Amicarbazone is a new herbicide registered for use in turfgrass under the trade name Xonerate.  Most previous research with amicarbazone has been conducted on creeping bentgrass golf greens and fairways.  Amicarbazone could become the first herbicide for selective annual bluegrass control in Kentucky bluegrass athletic fields, sod, and golf fairways during spring when the weed is most problematic, but more work is needed to determine the best treatment programs.  The objective of this study was to determine the appropriate application rates and timings of amicarbazone for controlling annual bluegrass in Kentucky bluegrass without causing unacceptable turf injury.

The trial was initiated on May 23, 2012 on an annual bluegrass infested Kentucky bluegrass fairway maintained at 1.2 cm.  Treatments consisted of amicarbazone applied four times at 49.0 g ai/ha at one-week intervals, twice at 98.0 g ai/ha at a two-week interval, twice at 98.0 g ai/ha at a three-week interval, twice at 147 g ai/ha at a two-week interval, and twice at 147 g ai/ha at a three-week interval.  An untreated check was included for comparison. 

All amicarbazone treatments controlled annual bluegrass greater than 93% 12 weeks after initial treatment (WAIT), with the 98.0 g ai/ha rate applied twice at a 3-week interval being significantly less (93%) than amicarbazone applied four times at 1-week intervals at the 49.0 g ai/ha rate (98%).  Amicarbazone applied at 147 g ai/ha injured Kentucky bluegrass the most 4 WAIT (20%) after the first application but decreased to 13% by 6 WAIT and recovered completely by 12 WAIT and after both applications had been made.  These data suggest that amicarbazone controls annual bluegrass at several different application rates and timings; however, the second application of the 98.0 and 147 g ai/ha rates seemed to be unnecessary in regards to annual bluegrass control.  Temperatures exceeded 85 F during most of the later application times and may have increased annual bluegrass and Kentucky bluegrass response to amicarbazone.  It was also noted that higher and seemingly drier areas of the trial exhibited more severe Kentucky bluegrass injury and more rapid annual bluegrass control.  In order to avoid unacceptable turf injury, amicarbazone should be applied at the 49.0 g ai/ha rate four times at 1-week intervals or the 98.0 or 147 g ai/ha rates once.  In cases of high annual bluegrass infestations, 49.0 g ai/ha of amicarbazone applied four times appears to be the most appropriate treatment in order to phase out annual bluegrass and prevent large, dead or void areas in the turf which may be unsightly to spectators or users of the athletic field.  Other research conducted in cooler weather suggests these lower rates may not effectively control annual bluegrass.  Future research should seek to develop a model that will allow turf managers to predict appropriate amicarbazone rates based on measured or expected temperature fluctuations.