Herbicide strategies for managing kochia (Kochia scoparia) resistant to ALS-inhibiting herbicides in wheat (Triticum aestivum) and soybean (Glycine max)

Kochia (Kochia scoparia) control dropped from about 90% to less than 20% af ter three consecutive annual applications of acetolactate synthase (ALS)-in hibiting herbicides in several northeastern South Dakota fields. Field expe riments were conducted to evaluate control of kochia resistant to ALS-inhib iting herbicides using combinations of herbicides applied postemergence (PO ST) in wheat (Triticum aestivum) and using herbicides applied preemergence (PRE), POST, and PRE plus POST in soybean (Glycine max). Thifensulfuron plu s tribenuron did not control kochia in wheat (> 600 kochia plants/m(2) at h arvest) and wheat yield was similar to yield of the untreated control. Dica mba plus 2,4-D (in 1996) and bromoxynil plus MCPA (in 1996 and 1997) contro lled kochia and increased yields by 15% or more. Kochia was sparse (less th an 1 plant/m(2)) in soybean plots, but grass weeds were prevalent. PRE plus POST or POST herbicide combinations that included ALS-inhibiting herbicide s controlled grasses, and soybean yields were as much as 20% greater than y ields with combinations that did not contain ALS-inhibiting herbicides. Add ing ALS-inhibiting herbicides to wheat herbicide combinations decreased pro fitability because treatment costs increased while yields did not. In contr ast, adding ALS-inhibiting herbicides to soybean treatments increased profi tability due to better grass control, resulting in increased yield and bett er return.