Glasshouse experiments were conducted to determine the effects of soil moisture content, irradiance, temperature and relative humidity on the efficacy of glyphosate applied to six isogenic lines of Avena fatua L. (wild oat) and four near‐isogenic lines of Urochloa panicoides Beauv. (liverseed grass). The variables examined were four soil moisture conditions (29%, 42%, 55% and 100% of field capacity), two levels of irradiance (400 and 800 μmol m−2 s−1), two levels of relative humidity (>92% and 65%) and four temperature regimes (20/15, 25/20, 30/25 and 35/30 °C: day/night), representing the environmental conditions of winter or summer fallows in the north‐east grain region of Australia. The efficacy of 360 g acid equivalent ha−1 glyphosate was greatest under well‐watered (100% of field capacity), warm (30/25 °C for A. fatua and 35/30 °C for U. panicoides) and humid (>92/90%) conditions. The efficacy was least under severe water stress (29% of field capacity), warm (30/25 °C for A. fatua and 35/30 °C for U. panicoides) and moderately humid (65/60%) conditions. Efficacy was not altered by the level of irradiance nor was it different between isogenic lines.

Glasshouse experiments were conducted to determine the effects of various environmental factors on glyphosate efficacy when applied to Echinochloa colona (L.) Link (awnless barnyard grass) plants. The variables examined were 4 soil moisture conditions (29, 42, 55, and 100% of field capacity), 2 levels of irradiance (400 and 800 µmol/m2· s), 3 temperature regimes (20/15, 30/25, and 35/30°C; day/night), and 2 levels of relative humidity (92 and 65%). The efficacy of 360 g acid equivalent glyphosate/ha was greatest when applied to well-watered (field capacity) plants that were placed under cool (20/25°C) and humid (92% relative humidity) conditions. The efficacy was least when applied to plants under severe water stress (29% of field capacity) that were placed under hot (35/30°C) and less humid (65% relative humidity) conditions. In all experiments, efficacy was not altered by the level of irradiance.

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

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