Canada Thistle Control and Competition in Sugarbeets

Canada thistle (Cirsium arvense (L.) Scop.) is an aggressive perennial weed found in sugarbeet (Beta vulgaris L.) fields throughout the northern half of the United States. Field trials were conducted at Powell, WY from 1988 to 1991 to evaluate the effectiveness of clopyralid (3,6-dichloro-2-pyridine carboxylic acid) for Canada thistle control in sugarbeet and to evaluate the effect of several Canada thistle densities on sugarbeet yield and sucrose percentage. Clopyralid provided good (83 to 89%) control of Canada thistle at 210 g ae/ha whether applied as a single or split treatment, alone or in combination with desmedipham (ethyl[3-[[(phenylamino)carbonyl]oxy]phenyl] carbamate) plus phenmedipbam (3-[(methoxy carbonyl)amino]phenyl(3-methylphenyl)carbamate) when at least half the rate of clopyralid was applied after all Canada thistle rosettes had emerged. Sugarbeet yield, sucrose percentage and grower net returns decreased as Canada thistle density increased. Each one thousand Canada thistle shoots/ha reduced root yield by 0.4 Mg/ha, sucrose content by 0.01% and grower net returns by $19.02/ha