Herbicide resistance in weeds isn’t exactly a new topic. But there’s some interesting news emerging about how weed species are becoming resistant.
A new study co-authored by University of Illinois weed scientist Patrick Tranel shows that Palmer amaranth populations in three states are now resistant to PPO-inhibiting herbicides (PPOs).
This follows PPO resistance that developed in another pigweed, waterhemp, in soybeans during the early 1990s — which took a lot of farmers by surprise because they had been using PPOs in the soil as a pre-emergence treatment along with glyphosate.
What’s interesting, says Tranel, is how this resistance develops. The genetic code of an organism, which determines all of its physical traits, is housed in its DNA in molecules known as nucleotides. Normally, mutations in genetic sequences that give rise to herbicide resistance happen at the scale of a single nucleotide.
In PPO-resistant waterhemp, however, Tranel's group found a different mutation. Instead of a change in a single nucleotide, they found the deletion of three nucleotides.
The mutation probably happened, he says, because the sequence of three nucleotides was repeated, and this repeat just happened to be in the right place in waterhemp's genetic code.
When they looked at the genetic code for Palmer amaranth, they found the repeat and predicted, correctly, that they'd eventually see Palmer amaranth developing resistance to PPOs.
That isn’t good news for growers in places like the Cotton Belt in Texas, which is experiencing some serious problems with Palmer.
“We predict PPO resistance is going to spread very rapidly in Palmer, like it did in waterhemp,” Tranel says. “If a farmer has Palmer, they should not rely on PPO as an effective herbicide. It might only work for a couple of years.”
In fact, when Tranel and his colleagues tested the effectiveness of the PPO inhibitor fomesafen on an Arkansas population of Palmer amaranth, they found that each successive generation had a higher frequency of the mutation and required more and more PPO to be applied to achieve a 50% growth reduction.
For the time being, he suggests farmers use as many different pre- and post-emergence herbicides, with as many modes of action as possible, in every field in every year.
But as you’ll read later in this e-newsletter, there might be some other solutions as well.
Researchers in Tennessee investigating different combinations of cover crops and pre-emergence herbicides to fight early-season Palmer concluded a grass and legume cover, along with fluometuron or acetochlor, reduced emergence of glyphosate-resistant Palmer amaranth by half.
Also in the video below, you can hear what Australian herbicide resistance expert Stephen Powles had to say about the current state of herbicide resistance across the globe and how farmers can get the most out of new technologies.