RUNOFF OF SULFOMETURON-METHYL AND CYANAZINE FROM SMALL PLOTS - EFFECTS OF FORMULATION AND GRASS COVER

To determine the effects of application rates, grass cover, and formulation type on herbicide losses in runoff, we applied 4.5 kg/ha cyanazine (2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl)]amino]-(2-methylpropa nenitrile) with 0.4 kg/ha sulfometuron-methyl (methyl-2[[[[(4,6-dimethyl-2-pyrimidinyl)amino] carbonyl] amino] sulfonyl] benzoate) to 1.2 by 2.4 m plots, using suspension concentrate (SC) and dispersible granule (DG) formulations of cyanazine, and SC and emulsifiable concentrate (EC) formulations of sulfometuron-methyl. The plots were established on a Tifton loamy sand soil (fine-loamy, siliceous, thermic Plinthic Paleudults) and had 3% slope. The plots were bare or covered with a mixed stand of common Bermudagrass[Cynodon dactylon (L.) Pers.] and Bahiagrass (Paspalum notatum Flugge var. suarae Parodi). On the day after the herbicides were applied, we simulated rainfall events of 69 mm/h intensity until 2 mm of runoff occurred. The runoff was analyzed for sediment and herbicides. The bare plots required one-third less rain to produce the same amount of runoff and yielded twice as much sediment as the grassy plots. However, losses of all formulations were 1 to 2% of the amounts applied regardless of grass cover and even though cyanazine rates were 11 times that of sulfometuronmethyl. Total losses of all formulations were sensitive to the length of time between rainfall initiation and runoff initiation, indicating that leaching made herbicide unavailable for runoff. These results suggest that, for these fomulations under conditions of similar runoff volumes, losses of pesticides are a fairly constant fraction of the amounts applied, with or without grass cover. For intense storms where the amount of rainfall is similar, chemical runoff from the grassed plots was predicted by computer simulation to be less than half of that from bare soil.