EFFICACY AND DISSIPATION OF DITHIOPYR AND PENDIMETHALIN IN PERENNIAL RYEGRASS (LOLIUM-PERENNE) TURF

Efficacy, movement, and degradation of dithiopyr and pendimethalin wer e monitored after application to perernnial ryegrass turf in 1990 and 1991, Dithiopyr at 0.6 kg ai ha(-1) reduced large crabgrass infestatio n by greater than or equal to 90% up to 87 DAT in both years, while pe ndimethalin at 1.7 kg ai ha(-1) provided similar control up to 59 DAT in 1 of 2 yr. Turf thinning due to disease and insect damage in 1990 a llowed more herbicide to reach the thatch, mat, and underlying soil in 1991. In both years, more pendimethalin but less dithiopyr nias found in thatch 7 DAT than 1 DAT, suggesting greater pendimethalin retentio n in verdure and thatch. Precipitation was greater after herbicide app lication in the second year, and concentration in the mat layer reache d a maximum by 7 DAT compared to 14 DAT in 1990. Maximum concentration s of dithiopyr and pendimethalin in soil at 0 to 5 cm deep were 20 and 56 mu g kg(-1), respectively, measured 7 DAT in 1991. Herbicide conce ntration 5 to 10 cm deep was less than or equal to 5 mu g kg(-1) at al l sampling dates in 1990. In 1991, dithiopyr and pendimethalin concent rations 5 to 10 cm deep did not exceed 4 and 20 mu g kg(-1), respectiv ely, measured within 1 DAT. Neither herbicide was detected 10 to 20 cm deep, nor in samples collected 30 cm outside of the experimental plot s in either year. At 126 DAT (final sampling date), little herbicide w as detected in verdure, but residues were found in most thatch and all mat samples. The estimated time for a 50% reduction in detectable res idues (DT50) of dithiopyr and pendimethalin from the sampling zone was 35 and 23 days, respectively.