Canada Thistle(Cirsium arvense)Control in Timothy(Phleum pratense)and Red Clover(Trifolium pratense)Sward

Abstract

Two of three herbicides were applied repetitively in each of the years to a dense infestation of Canada thistle [Cirsium arvense(L.) Scop.] in timothy(Phleum pratenseL.) and red clover(Trifolium pratenseL.) sward in Carroll County, Maryland during the period 1969 to 1971. Picloram (4-amino-3,5,6-tricloropicolinic acid), eliminated red clover from the sward after the first year, and controlled 95 to 100% of the Canada thistle in the third year of treatment. The addition of a non-phytotoxic oil to picloram did not improve its efficiency. Dicamba (3,6-dichloro-o-anisic acid) reduced 57 to 87% of the red clover and 72% of the Canada thistle in the first year at a dosage of 1.12 kg/ha, but effected only 42% control in the third year of treatment. Both an amine and ester formulations of 2,4-D [(2,4-dichlorophenoxy) acetic acid] effected variable Canada thistle control and reduced the red clover stand 10 to 15% following the first application. The timothy yield was not significantly reduced during the 3 years of repeated application of all herbicides, but some necrosis of leaves was noted with dicamba and the high rate of 2,4-D. Soybeans [Glycine max(L.) Merr. ‘Delmar’] planted 1 yr following the last picloram application had curled leaves initially, but outgrew the injury. Soybeans planted 2 yr following the last application of the herbicides showed no significant reduction in yield indicating herbicide residues had been reduced to tolerable levels.