RUNOFF LOSSES OF NORFLURAZON - EFFECT OF RUNOFF TIMING

Norflurazon was applied (2.24 kg/ha) in 1988 and 1989 to plots that co ntained subsurface drains 1 m deep. After soybeans were planted, runof f 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 da ys after application. In this period 50.7 g/ha norflurazon (2.3 % of t he amount applied) was lost from the plots in runoff. The highest conc entration in runoff in the first study was 6.6 mug/L on day 171. The h ighest concentration, 16.5 mug/L, for the second season was also measu red during the season's first runoff event, on day 22. The studies pro vided good examples of the direct relationship between time after appl ication of runoff-producing rainfall and the magnitude of pesticide lo ss in runoff.