The Influence of Nitrogen Fertility on the Tolerance of Kentucky Bluegrass (Poa pratensis L.) to Bispyribac-sodium Herbicide.

Bispyribac-sodium has been shown to control grassy weeds in cool-season turfgrass including creeping bentgrass (Agrostis stolonifera L.). Kentucky bluegrass (Poa pratensis L.) is a popular grass species used for turf but cultivars vary widely in their tolerance to bispyribac-sodium. The objective of this study was to evaluate the effects of nitrogen fertility on the tolerance of Kentucky bluegrass cultivars to this herbicide. The field trial was conducted in both 2004 and 2005, newly seeded each year at the Rutgers University Plant Biology and Pathology Research Center in Adelphia, NJ, on a well-drained Freehold sandy loam (fine-loamy, mixed, mesic, Typic Hapludult). Six cultivars were chosen to represent the full range of response to bispyribac-sodium. ‘Avalanche’ and ‘Washington’ represented susceptible cultivars with injury in excess of 80 percent, ‘Midnight’ and ‘Boutique’ represented a moderate response with injury of 34 and 38 percent, respectively, and ‘Lakeshore’ and ‘SR 2284’ represented tolerant cultivars that exhibited less then 25 percent injury. Zero or 50 kg N/ha were applied in a strip plot design using a homogenous 16-4-8 formulation, applied 4 days before initial herbicide applications. Bispyribac-sodium was applied at two rates (75 g.a.i./ha followed by 75 g.a.i./ha, and 150 g.a.i./ha followed by 150 g.a.i./ha) using a sequential application spaced 21 days apart in late June. Turfgrass injury was rated on a weekly basis and percent ground cover was noted at the completion of the study. Based on the results of this study, injury of tolerant cultivars is masked by the nitrogen application, injury in moderate cultivars is not significantly affected by the nitrogen application, and injury in the susceptible cultivars is increased with the application of nitrogen. The potential exists to safely use bispyribac-sodium herbicide on tolerant Kentucky bluegrass cultivars in conjunction with nitrogen fertility.