
Herbicide resistance has evolved since atrazine-resistant pigweed was discovered in the mid-Atlantic in 1972.
Two recent herbicide-resistant weeds found in New Jersey and Delmarva show how complicated a problem this is becoming for farmers, said Mark VanGessel, a weed specialist with University of Delaware Cooperative Extension.
What’s new?
Last year saw the first documented cases of glyphosate-resistant goosegrass in the region, VanGessel told a group of farmers attending Delaware Ag Week. It was confirmed this past summer in soybean and turf fields in southern New Jersey.
A summer annual, goosegrass is usually wiped out using Dual, a Group 15 herbicide. However, he said resistance to at least five different herbicide modes of action have been discovered.
"It looks pretty wimpy when it comes up. Oftentimes it looks like the plant is growing up out of the ground with one root going into the soil. It tends to fall over very quickly. It branches quite readily, even as seedlings. And then as it gets more mature, the center of the plant gets very white and gets real waxy. It grows very low to the ground," VanGessel said, adding that it can grow to about 6 or 7 inches tall.
Another herbicide-resistant weed that has been documented recently is Italian ryegrass.
“I’ve seen a lot more fields infested with it over the last couple of years," VanGessel said.
Also known as the weedy form of annual ryegrass, VanGessel said some farmers have reported poor control trying to kill it in early spring using glyphosate. But a recent study he led with researchers from the University of Maryland and Penn State shows that it might be gaining resistance to multiple herbicide modes of action.
The study involved collecting 41 seed heads from farms across the Delmarva Peninsula. Each seed was screened and tested for how it would react to certain herbicides — glyphosate, PowerFlex, Accent, SelectMax and Axial XL. The seed heads were collected from fields of corn and small grains.
All 41 seed samples died when treated with glyphosate or SelectMax, VanGessel said. But all the samples were resistant to PowerFlex. Another 25 samples were resistant to PowerFlex and Accent, and five samples had three-way resistance to PowerFlex, Accent and Axial XL.
PowerFlex and Accent are Group 2 herbicides, while Axial is a Group 5.
“So, we had cross resistance, meaning they were resistant to herbicides in the same group, and some had multiple resistance, resistant to different groups, or modes of action,” VanGessel said.
Why does it matter?
VanGessel said this highlights the fact that herbicide resistance is changing.
Target-site resistance and nontarget-site resistance are the two broad categories of herbicide resistance. Target-site resistance was discovered in the 1970s, he said. This involves a direct change in how the herbicide interacts with the weed it is targeting.
Think of it as a key (herbicide) that is no longer able to go into its keyhole (target protein of the weed). This happens, he said, because of a change in the protein binding site that prevents the herbicide from binding. There can also be overexpression of the target protein, meaning there isn’t enough herbicide to cover all the proteins.
"When we have target-site resistance, we generally have a really high level of resistance," VanGessel said. "Where the plant can withstand 10, 50, sometimes 100 times the normal use rate."
Nontarget-site resistance is a more recent phenomenon. VanGessel explains that this involves multiple processes, including reduced absorption and compartmentalization — restricting the herbicide’s ability to be toxic because of naturally occurring changes in the plant.
There is even something called the “Phoenix phenomenon,” which he said has been documented in glyphosate-treated giant ragweed in the Midwest. Simply put, the glyphosate kills the weed’s older leaves in just a few hours, but this also prevents the herbicide from translocating to the rest of the plant, so newer leaves can regrow.
But the one form of nontarget-resistance that worries him most is metabolic resistance. This involves several different processes that must come together, including the herbicide going through changes and then becoming inactive because of the plant’s metabolic process, to lead to resistance.
The challenge is when metabolic resistance shows up, it often confers resistance to other modes of action in a way that's not predictable, VanGessel said, and it can lead to multiple modes of resistance.
"And the big scary thing is, it may be leading to resistance in herbicides we haven't discovered yet," he said. "Since it is a naturally occurring process — it's a process the plant goes through to survive the environment — it's got this mechanism to deal with non-herbicidal chemicals in the environment. So, it may already be predisposed to resistance."
What does this all mean?
With no new herbicide modes of action on the horizon, VanGessel said this underscores the importance of preserving chemicals that are working.
“The message has been to use multiple modes of action, the idea being that using two different products to kill a weed makes it harder for the weed to develop resistance,” he said.
And combine it with nonchemical strategies such as using cover crops, going to narrower rows and adjusting seeding rates.
"So, tank-mixing multiple effective modes of action is our best option whether we have target- or nontarget-site,” VanGessel said. “It's just that the nontarget-site is not as long term a strategy as we would like it to be, and we need to supplement it with nonchemical strategies in order to prolong development of resistant biotypes.”
The GROW integrated weed management organization has put together a weed management planner that lists the effectiveness of nonchemical strategies on certain weeds. Find the planner by clicking here.
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