Palmer Amaranth (Amaranthus palmeri) and Common Waterhemp (Amaranthus rudis) Control with Very-Long-Chain Fatty Acid Inhibiting Herbicides

Field experiments were established in 2015 and 2016 near Manhattan, Hutchinson, and Ottawa, Kansas to assess residual control of Palmer amaranth (Amaranthus palmeri S. Watson) and common waterhemp (Amaranthus rudis Sauer) with very-long-chain fatty acid (VLCFA) inhibiting herbicides. Six VLCFA inhibiting herbicides and pendimethalin were applied at three different rates (high, middle, and low) based on labeled rate ranges for soybean [Glycine max (L.) Merr.]. All treatments were applied preemergence (PRE) in a non-crop scenario after the plot area was clean tilled with a field cultivator. The experiment was conducted one time in 2015 and four times in 2016 at two locations for a total of five site years. Percent Palmer amaranth and common waterhemp control was visually estimated at 4 and 8 weeks after treatment (WAT). At Manhattan, pyroxasulfone, S-metolachlor, and dimethenamid-P resulted in the greatest Palmer amaranth control at both 4 and 8 WAT. At Hutchinson and Ottawa, pyroxasulfone, S-metolachlor, and non-encapsulated acetochlor resulted in the highest Palmer amaranth and common waterhemp control at both 4 and 8 WAT. Pyroxasulfone and S-metolachlor were often the most effective herbicides; whereas, pendimethalin resulted in the least effective Palmer amaranth and common waterhemp control at all sites and observation times. The high use rate across all herbicides resulted in better control when compared to the low use rate across all herbicides at all sites and observation times. This research demonstrates the value of utilizing VLCFA inhibiting herbicides as an effective site of action for residual control of Palmer amaranth and common waterhemp as part of integrated weed management plan.