Response of Kentucky Bluegrass Cultivars and Annual Bluegrass Control with Mesotrione at Turfgrass Establishment.

Field studies were conducted in the fall of 2007 and 2008 to evaluate the response of newly seeded and seedling Kentucky bluegrass and annual bluegrass control to mesotrione applied at planting (PRE), and sequential treatments four and eight weeks after turfgrass emergence (WAE).  Mesotrione was applied at rates ranging from 0.14 to 0.56 kg ai/ha using a single nozzle CO₂ pressured sprayer calibrated to deliver a total 375 L ha^-1.  Nozzles used were 9504E and CO₂ regulators were set for 220 kPa. Experimental design was a randomized complete block with four replications.  Turfgrass chlorosis was rated on a percent scale where 0 equaled no chlorosis and 100 equaled complete chlorosis. Turfgrass cover and annual bluegrass control were rated visually on a percent scale. Overall, mesotrione caused minimal turfgrass cover reductions applied PRE and sequential applications caused minimal chlorosis and cover reductions. Control of winter annual broadleaf weeds such as chickweed, henbit, oxalis and veronica were nearly complete with all mesotrione treatments. Mesotrione exhibited potential to selectively control annual bluegrass applied PRE especially at the 0.28 and 0.56 kg/ha application rates. However, annual bluegrass control was greatest when mesotrione was applied sequentially especially at application rates of 0.2 kg/ha or higher.