Soybean Response to Dicamba: A Meta-Analysis

Abstract

A meta-analysis of 11 previously published field studies was conducted with the objectives being to (1) estimate the no observable effects dose (NOED) for dicamba on susceptible soybean; (2) evaluate available evidence for hormesis, or increased soybean yield in response to low doses of dicamba; (3) estimate the dose of dicamba likely to cause measurable soybean yield loss under field conditions; and (4) quantify the relationship between visible injury symptoms and soybean yield loss. All studies that included visible injury data (N=7) reported injury symptoms at the lowest nonzero dicamba dose applied (as low as 0.03 g ae ha−1), and therefore a NOED could not be estimated from the existing peer-reviewed literature. Based on statistical tests for hormesis, there is insufficient evidence to support any claim of increased soybean yield at low dicamba doses. Future research should include a range of dicamba doses lower than 0.03 g ha−1to estimate a NOED and determine whether a hormesis effect is possible at or below dicamba doses that cause visible injury symptoms. Soybean is more susceptible to dicamba when exposed at flowering (R1 to R2 stage) compared with vegetative stages (V1 to V7). A dicamba dose of 0.9 g ha−1(95% CI=0.08 to 1.7) at the flowering stage was estimated to cause 5% soybean yield loss. When exposed at vegetative stages, dicamba doses that cause less than 30% visible injury symptoms (95% CI=23 to 49%) appear unlikely to cause greater than 5% soybean yield loss; however, if soybean is exposed at flowering, visible injury symptoms greater than 12% (95% CI=8 to 16%) are likely to be associated with at least 5% soybean yield loss.

Peanut (Arachis hypogea) Response to Low Rates of Dicamba at Reproductive Growth Stages

“…Roundup Ready ® Xtend (Bayer CropScience LP, 800 Lindbergh Blvd.) is a dicamba-resistant (DT) cropping system widely adopted across the U.S. that allows producers to apply registered formulations of dicamba herbicides at preplant, planting, and POST timings [10][11][12]. The adoption of DT soybean and in-crop use of dicamba formulations has created great concern due to non-DT soybean expressing synthetic auxin symptoms in recent years [11,13].…”

“…In 2017 and 2018, >1.4 million and~1 million ha of soybean were damaged from off-target movement of dicamba in the U.S., respectively [11,13]. Applications of dicamba made during the growing season are prone to injure adjacent crops sensitive to dicamba from off-target movement [14][15][16].…”

Weed Control in Dicamba-Resistant Soybean with Glyphosate/Dicamba Applied at Various Doses and Timings

“…Tukey's adjustment was applied to pairwise comparisons before determining treatment variations at P < 0.05. e inverse link function or a back-transformation, where necessary, was applied to LSMEANS for presentation on the data scale. e DOSE by TIMING interaction was negligible for all parameters; therefore, only differences for the DOSE and TIMING main effects were determined [13]. e nontreated control, assigned a value of 0 for injury to the crop and weed control assessments, was excluded from the analysis because of no variance; differences between treatment means and the value zero were identified using the LSMEANS output.…”

Emerging Challenges for Weed Management in Herbicide-Resistant Crops

“…Plant response to these PGR herbicides is easily recognized as leaf cupping and crinkling and/or epinasty, and very low rates of herbicide may cause injury in susceptible plants [53]. Injury to soybean has been documented in the literature at the lowest published non-zero rate of 0.03 g ha −1 [54]. While low rates of PGR herbicides may not cause yield loss in soybeans, chemical trespass is illegal and may have other unintended consequences.…”