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item Koger Iii, Clifford
item Krutz, Larry
item THOMAS, W
item WILCUT, J

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2005
Publication Date: 9/28/2005
Citation: Koger III, C.H., Shaner, D.L., Krutz, L.J., Walker, W.W., Buehring, N., Henry, W.B., Thomas, W.E., and Wilcut, J.W. 2005. Rice (Oryza sativa) response to drift rates of glyphosate. Pest Management Science, Vol. 61, Issue 12, pp. 1161-1167.

Interpretive Summary: With the advent of transgenic glyphosate-resistant crops including Roundup Ready canola, corn, cotton, and soybean, glyphosate has become the most commonly applied herbicide worldwide. However, researchers have noted that drift of glyphosate onto rice causes significant yield reductions, and producers in major rice producing states are requesting a rapid method for identifying rice exposure to glyphosate and a means for estimating potential yield losses. Thus, field studies were conducted in commercial rice fields of the Mississippi River delta, MS to examine the response of rice to sub-lethal rates of glyphosate. Visual injury, shikimate levels, and rice yield reductions (8 to 92%) increased with increasing glyphosate rate. Visual injury coupled with measured shikimate levels were successfully employed to identify glyphosate exposure and estimate rice yield reductions.

Technical Abstract: Field studies were conducted in 2004 to investigate the response of rice to sub-lethal rates of glyphosate. Glyphosate was applied at 0, 0.013, 0.026, 0.053, 0.105, 0.21, and 0.42 kg AE ha-1 to 74-cm-tall rice in the internode separation growth stage. Visual injury, plant height, and leaf-tissue samples for shikimate analysis were collected at 3, 7, 14, 21, and 28 days after treatment (DAT). Rice yield was also determined. No visual injury was observed at 3 DAT. Rice injury from 7 to 28 DAT ranged from 0 to 50% and increased in a quadratic fashion (R2 = 0.97 to 0.99) with glyphosate rate. Little reduction in height (0 to 15%) occurred with respect to glyphosate rate. Shikimate levels were positively correlated with glyphosate rate (R2 = 0.76 to 0.98), and levels remained high in glyphosate treated plants across all sampling dates. Reduction in rice yield due to glyphosate drift ranged from 8% for the 0.013 kg AE ha-1 rate to 92% with the 0.42 kg ha-1 rate. Visual injury ratings and shikimate levels in treated rice were strongly related (R2 = 0.94 to 0.99) across time after glyphosate application. Shikimate levels in glyphosate-treated rice were strongly related to yield reductions (R2 =0.96 to 0.98). Moreover, shikimate levels were at least 2-fold higher in treated rice that exhibited greater than 10% visual injury. Thus, visual injury coupled with measured shikimate levels can be used collaboratively to identify glyphosate exposure and estimate subsequent rice yield reductions.