RUNOFF LOSSES OF NORFLURAZON - EFFECT OF RUNOFF TIMING

Norflurazon was applied (2.24 kg/ha) in 1988 and 1989 to plots that contained subsurface drains 1 m deep. After soybeans were planted, runoff losses of the herbicide were measured each season for about 300 days. The 1988-1989 season was atypical in that rainfall was insufficient to produce runoff until 171 days after application. From day 171 until day 302, 61 mm of runoff removed 1.5 g/ha (0.07 % of application) of herbicide from the plots. In 1989-1990, runoff was measured for 301 days after application. In this period 50.7 g/ha norflurazon (2.3 % of the amount applied) was lost from the plots in runoff. The highest concentration in runoff in the first study was 6.6 mug/L on day 171. The highest concentration, 16.5 mug/L, for the second season was also measured during the season's first runoff event, on day 22. The studies provided good examples of the direct relationship between time after application of runoff-producing rainfall and the magnitude of pesticide loss in runoff.