INFLUENCE OF ENVIRONMENTAL-FACTORS ON GLYPHOSATE EFFICACY WHEN APPLIED TO AVENA-FATUA OR UROCHLOA PANICOIDES

Glasshouse experiments were conducted to determine the effects of soil moisture content, irradiance, temperature and relative humidity on th e efficacy of glyphosate applied to six isogenic lines of Avena fatua L. (wild oat) and four near-isogenic lines of Urochloa panicoides Beau v. (liverseed grass). The variables examined were four soil moisture c onditions (29%, 42%, 55% and 100% of field capacity), two levels of ir radiance (400 and 800 mu mol m(-2) s(-1)), two levels of relative humi dity (> 92% and 65%) and four temperature regimes (20/15, 25/20, 30/25 and 35/30 degrees C: day/night), representing the environmental condi tions of winter or summer fallows in the north-east grain region of Au stralia. The efficacy of 360 g acid equivalent ha(-1) glyphosate was g reatest under well-watered (100% of field capacity), warm (30/25 degre es C for A. fatua and 35/30 degrees C for U. panicoides) and humid (> 92/90%) conditions, The efficacy was least under severe water stress ( 29% of field capacity), warm (30/25 degrees C for A. fatua and 35/30 d egrees C for U. panicoides) and moderately humid (65/60%) conditions. Efficacy was not altered by the level of irradiance nor was it differe nt between isogenic lines.