Horseweed Control with Preplant Herbicides after Mechanical Injury from Small Grain Harvest

Horseweed [ Conyza canadensis (L.) Cronquist] plants present during small grains harvest are variable in size, but tall plants are mechanically cut (“topped”) by the combine. Horseweed is best controlled prior to planting double‐cropped soybeans, which must be planted quickly after harvest to preserve yield potential. This urgency eliminates an opportunity for horseweed regrowth, leaving topped plants with reduced leaf area for herbicide spray interception, likely impacting herbicidal control, but little information has been reported in this regard. Horseweed control is further complicated by the prevalence of multiple herbicide‐resistant biotypes. Therefore, field research was conducted to evaluate herbicidal control of glyphosate‐susceptible horseweed that was topped (cut to 46 cm tall) prior to treatment. Additional greenhouse experiments evaluated glyphosate‐resistant horseweed at the rosette and bolt growth stages as well as topped plants prior to treatment. Paraquat alone or in combination with 2,4‐D or dicamba, resulted in 80 to 100% control of topped horseweed across field and greenhouse experiments. In greenhouse experiments, the rosette growth stage was most effectively controlled, followed by the bolt growth stage, which was similar or more effectively controlled, compared with topped horseweed. The authors recommend paraquat with 2,4‐D or dicamba for control of topped horseweed plants, due to efficacy and mixing of multiple effective herbicides. Core Ideas Small grains harvest equipment can cut horseweed, reducing herbicide efficacy. Paraquat containing treatments controlled rosette, bolt, and topped horseweed ≥80%. Dicamba and 2,4‐D controlled rosette stage glyphosate‐resistant horseweed >90%. Authors recommend paraquat with 2,4‐D or dicamba to control topped horseweed.