Application Timing Affects BAY MKH 6561 and MON 37500 Efficacy and Crop Response in Winter Wheat

Field experiments were conducted at four locations in Kansas and Wyoming during the 1998 to 1999, 1999 to 2000, or 2000 to 2001 winter wheat-growing seasons to compare BAY MKH 6561 and MON 37500 at four postemergence timings for weed control and crop response. BAY MKH 6561 at 45 g ai/ha and MON 37500 at 35 g ai/ha controlled cheat 95% or more when applied early-fall postemergence (EFP), late-fall postemergence (LFP), early-spring postemergence (ESP), or late-spring postemergence (LSP) at Hays, KS, in 1999 and 2001. At Manhattan, KS, in these years, cheat control was 98 to 100%, except for MON 37500 (83 to 90%) applied LSP. Downy brome control with BAY MKH 6561 or MON 37500 declined as application timing was delayed at Hays, Archer, WY, and Torrington, WY. Averaged over herbicides, downy brome was controlled 38 to 95% when herbicides were applied EFP and 10 to 79% when applied LSP. Late-season wheat stunting was 11% or less, regardless of herbicide treatment, at Manhattan each year; no late-season injury was visible at Hays, Archer, or Torrington. Yields did not differ between treated and untreated wheat at Manhattan in 1999 or 2001, at Archer in 2000, or at Hays in 1999. Wheat receiving BAY MKH 6561 EFP yielded 8% more grain than did wheat treated with MON 37500 LSP at Hays in 2001 and at Torrington. At these two locations, all herbicide-treated wheat yielded 10 to 20% more grain than did the untreated wheat.