Control and Cross-Resistance of an Acetolactate Synthase Inhibitor-Resistant Palmer Amaranth (Amaranthus palmeri) Biotype

Over two years, acetolactate synthase (ALS)-inhibiting herbicides and herbicides with different mechanisms of action were tested individually and in combination for control of ALS-resistant Palmer amaranth in soybean. As expected, ALS-inhibiting herbicides did not control the resistant Palmer amaranth. Lactofen at 210 g/ha and acifluorfen at 560 g/ha gave the best postemergence control. Tank mixes of lactofen with either imazethapyr or chlorimuron plus thifensulfuron did not significantly increase control over lactofen alone. Sequential treatment with a soil-applied herbicide, either SAN 582 or pendimethalin, followed by lactofen postemergence, controlled weeds best (greater than 85%). The extent of cross-resistance of Palmer amaranth to ALS-inhibiting herbicides was determined in the greenhouse. Sixteen POST ALS-inhibiting herbicides were sprayed at ½, 1, 2, 4, and 8X field use rates on resistant and susceptible biotypes. All ALS-inhibiting herbicides at all rates controlled the susceptible biotype. The resistant biotype was cross-resistant to all ALS-inhibiting herbicides. Metsulfuron and imazapyr at 8X rates were the only treatments that provided 80% or greater control of the ALS-resistant Palmer amaranth. Chlorsulfuron and tribenuron were the only other herbicides that gave partial control at the highest rate. Control with these four herbicides decreased as the rate was decreased. Experimental results suggest that cultural practices and herbicides with different mechanisms of action will be required to manage this ALS-resistant Palmer amaranth biotype.