Fertilization augments Canada thistle (Cirsium arvense L. Scop) control in temperate pastures with herbicides.
Integrated weed management combines treatments for improved weed control. Few studies have compared integrated approaches for longer-term control of Cirsium arvense in pasture. We evaluated the response of this weed in perennial pasture to one-time mowing or herbicide application (2,4-D ester, 2,4-D+mecoprop+dicamba, clopyralid, or picloram+2,4-D) at the early bud stage of growth, both with and without annual spring fertilization over a 3-year period in central Alberta, Canada. Regardless of fertilization, all herbicide treatments suppressed Cirsium arvense in the year of treatment. Fertilization without control measures decreased Cirsium arvense density but increased biomass, while a one-time mowing treatment increased shoot densities. Although all herbicide treatments tended to reduce Cirsium arvense into the second and third year, picloram+2,4-D at 0.24+0.89 kg ai/ha and clopyralid at 0.22 kg ai/ha provided the greatest weed suppression, particularly when fertilized annually. Overall, longer-term control of Cirsium arvense by herbicides was enhanced with annual spring fertilization, underscoring the importance of an integrated approach to controlling Cirsium arvense in temperate pastures.