An investigation of spray drift deposition of glyphosate from an herbicide spraying train and its potential impact on non-target vegetation and railway ditches.

Spray drift of glyphosate has the potential to affect non-target vegetation and surface waters close to the application area. To assess the likelihood of such impact along Swedish railways, four field experiments were conducted at three railway sites during 2019 and 2020. An herbicide spraying train applied herbicide Roundup Ultra (glyphosate) at speeds of 33 to 48 km/h. Quantitative filter papers were placed at 0.5, 1, 1.5, 2, 3 and 5 m distances to capture spray droplets. Wind speeds were low (0-2 m/s), but were found to be representative of normal operating conditions. Spray deposition decreased rapidly with distance, declining from 1800 g a.e./ha to an average of 5 g/ha within 1 m. Predicted 90th percentile drift rates suggested potential impact on vegetation within distances <1 m, where 90th percentile spray deposition would range from full dose to 18 g/ha. Beyond 1.5 m from the sprayed area, impact on vegetation was deemed unlikely. The potential concentrations in ditches near railways did not exceed the 100 μg glyphosate/L environmental quality standard even for ditches situated only 0.5 m from the sprayed area, indicating low risk to ditches or final recipients. Actual impact on vegetation was assessed using weed coverage data recorded by the herbicide spraying train itself. We extracted average weed coverages for 10 m sections around the edges of no spray zones and focused on the outermost sections surveyed, 0.35 to 1.4 m outside the application area. Predicted 90th percentile glyphosate deposition ranged from 565 to 6 g/ha, averaging 80 g/ha in this zone. By comparing no spray zones to adjacent track sections, and tracks treated with glyphosate in 2019 to those that were not, we demonstrate that there is a statistically significant but relatively minor effect of spray drift on non-target vegetation close to the track.