Assessing the Potential Use of TENACITY (mesotrione) Herbicide For use as a Control Agent for Poa annua In Conjunction With Fall Overseeding of Bermudagrass

Tenacity herbicide (mesotrione) was evaluated for use as both a pre-emergent and post- emergent herbicide in conjunction with fall overseeding of bermudagrass with perennial ryegrass. Tenacity herbicide, when applied @7 DBOS, 1 DBOS, at first mowing (2 WAOS) or 3 weeks after the first mowing (5 WAOS) did not cause a reduction in stand of perennial ryegrass compared to non-treated controls. There was essentially no difference in ryegrass emergence and cover for Tenacity when applied at the 2.0 oz. versus the 3.0 oz AI/A rate. When applied at these rates at both 7 DBOS and at the first mowing, treatments 1, 3, and 6 produced similar results. Overseeding occurred on October 16, 2007. Intense Poa annua pressure dominated the overseed ryegrass by late December, causing a decline in existing ryegrass cover. Only the latter applied treatments (of Prograss, or tank mixes which contained Prograss with Tenacity) regained sizable amounts of ryegrass by the end of February and early March. Percent weed control was ineffective for treatments which did not include Prograss herbicide. The split application of Prograss of 0.75 lbs AI/a applied @ 8 WAOS and again @ 12 WAOS resulted in the greatest amount of Poa annua control, and the greatest amount of ryegrass. Tenacity (mesotrione), when applied at rates and timings observed here, was safe for perennial ryegrass emergence, but ineffective on Poa annua pre-emergence and as a post emergent agent based on the subsequent growth of Poa annua.