Effect of Seasonal Timing of Glyphosate Application for Nonselective Control of Roughstalk Bluegrass.

Rough bluegrass (RBG, Poa trivialis L.) is a problematic weed in cultivated turfgrasses.  Chemical control of RBG can prove challenging as herbicides labeled for the selective removal of the species are limited.  Nonselective herbicides can eradicate RBG, and properly timed applications may offer better control.  Our objective was to evaluate seasonal timing of glyphosate (Glyphomate 41) for nonselective control of RBG.  Glyphosate was applied at 3.4 kg a.i. ha^-1 in spring, mid-summer, or late-summer in Manhattan, KS (2011 and 2012) and Mead, NE (2012).  Percent RBG cover was visually estimated.  Following glyphosate applications, RBG coverage was reduced to 0% in all treatments in Manhattan and Mead in 2011 and 2012.  After treatment in Manhattan in 2011, plots treated in spring, mid-summer, or late-summer averaged 1.3, 8.8, and 5.8% RBG cover, respectively, by 30 May 2012.  A spring glyphosate application resulted in less RBG cover than treatment in mid-summer.  Rough bluegrass treated in spring in Manhattan in 2012 had less cover (1.3%) than that treated in mid- (86.3%) or late-summer (46.8%) by 24 May 2013.  After treatment in Mead in 2012, RBG treated in spring (31.3%) had less cover by 3 June 2013 than those treated in midsummer (68%).   A single spring application of glyphosate offers better RBG control than application when RBG is not actively growing in mid-summer.