Submitted to: International Weed Control Congress Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 3/22/2004
Publication Date: 5/31/2004
Citation: Zablotowicz, R.M., Reddy, K.N. 2004. Effects of glyphosate on symbiotic nitrogen fixation, assimilation and soybean yield in glyphosate resistant soybean. [Abstract]. International Weed Control Congress Abstracts.
Technical Abstract: Glyphosate-resistant (GR) soybean has revolutionized weed control in soybean production. The effects of glyphosate on nitrogen fixation, nitrogen assimilation, and yield of GR soybean under weed-free conditions were determined in a two-year field study. Five glyphosate (0.84, 1.68, 2.52 + 2.52 and 0.84 + 0.84 kg ae/ha) treatments applied at 4 and 6 weeks after planting (WAP) were compared to no glyphosate (weed-free) control. Soybean plants were harvested between 4 and 8 WAP, and roots assessed for nitrogenase activity (acetylene reduction assay, ARA), respiration and nodulation, and foliar nitrogen content. Soybean seed yield and seed nitrogen content were determined. No consistent effect of glyphosate was observed on either ARA or root respiration. In 2002, both ARA and respiration were about a third of that in 2003, attributed to drought in 2002. All glyphosate treatments reduced foliar nitrogen content (26 to 42%) in 2002, and three glyphosate treatments reduced foliar nitrogen content (9 to 14%) in 2003, with the greatest reduction when glyphosate was applied at the highest rate. Soybean yield compared to nontreated control was reduced by 11% by two applications of 2.52 kg ae/ha glyphosate in 2002, but seed yield was not affected in 2003. Total seed nitrogen harvested was reduced by 32 and 17% compared to nontreated soybean respectively, when 2.54 kg ae/ha glyphosate was applied in 2002 and 2003. These studies indicate that nitrogen fixation and/or assimilation in GR soybean was consistently reduced at high rates of glyphosate and the greatest reductions occurred with soil moisture stress following glyphosate application.