Soybean dose-response to 2,4-D and dicamba at vegetative and reproductive growth stages.

BACKGROUND: Field experiments were conducted across multiple sites in 2012 and 2013 to describe sensitivity of soybean to 2,4-D (six doses) and dicamba (seven doses) at V3 and R1 growth stages. Further experiments were conducted under greenhouse conditions in 2017 and 2018 to compare soybean response to several dicamba herbicides across a broader range of doses than those tested in the field. RESULTS: Soybean yield loss was 6.1-fold greater from 2,4-D exposure at V3 compared to R1 and 1.4 times greater from dicamba exposure at R1 than at V3. In V3 exposures, soybean was 15.4 times more sensitive to dicamba than 2,4-D and 134.4-fold more sensitive to dicamba when exposed at R1. Plant injury and height correlations to grain yield resulted in coefficients ranging from 0.65 to 0.91. In greenhouse experiments, five dicamba products were tested at up to 19 doses and as low as 0.002 g ae ha-1 (3.6 × 10-6 % of maximum single use-rate); however, no differences were observed among formulations used in dicamba-resistant crops versus traditional formulations. A no observable effects dose was not identified due to responses observed even at the lowest doses tested, although hormesis effects were observed in plant height. CONCLUSION: These data suggest that the sensitivity of soybean to dicamba is much greater than what has previously been reported. However, as has been indicated by previous work, that injury does not always result in yield loss. © 2022 Society of Chemical Industry.

Droplet size and physicochemical property effects on herbicide efficacy of pre-emergence herbicides in soybean (Glycine max (L.) Merr).

BACKGROUND: Unlike post-emergence herbicides (POSTs), little is known about droplet size effect on pre-emergence herbicide (PRE) efficacy. Four nozzle types were used to apply different PRE herbicides on eight soybean fields in Missouri and Mississippi in 2017 and 2018, respectively. Pendimethalin, metribuzin, clomazone, imazethapyr and pyroxasulfone were selected based on their physicochemical characteristics (adsorption, volatility and solubility) and were sprayed using XR11002, ULD12002, TTI6011002, and TTI11002 nozzles. RESULTS: The XR nozzle produced the smallest droplet size (DV0.5 ), 204 µm, followed by the ULD, TTI60 and TTI with DV0.5 of 468, 646, and 794 µm, respectively. Droplet size, spray coverage, nozzle type or physicochemical characteristics showed no effect on PRE herbicide efficacy, except in the Monroe County, MS, field, with pendimethalin. The TTI60 twin fan nozzle enhanced pendimethalin weed control (up to 91%) in comparison with pendimethalin sprayed with the TTI nozzle (64%), in a high organic matter (OM) soil composed of large soil clods and high weed pressure. It was hypothesized that improved herbicide penetration assisted by the TTI60 dual fan pattern increased herbicide-moisture contact and clod coverage by the herbicide. Under soils with higher OM content (>2%) pendimethalin weed control was reduced. In soils with low OM (<0.7%), low cation exchange capacity (CEC) (<13.1%) and rainfall of 12.2 mm within 3 days after application, metribuzin also resulted in reduced weed control. CONCLUSION: The results indicate that droplet size does not affect PRE herbicide efficacy regardless of physicochemical herbicide properties including adsorption, volatility and solubility. © 2019 Society of Chemical Industry.