Combining Mowing and Fall-Applied Herbicides to Control Canada Thistle (Cirsium arvense)1

Experiments were conducted in subirrigated and upland Colorado pastures to compare herbicides applied alone in fall to the same herbicides preceded by one, two, or three mowings. Picloram controlled Canada thistle (Cirsium arvense) well at both sites, and in general, mowing did not improve its performance. Picloram at 560 g ai/ha and picloram plus 2,4-D at 280 plus 1,120 g ai/ha were the lowest rates that eliminated Canada thistle. Mowing did not improve chlorsulfuron performance at either site, but chlorsulfuron eliminated Canada thistle at the subirrigated site. Dicamba controlled 97% of Canada thistle at the subirrigated site, and mowing did not improve its performance; two or three mowings before spraying dicamba at the upland site improved performance, but 37% remained uncontrolled. Control from the reduced rate and the two lowest recommended rates of clopyralid plus 2,4-D was improved by two or three prior mowings at the subirrigated site, but only the highest rate benefited from two or three mowings at the upland site. Mowing alone three times per year for 2 yr controlled 85% of Canada thistle at the subirrigated site, but failed to control it at the upland site. Variability in results between experiments likely was due to a high water table at the subirrigated site, which may have restricted Canada thistle root growth and possibly made it easier to control. Inconsistent results prohibit concluding that mowing before spraying will consistently improve Canada thistle control, and such a treatment combination should not be commonly recommended.Nomenclature: Clopyralid, 3,6-dichloro-2-pyridinecarboxylic acid; chlorsulfuron, 2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide; dicamba, 3,6-dichloro-2-methoxybenzoic acid; picloram, 4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid; 2,4-D, (2,4-dichlorophenoxy)acetic acid; Canada thistle, Cirsium arvense (L.) Scop. #3 CIRAR.Additional index words: Integrated management, noncrop weed management, CIRAR.