Influence of application timing, surface temperature inversions, and new formulations on dicamba air concentrations following treatment

Authors: FARRELL, S - University Of Missouri; DINTELMANN, B - University Of Missouri; OSELAND, E - University Of Missouri; BISH, M - University Of Missouri; Lerch, Robert; BRADLEY, K - University Of Missouri

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 11/29/2017
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Few studies have been conducted to understand the extent to which newly-labeled dicamba formulations are present in the air following application. The objectives of this research are to determine the effects of time of application, surface temperature inversions and new formulations on the concentration of dicamba detected in the air following application. A series of field experiments were conducted near Columbia, Missouri during the summer of 2017. Air samplers were placed equidistantly within 6 x 31 m plots and 31 cm above the canopy prior to dicamba applications to obtain background levels of dicamba. Two or three air samplers were utilized per treatment per experiment depending on availability of samplers. The samplers were removed immediately prior to dicamba application, and then returned to the treated field 30 minutes following application. Applications were made at the 1X rate for each product, and plots were a minimum of 480 meters apart. Glass fiber filters and polyurethane foam substrates (PUF plugs) from the air sampling machines were replaced at set intervals throughout the experiments, which extended up to 72 or 96 hours following application. A methanol wash was used to extract dicamba from the filter paper and PUF plugs, and HPLC-UV was utilized to detect dicamba. The recorded concentrations for each of the air samplers were averaged together in all experiments. Preliminary results for two experiments in which Xtendimax plus VaporGrip and Engenia were applied at the same time on the same evening showed the majority of dicamba, regardless of formulation, was detected in the first 0.5 to 8 hours after treatment (HAT); the average concentration of dicamba for the Xtendimax treatment was 26.5 and 31.1 ng/m3 while that for Engenia was 18.4 and 20.5 ng/m3. By 24 to 48 HAT, dicamba levels had declined to less than 3 ng/m3 for each treatment in both experiments. Given spatial limitations and the need to further investigate dicamba concentrations in the air following on-label and off-label applications, only one dicamba formulation, Xtendimax plus VaporGrip, was utilized for the additional inversion experiments. Across two experiments in which Xtendimax plus VaporGrip was applied during inversion conditions, dicamba concentrations were 31.7 ng/m3 0.5 to 8 HAT, which was higher than any other sampling timepoint. The 16 to 24 HAT samples, which corresponded to the afternoon of the day following application, resulted in dicamba concentrations of 10.8 ng/m3. Samples from all other timepoints averaged less than 5 ng/m3. Xtendimax plus VaporGrip applications were also made on-label and during the day prior to the evening applications. Across two experiments, dicamba concentrations for 0.5 to 8 HAT were 2.64 ng/m3. Concentrations for the 8 to 16 HAT samples, which would correspond to the overnight hours, were 18.24 ng/m3, and these concentrations were statistically higher than those from any other sampling timepoint (< 2.63 ng/m3). To quantify the stability of the atmosphere during air sampling times, lapse rates were calculated. The more negative the lapse rate, the more likely the atmosphere is stable or favors inversion-like conditions. Spearman’s correlation was used to study relationships between dicamba concentrations, lapse rates, and maximum wind speeds for 0.5 to 8 HAT, 8 to 16 HAT, and 16 to 24 HAT samples in which Xtendimax was applied alone without glyphosate (n=42). A correlation coefficient of -0.54562 (P < 0.0005) was observed between lapse rate and dicamba concentration, suggesting a trend between air stability and dicamba concentration. Another indicator of a stable atmosphere is reduced wind. The correlation coefficient between maximum wind speed and dicamba concentration was -0.69683 (P < 0.0001). These preliminary results indicate that dicamba can be detected in the air following evening applications and to