Effect of relative humidity on the uptake, translocation, and efficacy of glufosinate ammonium in wild oat ( Avena fatua)

Abstract

Relative humidity (RH) can greatly affect the uptake and efficacy of glufosinate ammonium ( d, l-homoalanine-4yl-(methyl)-phosphinate) in wild oat ( Avena fatua L.). Exposure to high (>95%) RH as opposed to low (40%) RH increased glufosinate ammonium efficacy and it was high RH within 12 h of spraying that was most crucial in affecting the efficacy. Subsequent dose–response experiments at 40% RH indicated that exposure of wild oat to high RH for as little as 30 min before and after spraying significantly increased the efficacy when compared to plants grown continuously at 40% RH. Furthermore, when [ 14C]glufosinate ammonium was applied as a spray to wild oat plants grown at 40% RH, there was a significant increase in uptake into wild oat plants exposed to high RH for 30 min before and after treatment compared to those left continuously at low RH. Conversely, when the same experiment was conducted with [ 14C]glufosinate ammonium applied as ten 1-μl droplets, exposure to high RH for 30 min before and after treatment did not increase the herbicide uptake. Based on these results, we hypothesize that droplet size and hence drying time is the major factor in effecting changes in glufosinate ammonium uptake in response to exposure to high versus low RH. Applying [ 14C]glufosinate ammonium as large droplets resulted in an overestimation of uptake and masked the inhibitory effect of low RH on glufosinate ammonium as determined by dose–response experiments. Conversely, spray application of [ 14C]glufosinate ammonium produced data that correlated with those from dose–response experiments.