Development and Testing of a Laboratory Spray Table Methodology to Bioassay Simulated Levels of Aerial Spray Drift

The objective of this work was to develop a repeatable methodology for bioassaying simulated levels of aerially applied glyphosate at deposition levels ranging from 1∕3 to 1∕100 of labeled rate at droplet sizes of 100 μm in a spray table environment. These drift deposition levels are consistent with downwind drift measurements out to 200 m seen in previous field studies focusing on quantitative drift assessment. Additionally, full rate applications were included for comparative purposes. The deposition levels were obtained by varying nozzle traverse speed and plant location under the nozzle. Ten replications were conducted at each targeted rate applying glyphosate to container grown-plant samples. Deposition was measured on Mylar cards through fluorometric analysis. Plant health measures [height and normalized difference vegetation index (NDVI)] were taken at 0, 1, 3, 5, 7, and 14 days after treatment. An equal number of nontreated control plants were analyzed alongside treated plants. Deposition and plant health data were used to generate dose-response relationships. Dose-response curves relating change in plant height and change in measured NDVI values corresponding to deposition levels were generated. This methodology is one that can be implemented across a wide variety of plant and pesticide combinations. Collected data from this and future studies will be tested under field conditions and ultimately be included in application decision support systems that integrate spray drift modeling results with established dose-response relationships.