INTERVAL BETWEEN SEQUENTIAL HERBICIDE TREATMENTS FOR SOURGRASS MANAGEMENT

ABSTRACT More than one herbicide application is usually necessary to manage glyphosate-resistant sourgrass in advanced stages of development efficiently during off-season fallow periods. The objective of this study was to determine the best interval between two sequential applications to control sourgrass, based on the number of days and tiller-height after the first treatment. Two experiments were conducted based on these criteria. Experiment 1 consisted of one application of glyphosate + clethodim (1140 ae ha-1 + 108 g ha-1) followed by glyphosate + clethodim or paraquat (400 g ai ha-1) at an interval of 10, 17, 24, 31, 28, or 45 days. Experiment 2 was conducted with the same herbicide treatments, but using the tiller-height as the criteria for the second application, which were 2-5, 6-10, 11-20, 21-30, and >30 cm. None of the treatments resulted in total sourgrass control during the evaluation period. Overall, treatments with glyphosate + clethodim in the second application were more efficient than paraquat. The most effective interval between sequential applications of glyphosate + clethodim was observed at 17 to 24 days. For paraquat, the best interval for the second application was 6-10 days. The most effective performances based on the tiller-height were found at 620 cm tall for glyphosate + clethodim and 6-10 cm tall for paraquat.