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In 2016 many states saw a rapid increase in acres planted to dicamba-tolerant soybeans. Many of these states experienced numerous cases of dicamba off-target movement due to vapor-drift, physical drift, tank-contamination and/or use of an improper dicamba formulation. In this article we will focus on how to identify dicamba symptomology on non-dicamba-tolerant soybeans and the estimated yield impact of an off-target application of dicamba to non-dicamba-tolerant soybeans.

Dicamba symptomology is often not expressed until 7 to 14 days after application on non-dicamba tolerant soybeans.  After 7 to 14 day after application, non-dicamba tolerant soybean may exhibit, crinkling of leaf tips, leaf cupping up or down, leaf puckering with blunt tips, upper leaves can be strapped and the stem can twist, swell, and split.

See the image below or this follow this link to view symptoms: 

Dicamba soybean injury at 40 times less than the 1X rate compared with an untreated check

Dicamba yield impact on non-dicamba tolerant soybeans will vary by the rate, the soybean growth stage and the environmental conditions at the time of exposure.  In general, the soybean flowering stages are more sensitive to dicamba vs. the vegetative stages.  A 100 trial meta-analysis summary indicated that the soybean flowering stages exposed to dicamba at rates of:

• 1000 times less than the 1x use rate-(“Vapor Drift” rate), showed a 1% soybean potential yield loss;
• 100 times less than the 1x use rate-(“Physical Drift” rate), experienced an 8.7% soybean potential yield loss;
• 10 times less than the 1X use rate-(“Tank Contamination” rate), showed a 48.5% soybean potential yield loss- (see table below). 
•  The “Tank Contamination” rate, 10 times less than the 1X use rate, showed between a 48.5% to a 58.6% soybean yield reduction for the soybean vegetative, flowering and the pod stages- (see table below).

 Yield loss estimated from dicamba drift and contamination

Basic tips to avoid potential injury:

• Properly clean sprayers between loads
• Check planting records to ensure correctly traited varieties were planted in that field
• Scout fields before and after application

For a review of plant growth regulator injury to soybeans visit http://weeds.cropsci.illinois.edu/extension/factsheets/PGR.pdf.

Todd Thumma is a product development agronomist for Syngenta.