Effects of Simulated Glyphosate Drift to Native Prairie Plants and Canola-Compatible Brassicaceae Species of North Dakota, United States

This study evaluated effects to native plant and weedy Brassicaceae species growing in areas potentially affected by drift of glyphosate used with glyphosate-resistant canola (Brassica napus). Ten native grass and forb species were selected based on importance in prairie areas of North Dakota, US; and four introduced Brassicaceae species (Brassica juncea, Brassica nigra, Brassica rapa, and Sinapis arvensis) were selected based on their ability to cross with B. napus. Greenhouse-grown seedlings were treated with 0 (carrier control), 0.00056, 0.0032, 0.018 and 0.1 × a field application rate (FAR) of 829 g ha-1 acid glyphosate (g acid equivalent or a.e. ha-1), along with no spray plants; with each treatment repeated in two experiments. Shoot dry weight and height were measured 14 days after treatment, and data were subjected to analysis of variance or covariance followed by a Dunnett’s multiple comparison test to obtain No-Observed-Adverse-Effect-Rates (NOAERs) for both parameters. A Weibull regression was used to obtain the rate producing a 25% reduction (ER25) for shoot dry weight or height for a limited number of species and experiment combinations. Based on NOAER values for both shoot dry weight and height, most native species had reductions in growth with 0.1 x FAR resulting in NOAERs of 0.018 × FAR for at least one experiment. Nassella viridula was the most sensitive native species, with a NOAER of 0.0032 x FAR for shoot dry weight and one experiment for height. The Brassicaceae species responded similarly to glyphosate as the native species, with NOAER values ranging from 0.0032 to 0.018 × FAR. Only four species had valid regression analyses for shoot dry weight or height resulting in ER25 values between 0.007 and 0.054 x FAR. Pascopyrum smithii and Schizachyrium scoparium were not affected by glyphosate as indicated by NOAER values. This study indicated that drift concentrations between approximately 0.003, but more commonly ≥ 0.1 × FAR (2.49 and 82.9 g ha-1 acid glyphosate, respectively) may affect the growth and potential competitiveness of selected native plant species, and Brassicaceae species sexually compatible with glyphosate-resistant B. napus in North Dakota.