In agricultural practice, most of the diquat spray is initially deposited on plant surfaces and part of it on the soil surface. According to Black et al. (1966), photochemical degradation is responsible for the rapid decrease in the concentration of diquat following the spraying of herbage. Application of 0.284 kg/ha resulted in 12 - 48 mg diquat/kg dry herbage on the first day, 2.5 - 10.9 mg/kg after 3 - 4 days, and 1.0 - 5.7 mg/kg, 7 days after treatment