Persistence and accumulation of clopyralid (3,6-dichloropicolinic acid) in soil Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/76537530j

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  • Field studies were started at Hyslop Farm, Corvallis, Oregon in 1984 to determine the soil persistence of the herbicide clopyralid (3,6-dichloropicolinic acid) under cropping situations. The herbicide was sprayed on bare soil at the proposed use rate of 0.14 kg/ha in the spring and on the same plots at a high rate of 0.56 kg/ha in the summer. Winter wheat (Triticum aestivum L. 'Stephen') was seeded in the same fall. Treated plots did not yield differently than untreated plots. Safflower (Carthamus tinctorius L.), a crop known to be sensitive to clopyralid, was planted in the spring of 1985 to the area treated with clopyralid in 1984. Safflower grew normally and fresh weights were not reduced. Greenhouse bioassays were conducted on soil samples collected from two separate sets of plots treated with clopyralid in the fall of 1984 or the spring of 1985. The application rates were 0.56 and 1.12 kg/ha. Lentil (Lentilla lens L.), safflower, and peas (Pisum sativum L.) were used as indicator plants. From the plots sprayed in the fall, soil samples were collected at 0, 14, 34, 54, 114, 220, and 287 days after treatment. Clopyralid disappeared faster in the second depth (10 to 20 cm depth) than in the first depth (0 to 10 cm depth). No herbicide was detected in the second depth 220 days after treatment but in the first depth there was sufficient herbicide to cause growth reduction in all of the indicator species. In soil sampled 287 days after applying either 0.56 or 1.12 kg/ha, enough herbicide remained in the first depth to produce slight symptoms on lentil. No herbicide injury was observed on peas or safflower. From plots sprayed in the spring, soil samples were collected 0, 14, 28, and 56 days after treatment. In the spring, clopyralid dissipated more slowly than in the fall, based on the observation that the indicator plants were more severely injured when grown in soil samples collected following the spring application than they were following the fall application. Adding 2,4-D did not affect clopyralid persistence. Clopyralid and XRM-3785 (clopyralid + 2,4-dichlorophenoxy acetic acid) at the same dosage of clopyralid disappeared from the soil at approximately the same time.
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