We know why weed resistance occurs, and we know it is happening now, so why aren’t we doing a better job of delaying it?
That’s the question asked by University of Georgia Extension Weed Scientist Eric Prostko during the recent Beltwide Cotton Conferences in Atlanta.
“We know we get weed resistance due to the repeated application of herbicides with the same mode of action. This occurs more in annual weeds that have high populations or are highly distributed and are prolific seed producers, such as Palmer amaranth pigweed,” he says.
Everyone at the Beltwide and at other conferences and meetings is talking about resistance, but it’s not a new phenomenon, says Prostko. “It has been happening since 1968. My point is we know why it happens, and we know that it does happen, but it’s not getting any better. So why can’t we do a better job of delaying it? We can’t prevent it because we can’t prevent what Mother Nature is going to do,” he says.
One of the hindrances to resistance management is economics, he says.
“A lot of growers aren’t thinking long-term, they can’t. They’re thinking about how they’ll make it to next year and pay the banker,” says Prostko.
Another hindrance has been reluctance to change, he adds. “Think about what has happened from about 1997 to 2004. Weed control became very easy and very cheap or at least relatively cheap. Now we’re talking about going back to some things we used to do, and that’s significant. We’ve expanded acres and people don’t cultivate anymore, so to go backwards is a hard pill to swallow.”
There’s also denial about having or ever getting the problem, says Prostko. “There’s also a belief out there that there’s a new herbicide on the horizon, but there’s not, at least not a new mode of action that will give us some relief from the load that we’re putting on current herbicides.”
In Georgia, he says, resistance management is taking several forms. “We hear a lot about crop rotation, and we know when you rotate crops you can do other things. If you look at the data, cotton is the least competitive crop with Palmer amaranth. Going to a crop that is more competitive is one reason for going with a rotation, along with some of the other benefits you receive.”
Tillage and cultivation also are receiving more interest, he says, maybe not plowing every year but at least once every three or four years.
“We’re also doing some extreme cover crops. This is where a grower will plant a rye cover crop in the fall and manage it to get to the maximum height and then terminate it in the spring with herbicides and rolling, and then strip-till into it. That cover crop has a suppressing effect on Palmer amaranth, but that’s not something everyone can do.”
Row spacing is another factor, says Prostko. “While we don’t grow a lot of soybeans in Georgia, we still grow a few, and we plant some of those in wide rows. Palmer amaranth doesn’t like shade, so we know if we go to a narrower row, we can pick up some benefits.”
Everyone knows by now, he says, that we can’t rely strictly on herbicides. Other things must be done in the battle against resistant Palmer amaranth pigweed.
“One of my responses to what can we do about Palmer amaranth is to dig a well,” says Prostko. “We need residual herbicides, and the only way we can get them to work is with water. In Georgia, about 50 percent of our cotton and 50 percent of our peanuts are irrigated. We should be able to do a better job.”
Modes of action can be a scary term because it has to do with chemistry, he says, and no one likes chemistry, but you don’t have to be a scientist to understand what it means.
“Some herbicide labels will have it clearly on the label. The University of Georgia has put together a master list of herbicides with their common name, trade name and mode of action, as well as putting the modes of action in our recommendations.
“We also want to encourage growers to put together a long-term resistance plan or mode of action plan. This is where we write down the rotation or the sequence and fill in the blanks with the herbicides you’ll use. When you come to the end of the column, you can figure out how many modes of action you have, and you can see where you’re making two or three applications of the same mode of action.”
Other things to be considered are tank-mixtures or pre-mixtures, says Prostko. “For these mixtures to be most effective in delaying the evolution of resistance, ideally they must have different sites of action, they should have similar efficacy and persistence but also different propensities for selecting for resistance or different modes of action.”
Many say that residual herbicides are the key to survival, and that every acre that is planted should have a residual on it, he says. “As a weed science community, we should never promote the absence of residuals. And in a lot of cases, one is not enough. You might use two or in cotton you might need three to do the job.”
More emphasis should be placed on removing escaped plants so they don’t put seed back into the field, says Prostko. Also, in Georgia, there has been a significant increase in the interest and use of non-selective applicators.
“We’re not actively promoting these with cotton and soybeans, but we are with peanuts. We feel we can get the height differential in peanuts to give us the kind of control we need where we can’t consistently with other crops. There’s still a lot of work to be done.”
And of course, hand-weeding is another option, he says.
As for post-harvest treatments, some of this has been done previously in Georgia for tropical spider wort, he says.
“We’ll start harvesting corn at the end of July, early August, and we might not get our first frost until mid-November. If you look at the published data, pigweed can go from seed to seed in 35 days. We have ample time between corn harvest and first frost for another crop of pigweed to become established. So we have to start doing something about that if we’re going to get a handle on it.”
But it depends, he says, upon the size of the weed, and if you’re going to plant a small grain. “I don’t think we can get very effective control if a pigweed is bigger than 6 inches. Gramoxone, 2,4-D and dicamba have been used quite a bit. You can put out a residual in the fall, but I wouldn’t if I planned on planting a small grain in November.”
Georgia growers can finally stop worrying about Palmer amaranth at about the middle of October, he says.
As for the future, Prostko says he’s extremely concerned about developing resistance to the PPO herbicides.
“In Georgia, we’re using a lot of Reflex and Valor, and we’re extremely concerned about keeping the longevity of those products. Some weeds have already developed resistance to this mode of action, including two species of pigweed. It will happen, and it may be out there already and we haven’t seen it. You could conceivably use 16 PPO herbicides in four years — that’s definitely a bad idea.”
He’s also concerned about the abuse of Ignite.
“Abuse is a harsh word, but the weed science community was not harsh enough in the case of Roundup Ready. I want to be extremely harsh and say we’re abusing Ignite.
“In some cases, we’re seeing three applications made on crops, and that’s a recipe for resistance. We already have resistance to Italian ryegrass and goosegrass to Ignite. It will happen. I hope we learned our lesson from what happened with Roundup.”