Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/2006
Publication Date: 4/1/2006
Citation: Bellaloui, N., Reddy, K.N., Zablotowicz, R.M., Mengistu, A. 2006. Simulated glyphosate drift influences nitrate assimilation and nitrogen fixation in non-glyphosate resistant soybean. Journal of Agriculture and Food Chemistry. Vol 54: 3357-3364 Interpretive Summary: Most of the soybean grown varieties in the USA in 2005 were resistant to damage caused by the herbicide or weed killer called glyphosate. Glyphosate can kill most green plants, but some crops such as soybean have a gene transferred from bacteria that allow these plants to resist damage caused by the herbicide. A small portion of the soybean crop, called conventional soybean, is still susceptible to damage by glyphosate because these plants do not contain the gene. Thus, conventional soybean can be damaged when glyphosate applied to another field drifts onto the conventional soybean. Nitrogen metabolism in conventional soybean was reduced by glyphosate drift, and conventional soybean was most sensitive to the herbicide drift when soybean was in early vegetative growth. However, when the plants matured, there were no yield, seed nitrogen, protein or oil content differences between conventional soybean that were damaged by herbicide drift and those plants that had no damage. This research demonstrates that conventional soybean has the potential to recover fully from stress caused by drift of the herbicide glyphosate.
Technical Abstract: Non-target injury to conventional crops 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, nodule mass, and yield of non-GR soybean were assessed in a field trial at Stoneville, MS. A non-GR soybean cultivar ‘Delta Pine 4748S’ was treated with glyphosate at 12.5% of use rate of 0.84 kg ae/ha at 3 (V2), 6 (V7), and 8 (R2, full bloom) weeks after planting (WAP) soybean to simulate glyphosate drift. Untreated soybean was used as a control. Soybeans were sampled between 4 and 10 WAP. Nitrate assimilation and N2 fixation potential were assessed using in vivo nitrate reductase activity (NRA) in leaves, stems, roots, and in nodules, and the acetylene reduction assay (ARA), respectively. Nitrogen content of leaves, shoots, and seed and soybean yield were also determined. In the first sampling date (4 WAP), glyphosate drift 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 ARA activity was reduced 36 to 58% by glyphosate treatment at 3 or 6 WAP. However, no effect on ARA activity was observed during the last two sampling dates. Nitrogen content was only affected by glyphosate application in shoots after the first application. No yield, seed nitrogen, protein or oil content differences were detected. These results suggest that nitrate assimilation and nitrogen fixation potential were significantly reduced by glyphosate drift and the greatest sensitivity was during early in vegetative growth. Soybean has the ability to recover from the physiological stress caused by glyphosate drift.