use of synthetic auxin herbicides has increased across the United States Midwest following adoption of synthetic auxin-resistant soybean traits in addition to extensive use of these herbicides in corn. Off-target movement of synthetic auxin herbicides such as dicamba can lead to severe injury to sensitive plants nearby. Previous research has documented effects of glyphosate on spray solution pH and volatility of several dicamba formulations, but our understanding of the relationships between glyphosate and dicamba formulations commonly used in corn and for 2,4-D remains limited. The objectives of this research were to i) investigate the roles of synthetic auxin herbicide formulation, glyphosate and spray additives on spray solution pH, ii) assess the impact of synthetic auxin herbicide rate on solution pH, and iii) assess the influence of glyphosate and application time of year on dicamba and 2,4-D volatility using soybean as bioindicators in low-tunnel field volatility experiments. Addition of glyphosate to a synthetic auxin herbicide decreased solution pH below 5.0 for four of the seven herbicides tested (initial pH of water source = 7.45 to 7.70). Solution pH of most treatments was lower at a higher application rate (4× the labeled POST rate) than the 1× rate. Among all treatment factors, inclusion of glyphosate was the most important affecting spray solution pH; however, the addition of glyphosate did not influence Area Under the Injury over Distance Stairs (AUIDS; p=0.366) in low-tunnel field volatility experiments. Greater soybean injury in field experiments was associated with high air temperatures (maximum >29 C) and low wind speeds (mean 0.3 to 1.5 m s-1) during the 48-h period following treatment application. The two dicamba formulations (diglycolamine with VaporGrip® and sodium salts) showed similar levels of soybean injury for applications that occurred later in the growing season. Greater soybean injury was observed for dicamba than 2,4-D treatments.

Enlist E3TM soybean cultivars permit over-the-top application of labeled glyphosate, glufosinate and 2,4-D choline products. Increased Enlist E3TM trait adoption and use of 2,4-D choline POST across US soybean production systems raise concerns regarding potential for 2,4-D off-target movement (OTM). A large-scale drift experiment was established near Sun Prairie, WI and Arlington, WI in 2019 and 2020, respectively. A 2,4-D-resistant soybean cultivar was planted in the center of the field (˜3 ha), while the surrounding area was planted with a 2,4-D-susceptible cultivar. An application of 785 ae ha−1 2,4-D choline plus 834 g ae ha−1 glyphosate was completed within the center block at R2 and V6 growth stage on Aug 1, 2019 and Jul 3, 2020, respectively. Filter papers were placed in-swath and outside of the treated area in one upwind transect and three downwind transects to estimate particle deposition. Low volume air samplers ran for the 0.5 to 48 h period following application to estimate 2,4-D air concentration. Injury to 2,4-D-susceptible soybean was assessed 21 d after treatment (0 to 100% injury). The 2,4-D deposition in-swath was 9,966 and 5727 ƞg cm−2 in 2019 and 2020, respectively. Three-parameter log-logistic models estimated the distance to 90% reduction in 2,4-D deposition (D90) was 0.63 and 0.90 m in 2019 and 2020, respectively. In 2020, the 2,4-D air concentration detected was lower for the upwind (0.395 ƞg cm−3) than the downwind direction (1.34 ƞg cm−3), although both were lower than the amount detected in-swath (4.01 ƞg cm−3). No soybean injury was observed in the downwind or upwind directions. Our results suggest that 2,4-D choline applications following label recommendations pose little risk to 2,4-D-susceptible soybean cultivars; however, further work is needed to understand 2,4-D choline OTM under different environmental conditions and the presence of other susceptible crops.

The use of pre-emergence (PRE) herbicides is resurging as a standard practice for weed management in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] in the U.S. north-central region. The increased use of PRE herbicides warrants investigation on the impact of spray carrier volume on weed control. Field experiments were established at Arlington, WI, in 2018 and 2019 and Janesville, WI, in 2018 to evaluate the influence of carrier volume on weed control of PRE herbicides in conventional tilled corn and soybean. Each experiment included three PRE herbicide premixes applied at crop planting at five carrier volumes (2.5, 5, 10, 15, and 17.5 gal acre −1 ). A nontreated control was also included in the experiment. Carrier volume did not influence weed control but PRE herbicide selection did. In corn, acetochlor + clopyralid + mesotrione and bicyclopyrone + mesotrione + S-metolachlor provided 10-13% better annual grass, common ragweed (Ambrosia artemisiifolia L.), and giant ragweed (A. trifida L.) control than fluthiacet-methyl + pyroxasulfone. In soybean, dimethenamid-P + saflufenacil resulted in similar common ragweed control as the flumioxazin + pyroxasulfone treatment (79-85%), and 6 and 14% higher annual grass control and biomass reduction, respectively, compared to metribuzin + chlorimuron-ethyl at Arlington. At Janesville, due to the high giant ragweed density in soybean, all herbicide treatments provided low levels of giant ragweed control (48-56%). Results from these field experiments suggest that lower carrier volumes could be used for PRE herbicide applications without compromising weed control in conventional tillage systems.

Roundup Ready 2 Xtend® [glyphosate- and dicamba-resistant (DR)] soybean is a novel trait option for postemergence (POST) control of herbicide-resistant broadleaf weeds in soybean. With increased use of labeled dicamba products POST in DR soybean and recommendations to include a soil-residual herbicide POST (e.g., layered residual approach), research on how combinations of these approaches influence weed control, weed seed production, and soybean grain yield is warranted. The objective of this research was to evaluate the effects of (1) flumioxazin applied preemergence (PRE) followed by (fb) dicamba plus glyphosate applied POST at different crop developmental stages and (2) acetochlor POST as a layered residual approach on weed control, weed seed production, and soybean yield to determine the optimal POST timing in DR soybean. A field study was conducted in Wisconsin at three sites in 2018 and four sites in 2019 to evaluate flumioxazin (43.4 g ai ha−1, WDG 51%) PRE fb dicamba (560 g ae ha−1, SL) plus glyphosate (1,101 g ae ha−1, SL) POST in DR soybean at three stages: early-POST (EPOST, V1-V2), mid-POST (MPOST, V3-V4), and late-POST (LPOST, V5-V6/R1) with or without a soil-residual herbicide POST (acetochlor, 1,262 g ai ha−1, ME). Weed community composition was site-specific; difficult-to-control broadleaf species included giant ragweed (Ambrosia trifida L.) and waterhemp [Amaranthus tuberculatus (Moq.) J.D. Sauer]. Dicamba plus glyphosate applied MPOST and LPOST provided greater control, weed biomass reduction, and density reduction of giant ragweed and waterhemp when compared with EPOST treatments. Giant ragweed and waterhemp had not reached 100% cumulative emergence at EPOST, and plants that emerged after EPOST produced seed. There was some benefit to including acetochlor as a layered residual at EPOST as indicated by a residual by POST timing interaction for waterhemp density reduction. Complete waterhemp control was not attained at one site-year. For remaining site-years, dicamba plus glyphosate applied MPOST (V3-V4) provided season-long weed control, reduced weed seed production, and optimized soybean grain yield compared with other POST treatments. Results highlight the importance of timely POST applications and suggest utilization of a POST layered residual needs to be timed appropriately for the window of active weed species emergence.