The influence of soil moisture content on glyphosate efficacy for the control of annual grasses in fallow land

In order to determine the effects of soil moisture content on the efficacy of glyphosate, field experiments were conducted on three annual grass weeds [Avena fatua L., Urochloa panicoides Beauv. and Echinochloa colona L. (Link)]. The soil moisture conditions were as follows: absence of rainfall using a rain-out shelter, natural rainfall and natural rainfall plus irrigation (2.5 cm week^-1). These conditions were selected as they are representative of those in winter and summer fallows in the north-east grain region of Australia and had been previously identified by glasshouse experiments to be conditions that influence glyphosate efficacy. As predicted, efficacy of 360 g acid equivalent ha^-1 glyphosate when applied to all three species was greatest under the irrigated and normal rainfall conditions and significantly less under the exclusion of rainfall condition. This response was the same for all near-isogenic lines of A. fatua and U. panicoides studied. As mild and severe forms of water stress are common events in the region, it is highly likely that soil moisture level will interfere with efficacy of glyphosate application in summer and winter fallows.