Spray solution pH and soybean injury as influenced by synthetic auxin formulation and spray additives

Abstract

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.

Large-scale evaluation of 2,4-D choline off-target movement and injury in 2,4-D-susceptible soybean

“…Downwind particle drift was identified as the primary source of OTM of 2,4-D choline. Moreover, Striegel et al (2021) reported no soybean injury from 2,4-D in low-tunnel field volatility experiments. Therefore, wind speed and direction are important considerations for 2,4-D choline applications.…”

“…Herbicides can volatilize during application or after reaching the target area, and herbicide volatility is highly influenced by herbicide formulation, spray nozzle type, and meteorological conditions (Havens et al 2018;Strachan et al 2010). Although 2,4-D choline is a low volatility formulation (Simpson 2019;Sosnoskie et al 2015;Striegel et al 2021), vapor drift of synthetic auxins is known to increase during high-temperature conditions (Behrens and Lueschen 1979;Bish et al 2019aBish et al , 2019bJones et al 2019). Vapor drift typically occurs within the first 48 h following herbicide application (Jones et al 2019;Soltani et al 2020); therefore, 2,4-D concentration detected by air samplers during the 48-h period following application serve as a good estimate of vapor drift.…”

“…Herbicides can also move to a neighboring area in the form of vapor within a few hours and up to ≥48 h after application (Matthews et al 2014;Soltani et al 2020). Herbicide physiochemical properties (i.e., vapor pressure) are key in predicting vapor drift potential; however, numerous environmental conditions such as temperature, humidity, wind, and the treated surface conditions (e.g., bare soil, dry vegetation, actively growing vegetation) can also influence herbicide volatility (Mueller and Steckel 2021;Strachan et al 2010;Striegel et al 2021;Sosnoskie et al 2015). The 2,4-D choline formulation is reported to be 88% and 96% less volatile than the amine and ester formulations, respectively, and application with an approved nozzle could reduce spray drift up to 90% (Havens et al 2018;Hillger et al 2012).…”

Hooded broadcast sprayer for particle drift reduction

“…The AUC is commonly used for plant disease progress 36,37 but also has been used to calculate herbicide injury. 38,39 Data were analyzed using a mixed model using the brm function. The model structure, family, priors, posterior estimates and treatment comparisons were performed as described previously for the in-swath spray deposition.…”

Evaluating efficacy of preemergence soybean herbicides using field treated soil in greenhouse bioassays

“…If ANOVA indicated herbicide × bioindicator species interaction or main effects to be significant (P ˂ 0.05), means were separated accordingly using Fisher's protected LSD test. AUDPC is a valuable tool commonly used in the field of plant pathology to estimate disease progress over time (Madden et al 2007) and has been previously adopted by weed scientists to estimate crop injury from POST herbicides across distance or over time (Striegel et al 2020;VanGessel et al 2016). The AUBPC allowed estimation of a single response variable, and to thus rank the overall PRE herbicide impact on biomass of each bioindicator species over the period evaluated (0 to 50 DAT; 0 to 900 GDD), further supporting the linear regression results.…”