Vapor movement of synthetic auxin herbicides can injure desirable plants outside the treatment zone. Vapor movement of the synthetic auxin herbicides aminocyclopyrachlor and aminocyclopyrachlor methyl was compared with that of the relatively volatile herbicide dicamba and the low volatile herbicide aminopyralid with a soybean bioassay under greenhouse and field conditions. Soybean is very sensitive to these active ingredients. Under greenhouse conditions, 82 (61 to 104) mg ae ha(-1) of aminocyclopyrachlor, 26 (18 to 33) mg ae ha(-1) of aminocyclopyrachlor methyl, 82 (69 to 95) mg ae ha(-1) of aminopyralid, and 61 (47 to 75) mg ae ha(-1) of dicamba produced an estimated 25 visual soybean phytotoxicity response when soybean was treated POST at the V3 growth stage (GR(25) 95 confidence interval). In field studies, aminocyclopyrachlor, aminocyclopyrachlor methyl, and aminopyralid were applied at 70 g ae ha(-1) and dicamba was applied at 560 g ae ha(-1) (labeled application rates) to soybean at the V3 growth stage. All herbicides were applied within an enclosed chamber (3 m by 3 m by 1 m) to mitigate movement of spray droplets. The enclosures were removed shortly after spray application and soybean response immediately surrounding the treated area was recorded in each of eight directions approximately 10 d after treatment. On the basis of bioassay responses, relative amount of vapor movement was dicamba > aminocyclopyrachlor methyl > aminopyralid approximate to aminocyclopyrachlor. Vapor movement of aminocydopyrachlor was very low indicating that the risk of phytotoxic response of sensitive plants due to volatility of aminocyclopyrachlor is negligible.
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