One of the greatest threats to cotton and soybean producers is Palmer amaranth, an invasive and aggressively growing weed. The weed has developed resistance to the widely used weedkiller glyphosate and now Palmer amaranth populations in Arkansas are resistant to a class of herbicides known as PPO inhibitors, compounding the challenge of weed control, says a University of Illinois researcher.
“We predict PPO resistance is going to spread very rapidly in Palmer, like it did in waterhemp. By now, we know PPO-resistant Palmer is present in at least three states. If a farmer has Palmer, they should not rely on PPO as an effective herbicide. It might only work for a couple of years,” says U-Illinois weed scientist Patrick Tranel. Like other experts, Tranel says farmers should try different pre- and post-emergence herbicides with as many modes of action as possible to increase their chances of finding a way to kill the weed.
Purdue suggests crop rotation, deep tillage, cover crops and hand weeding as additional steps.
In a study, Tranel and other researchers say they found the process by which Palmer amaranth, like its cousin waterhemp, developed resistance to PPOs. It is a mutation in which three nucleotides in the weed’s DNA molecule are deleted. Normally, mutations that result in herbicide resistance occur in a single nucleotide.