USE OF FIELD EVALUATIONS TO BETTER UNDERSTAND DICAMBA VOLATILITY. C. Brabham*1, J. K. Norsworthy1, M. Zaccaro1, V. K. Varanasi1, T. Mueller2; 1University of Arkansas, Fayetteville, AR, 2University of Tennessee, Knoxville, TN (336)


Nine field trials were conducted during the 2018 growing season to quantify off-target movement of dicamba using air samplers and greenhouse-grown dicamba sensitive soybean. The aim of our research was to better understand how environmental factors promote or lessen dicamba volatility and to correlate the amount of dicamba volatility captured (ng/m3/day) to soybean injury using the Behrens and Lueschen scale. Briefly, dicamba (XtendiMax) at 560 g ae ha-1, glyphosate (Roundup PowerMax II) at 860 g ae ha-1, and the drift retardant Intact at 1% v/v were applied using a high-clearance sprayer to a 0.37 ha plot. At 30 to 60 min after application, three high flow (6.5 L/min) air samplers were placed centrally in the treated area. Every 24 hrs after application (24, 48, 72, and 96), the filter paper and puffs from air samplers were collected and replaced with new ones. Only data from filter paper have been measured and will be presented. Greenhouse-grown dicamba sensitive soybean were used as bio-indicators and these plants remained in the field for the following periods: 0.5 – 24, 0.5 – 48, 0.5 – 72, 0.5 – 96, 24 – 48, 48 – 72, and 72 – 96 hours after application. Soybean were rated for injury at 14 and 21 days after treatment with height and dry weight also taken at 21 days after treatment. As much as 27 ng/m3/ day of dicamba acid was detected and generally the concentration decreased over time. In five of nine experiments, dicamba volatility was detected at 96 hrs after treatment. In four of nine experiments, a rain event occurred (0.35 to 1.4 cm) after application, and in these experiments, dicamba volatility ceased or was less than 1 ng/m3/day. Soybean typically showed dicamba symptomology any time the dicamba acid was detected with air samplers, even at concentrations below 1 ng/m3/day.