Effect of simulated rain on retention, distribution, uptake, movement and activity of difenzoquat applied to Avena fatua

In glasshouse studies the degree of control of A vena fatua increased as the period between application of difenzoquat and the onset of simulated rain was prolonged. 0.5 mm of ‘rain’ removed 29% of the herbicide deposit without adversely affecting performance at the recommended dose of 1 kg/ha. A further 30% was removed by 2.0 mm of ‘rain’, resulting in a marked reduction in acrivity. With lower amounts of ‘rain’ (0.16 mm), some of the spray deposit was redistributed from the leaf lamina to the leaf base/ligule area. The rate of penetration of ¹⁴C-difenzoquat was much greater when applied to the inner surface of the leaf sheath than when the leaf blade and outer sheath areas were treated. Translocation from the ‘inner sheath’ to other parts of the plant was up to 100 times greater than from other areas. It is suggested that the performance of difenzoquat is not reduced by low amounts of rain because: (1) the spray deposit is removed principally from the leaf blade, whilst in the more responsive ligule/leaf sheath area the herbicide remains in solution, (2) the recommended dose of 1 kg/ha allows for some loss of active ingredient without reduction in performance. The practical implications of the work are discussed and further topics for research are outlined.