Selective Application of Herbicides for Canada Thistle (Cirsium arvense) Control in Birdsfoot Trefoil (Lotus corniculatus)

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Abstract
Canada thistle [Cirsium arvense[L.] Scop. # CIRAR] is a major weed problem in birdsfoot trefoil (Lotus corniculatusL. ‘Norcen’) seed production in northern Minnesota. Several systemic herbicides applied with selective applicators (roller and ropewick) were evaluated for Canada thistle control in birdsfoot trefoil. Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), clopyralid (3,6-dichloro-2-pyridinecarboxylic acid), and glyphosate [N-(phosphonomethyl)glycine] controlled Canada thistle better than MCPA [(4-chloro-2-methylphenoxy)acetic acid] and dicamba (3,6-dichloro-2-methoxybenzoic acid). Glyphosate and MCPA did not injure birdsfoot trefoil foliage or bloom, whereas, picloram, clopyralid, and dicamba injured both. Although selective applications of glyphosate controlled Canada thistle for a short term with the least birdsfoot trefoil injury, long-term Canada thistle control in birdsfoot trefoil does not appear feasible with selective herbicide applications.