Annual bluegrass is the most prevalent weed problem on creeping bentgrass golf putting greens.  There are no effective selective controls to manage it well.  Most superintendants rely on plant growth regulators to slow the growth of annual bluegrass and effectively suppress seedhead production.  A plant growth regulator program requires repeated applications throughout the season to maintain control.  For example paclobutrazol and trinexapac ethyl are typically applied together 6 to 8 times at three week intervals beginning in late spring and continuing into the fall.  Patents recently expired for both paclobutrazol and trinexapac ethyl and new formulations have been registered. 

The objective of this study was to compare PGR programs using new generic formulations of paclobutrazol and trinexapac ethyl compared to the original proprietary products.  Studies were established as randomized complete block designs at two sites in 2011, Draper Valley Country Club in Draper, VA and Spotswood Country Club in Harrisonburg, VA.  Trials were initiated May 13th and May 14th, respectively and treatments were repeated every three weeks.  Treatment programs included the following:  1) paclobutrazol (Trimmit 2SC) applied twice in spring and three times in fall at 0.28 kg ai/ha and three times in summer at 0.14 kg ai/ha plus trinexapac ethyl (Primo Maxx) at 0.05 kg ai/ha; 2) paclobutrazol (Tide Paclo) applied at same rates and timings as treatment 1 with the exception that the summer addition of trinexapac ethyl consisted of the product T-Nex instead of Primo Maxx; 3) same as treatment 1 except all paclobutrazol rates are reduced by half; 4) same as treatment 2 except all paclobutrazol rates are reduced by half; 5) flurprimidol (Cutless 50WP) applied twice in spring and three times in fall at 0.3 kg ai/ha and three times in summer at 0.15 kg ai/ha plus trinexapac ethyl (Primo Maxx) at 0.05 kg ai/ha.  A nontreated check was also included and treatments were replicated three times at each site.  Annual bluegrass cover was 35-47% at Draper when the trials were initiated.  In mid July, annual bluegrass cover at Draper was 62% in the nontreated check and 13 to 27% and equivalent for both formulations at the low paclobutrazol rate and 4.0 to 4.7% and equivalent in both formulations at the high paclobutrazol rate and the flurprimidol program.  Thus, there was a significant rate response and all treatments reduced annual bluegrass cover compared to the nontreated control but product formulation did not significantly impact annual bluegrass cover.  Turf injury was never evident and quality was never significantly different from the nontreated check at Draper.  Annual bluegrass cover ranged from 58-67% at Spotswood at trial initiation.  On June 23, the nontreated control had 50% annual bluegrass cover and all treatments significantly reduced cover to 21 to 33% with no differences between treatments.  In July and August, no differences were noted in annual bluegrass cover at Spotswood.  Although creeping bentgrass was never injured by various treatment programs, on June 15 turfgrass quality was significantly decreased by both high-rate paclobutrazol programs due to phytotoxicity to annual bluegrass.  On June 23, all treatment programs significantly increased turf quality compared to the nontreated control, presumably due to the addition of trinexapac ethyl for summer treatments.  These programs are undergoing fall treatments now and fall ratings will be discussed at the annual meeting.