Wednesday, November 15, 2006 - 8:30 AM

Effects of Glyphosate Drift on Nitrate Assimilation and Nitrogen Fixation in Glyphosate-Sensitive Soybean.

Nacer Bellaloui, Krishna Reddy, Robert Zablotowicz, and Alemu Mengistu. USDA-ARS, USDA-ARS, 141 Experiment Station Road, Stoneville, MS 38776

Nontarget injury from glyphosate drift is a concern among growers using non-glyphosate-resistant (non-GR) cultivars. The effects of glyphosate drift on nitrate assimilation and nitrogen fixation potential, nitrogen content, yield, and seed composition of non-GR soybean were assessed. A non-GR soybean cultivar ‘Delta Pine 4748S’ was treated with glyphosate at 0.105 kg ha-1 at 3 (V2), 6 (V7), and 8 (R2) weeks after planting (WAP) soybean to simulate glyphosate drift. Untreated soybean was used as a control. Nitrate assimilation was assessed using in vivo nitrate reductase assay in leaves, stems, roots, and nodules. Nitrogen fixation potential was assessed using the acetylene reduction assay for nitrogenase activity. At 4 WAP, glyphosate caused a significant decrease in NRA in leaves (60%), stems (77%), and nodules (50%), with no decrease in roots. At later growth stages, NRA in leaves was more sensitive to glyphosate drift than stems and roots. Specific nitrogenase activity was reduced 36-58% by glyphosate treatment at 3 or 6 WAP, but unaffected at 8 WAP. Nitrogen content in shoots was affected by glyphosate application only after the first application. No yield, seed nitrogen, protein, or oil concentration differences were detected. These results suggest that nitrate assimilation and nitrogen fixation potential were significantly reduced by glyphosate drift, with the greatest sensitivity early in vegetative growth. Soybean has the ability to recover from the physiological stress caused by glyphosate drift.