Nitrogen Enhanced Efficacy of Flazasulfuron for Controlling Annual Bluegrass (Poa annua L.).

Annual bluegrass (Poa annua L.) commonly invades overseeded bermudagrass stands.  Sulfonylurea herbicides like flazasulfuron can be used to remove overseeded cool-season species when environmental conditions favor bermudagrass growth in spring. Although turf managers commonly apply nitrogen (N) fertilizers each spring, the effects of N on the efficacy of flazasulfuron have not been determined.   A two-year study was initiated at the University of Tennessee (Knoxville, TN) in 2008 to evaluate the efficacy of flazasulfuron for control of overseeded perennial ryegrass (Lolium perenne L.) contaminated with annual bluegrass.  In 2009, the study was replicated at the University of Georgia (Griffin, GA) and Louisiana State University (Baton Rouge, LA).  Flazasulfuron was applied at rates of 4.4, 8.8, and 17.5 g a.i. ha^-1 alone, in combination with ammonium sulfate at 2.2 kg N ha^-1, and following two applications of urea at 73.0 kg N ha^-1.  Ammonium sulfate was delivered as part of the spray solution. Urea was applied granularly immediately prior to treatment and 4 weeks later.  At all rates in 2008, flazasulfuron treatments applied following an application of urea provided a greater level of perennial ryegrass control at 14 and 21 days after treatment (DAT) than those applied alone or in combination with ammonium sulfate. On every rating date in 2008, all treatments applied to turf treated with urea based N provided greater annual bluegrass control than those applied alone or in combination with ammonium sulfate. Similar results were observed at all locations in 2009.  Annual bluegrass control with flazasulfuron at 17.5 g a.i. ha^-1 was significantly less than that which was observed following applications of flazasulfuron at 4.4 g a.i. ha^-1 made to turf treated with urea at 73.0 kg N ha^-1. These results suggest that applications of urea N enhance the efficacy of flazasulfuron for controlling annual bluegrass in bermudagrass stands overseeded with perennial ryegrass.