Off-Target Dicamba Exposure Differentially Affects Non-DT Soybean Genotypes
For the last several years, off-target movement of dicamba that was applied to dicamba-tolerant (DT) soybean has adversely affected production of nearby non-DT soybean. This is because non-DT soybean is naturally highly sensitive to dicamba, a synthetic auxin herbicide.
Numerous studies have investigated the effect of the following on the severity of exposure symptoms and accompanying yield loss in soybean: 1) growth stage at time of exposure to dicamba; 2) the dosage of dicamba that is experienced from off-target dicamba movement; and 3) the frequency and duration of exposure to dicamba. Since there has been little research that investigated the effect of soybean genetics on the response to such exposure (in essence the variability in varietal response), a study was conducted for 3 years (2019-2021) in southeast Missouri at the Lee Farm in Portageville (36°23' N lat.) to estimate yield losses caused by prolonged exposure to off-target dicamba movement, and to identify possible variable response among soybean genotypes to such exposure.
A total of 553 non-DT soybean genotypes and 18 commercial varieties (14 DT and four non-DT, all glyphosate tolerant) were used in the study. The non-DT genotypes had been derived from 239 unique parental backgrounds, and ranged in maturity from early MG 4 to late MG 5. The location of the research had a history of damage resulting from exposure to off-target dicamba movement since 2017 as evidenced by significant yield losses among non-DT breeding lines grown there.
Results from this research are reported in an article titled “Differential responses of soybean genotypes to off-target dicamba damage” that was published in the journal Crop Science in June 2022 (Vol. 62:472-1483). A summary of pertinent results from that research follows.
• The purpose of the study was to replicate a season-long exposure to off-target dicamba that is experienced by growers of non-DT soybean varieties in order to provide realistic estimates of yield losses caused by varying degrees of observed dicamba damage across non-DT genotypes of varied genetic backgrounds that received equal exposure to the off-target dicamba.
• Each year, plots were visually assessed in the early reproductive stages (between R1 and R3) for off-target dicamba damage using a scale of 1-4, where genotypes with a 1 rating showed similar growth and development as the DT control variety in the trial, and a 4 rating showed extreme dicamba damage symptoms that included severe leaf cupping, reduced vegetative growth, and reduced canopy coverage.
• Yields of the tested genotypes were standardized to the relative yield of the DT control variety in the study (% of the DT control). Overall, the mean relative yield ranged from 48.4% to 96.1%, which indicated that the study environments were exposed to off-target dicamba.
• Across all 3 years of the study, there was a significant effect of off-target dicamba damage on soybean yield.
• Non-DT lines that displayed minimal visual dicamba damage and similar plant development as the DT control varieties still showed yield losses between 7% and 10.6%, and this high-tolerance response was independent of MG.
• In this study, the interaction between MG and damage was more pronounced among genotypes that showed a higher sensitivity to dicamba drift.
• The results from this study indicate that the variation in symptomology and the degree of yield losses associated with off-target dicamba movement is related to genetic factors–i.e., genetic components of the genotypes, and that environmental factors had little effect on the variation in response among the genotypes.
• The authors concluded that natural tolerance to off-target dicamba movement may be caused by physiological mechanisms inherent in the genotypes rather than the length of time for recovery. In other words, tolerant gentoypes may have the ability to detoxify low doses of dicamba more rapidly than sensitive genotypes.
• Finally, these results indicate that the identification and selection of non-DT soybean genotypes with greater natural tolerance to off-target dicamba may sustain and improve the production of these genotypes.
These results are interesting, but may pose an insurmountable problem. That is, should all breeding programs that involve the development and release of non-DT varieties include a screening of promising varieties against low doses of dicamba? This will not easily be done since the definition of the “low dose” to use in such a screening is not and may never be defined. Also, such activity may not be practical or feasible. The Univ. of Arkansas conducts such a screening of available varieties for tolerance to metribuzin, but the rate of metribuzin to use is defined since it is a PRE herbicide and does not involve a variable dose that would be common in off-target drift from a POST herbicide such as dicamba.
Takehome Message. Producers can at least ask a seed supplier if there is a documented record of tolerance to dicamba of a chosen non-DT soybean variety. This information will not likely be available for many if any varieties, so it behooves a producer who grows a non-DT variety to document the estimated yield loss of such varieties that have been exposed to off-target dicamba movement. At least the results from the above-cited study show that there is variability in sensitivity to dicamba exposure among non-DT soybean genotypes, and this information can be used by producers to select less-sensitive non-DT varieties (if available) that may potentially be subjected to off-target dicamba movement. Also, a small test that involves the more prominent non-DT varieties that are grown in the Midsouth could be conducted at one location in the region each year with an agreed-on low rate application of dicamba to determine which, if any, of these varieties are more or less sensitive to dicamba. This would at least inform producers if the non-DT variety or varieties they plan to grow will be subject to minor or major damage from potential or expected dicamba drift.
Composed by Larry G. Heatherly, Aug. 2022, larryh91746@gmail.com