Management of Annual Bluegrass (Poa annua .L) with Bispyribac-Sodium On Creeping Bentgrass (Agrostis stolonifera .L) Putting Greens.

Annual bluegrass (Poa annua L.) is a troublesome weed on creeping bentgrass (Agrostis stolonifera L.) putting greens.  Bispyribac-sodium effectively removes annual bluegrass in creeping bentgrass fairways, but turf tolerance to the herbicide is lower as mowing heights are reduced from fairway to green heights (12.5 to 3 mm).  The objective of this research was to determine if bispyribac-sodium applied in late spring/summer months in the transition zone can reduce P. annua infestations over time without compromising turf quality.  Field studies were established on a golf course putting green in Columbia, Missouri in 2010 and 2011, and were designed as a randomized complete block with four replications.  Treatments included two rates of bispyribac-sodium (12.4 and 24.8 g ai ha^-1).  The lower rate was applied on 14 (6 total applications) and 21 day (4 total applications) intervals, while the higher rate was applied every 28 days (3 total applications).  Treatments also included the addition of the plant growth regulators (PGRs) paclobutrazol (224 g ai ha^-1) and trinexapac-ethyl (57 g ai ha^-1) every 28 days (3 total applications).  Bispyribac-sodium effectively reduced annual bluegrass to less than 10% incidence per plot by early fall, while the non-treated control plots had ~20% Poa infestation.  During the application period of treatments, turf quality was not affected by the low rate of bispyribac-sodium application, although transient yellowing was observed on plots that received the high rate of bispyribac-sodium.  Addition of trinexapac-ethyl to bispyribac-sodium did not improve annual bluegrass control or creeping bentgrass turf quality.  Combinations of bispyribac-sodium and paclobutrazol resulted in the most effective control of annual bluegrass (<5% incidence), although addition of paclobutrazol resulted in turf phytotoxicity within 2 weeks of application.  Results suggest that optimum management of annual bluegrass with the least impact on turfgrass quality follows applications of bispyribac-sodium at rate of 12.4 g ai ha^-1 on 14 or 21 day intervals