Influence of Methiozolin Rates and Timings on Annual Bluegrass Control.

Methiozolin is an experimental post-emergent herbicide designed to selectively control annual bluegrass (Poa annua L.; ABG).  The objective of this study was to evaluate the herbicide’s efficacy to control ABG when applied at two rates and initiated at three seasonal timings.  A field study was initiated in 2011 and repeated in 2012 on a golf course or research putting green, respectively. Each site consisted of a mixed stand of creeping bentgrass (Agrostis stolonifera L.; CBG) and ABG at a ratio of approximately 40%/60% and 25%/75%.  Plots measured 0.9 x 1.8 m and were set up as a 2 x 3 incomplete factorial and arranged in a randomized complete block design with four replications.  Main factors included methiozolin rates (0.5 kg a.i. ha^-1 and 1.0 kg a.i. ha^-1) and timings (spring, summer and autumn).  In each year, treatments were initiated in early May, mid-June or late August and applied every 2 weeks until the total herbicide load reached 3.0 kg a.i. ha^-1.  When rated the following spring, plots in which methiozolin applications were initiated in late August had the lowest percent ABG (1% to 3%), regardless of application rate. In the 2011 study, methiozolin treatments initiated during the summer months resulted in no reduction in percent ABG when compared to the untreated control. The same treatments initiated in 2012 resulted in a 55% to 65% reduction in percent ABG when compared to the untreated control. In 2011 and 2012, plots treated with methiozolin in the spring resulted in a 29% to 43% and 73% to 80% reduction in percent ABG when compared to the untreated control, respectively. Except when applied in the spring of 2011, methiozolin rates did not influence percent suppression of ABG. Although ABG suppression was possible, overall turfgrass quality declined in select treatments and injury was moderate to severe at times.  Methiozolin rates and application timings should be considered when a slow and non-invasive conversion to CBG is desired.