Balancing Residual Herbicide Cost and Application Timing for Maximum Late Season Waterhemp Control.

Herbicide resistant waterhemp has continued to spread and be major pest for Kentucky soybean growers. The introduction of multiple herbicide tolerant soybean varieties (i.e. RR2Xtend, Enlist E3, and LLGT27) that offer effective postemergence herbicide option has decreased farmer incentive to costly soil residual herbicides. The elimination soil residuals or reduction to single active residual herbicides potentially increases herbicide resistance selection pressure on postemergence herbicides, as well as allows for late season waterhemp escapes. University Weed Scientist recommendations for complete waterhemp control can have input cost of upwards of $30 for residual herbicides alone and often are not observed as feasible by growers. Research trials are to be conducted at two grower locations infested with herbicide resistant waterhemp. Experimental treatments will evaluate a range of soil residual herbicides with variable number of effective sites of action as well as variable timings of application and overall cost. Treatments will be evaluated for end of season waterhemp densities as well as waterhemp seed production. Overall treatment cost will also be calculated considering total number of postemergence applications applied on an as needed basis.