Studies were conducted to determine if mesotrione alone or in mixtures with low rates of atrazine would control Canada thistle. In the field, mesotrione applied alone did not adequately control Canada thistle, although smaller plants in the rosette stage of growth were more susceptible than plants in the bolting stage. A mixture of mesotrione at 105 g ai ha−1 and atrazine at 280 g ai ha−1 improved control of Canada thistle over that with mesotrione alone. In the greenhouse, mixtures of mesotrione plus atrazine at 560 g ha−1 reduced Canada thistle regrowth more than mesotrione alone or mesotrione plus 280 g ha−1 atrazine. Mesotrione plus atrazine mixtures increased the rate of tissue necrosis compared with the slower development of bleaching symptoms normally associated with mesotrione alone. Uptake, translocation, and metabolism of 14C-mesotrione in Canada thistle were generally slow, and results did not explain the increased control associated with mesotrione plus atrazine mixtures. However, higher levels of absorption and translocation and reduced root metabolism of mesotrione in rosette stage plants compared with bolting plants may explain the greater susceptibility to mesotrione in the rosette stage. The changes in symptomology and increased control with mixtures of mesotrione and atrazine were likely due to the interrelationship between the modes of action of these herbicides.
Nomenclature: Atrazine; mesotrione; Canada thistle, Cirsium arvense (L.) Scop. CIRAR.