Submitted to: Proceedings of Southern Weed Science Society
Publication Type: Abstract Only
Publication Acceptance Date: 12/1/2003
Publication Date: 2/26/2004
Citation: Zablotowicz, R.M., Reddy, K.N. 2004. Effects of glyphosate on symbiotic nitrogen fixation and yield in glyphosate-resistant soybean. Proceedings of Southern Weed Science Society. pp.
Technical Abstract: Glyphosate-resistant (GR) soybean has revolutionized weed control in soybean production. This field study continues our laboratories risk assessment of GR soybean examining effects of glyphosate on nitrogen fixation and yield of GR soybean under weed-free conditions. A 2-yr field study using Roundup Ready soybean was conducted at Stoneville, MS evaluating five treatments (glyphosate at 0.84, 1.68, 2.52 + 2.52 and 0.84 + 0.84 kg ae/ha and hand weeded, weed-free control). Glyphosate was applied at 4 WAP and 6 WAP and soybean were harvested at five to seven samples between 4 and 8 WAP, and roots were assessed for nitrogenase activity (acetylene reduction assay, ARA), respiration and nodulation. Shoot and root biomass were determined at the final vegetative sample and foliar nitrogen content determined using a CNS analyzer. Soybean was harvested from all 4 rows and yield adjusted to 13% moisture and nitrogen content of soybean seed determined. Little consistent effect of glyphosate was observed on either ARA or root respiration. Root respiration and ARA observed in 2002 were about a third of that in 2003, attributed to water deficit following glyphosate application in 2002. All glyphosate treatments reduced foliar nitrogen content by 26 to 42% in 2002, while three glyphosate treatments reduced foliar nitrogen content by 9 to 14% in 2003. The greatest reduction in foliar N was when glyphosate was applied at the highest rate. Soybean yield compared to untreated control was reduced by 11% by two applications of 2.52 kg ae/ha glyphosate in 2002, but no effect on seed yield was observed in 2003. Total soybean seed nitrogen harvested was reduced by 32 and 17% compared to untreated soybean respectively, when 2.54 kg ae/ha glyphosate was applied in 2002 and 2003. These studies indicate that nitrogen fixation or assimilation in GR soybean was consistently reduced by high glyphosate applications compared to untreated control under weed-free conditions and the greatest reductions were observed when soil moisture deficit occurred following glyphosate application.