Canada Thistle (Cirsium arvense) Control in Sugarbeet with Clopyralid

Sugarbeet tolerance to and Canada thistle control from tank mixtures of clopyralid plus other postemergence herbicides and/or petroleum oil adjuvant applied at various times were determined. Herbicides were applied to 5 to 20 or 20 to 70-cm tall Canada thistle. Clopyralid at 0.14 and 0.21 kg ai ha-1alone or with petroleum oil adjuvant did not injure sugarbeet. Tank mixing clopyralid with desmedipham/phenmedipham plus endothall injured sugarbeet 7 d after application in 2 of 3 yr but not when observed 14 d after application. Herbicides applied early postemergence in 1987 and late postemergence in 1988 controlled Canada thistle poorly due to rainfall 1 h after application and drought conditions, respectively. The addition of petroleum oil adjuvant enhanced control of Canada thistle by clopyralid at 0.14 kg ha-1, and control equaled that of clopyralid at 0.21 kg ha-1alone. Addition of desmedipham/phenmedipham plus endothall to clopyralid did not reduce Canada thistle control. Petroleum oil adjuvant added to clopyralid at 0.14 kg ha-1increased Canada thistle control to that achieved by clopyralid at 0.21 kg ha-1plus desmedipham/phenmedipham plus endothall.