So far, Arkansas has escaped the wily weed. But glyphosate-resistant horseweed has been found in neighboring Tennessee and this “escape” doesn't mean it's time for Arkansas producers to breathe easy. In fact, it may be time to check fields with a keener eye.
Ken Smith, Arkansas Extension weed scientist, isn't terribly optimistic the state will escape a glyphosate-resistant weed forever.
“We were certainly concerned before. But since Tennessee announced it has glyphosate-resistant weeds, we've got be even more vigilant in Arkansas.”
Smith has alerted Extension agents and growers to the possibilities.
“I tell them all, ‘If you spray it and don't kill it, go out and spray again. Make sure you spray it right. If you don't kill it after the second spraying, call me. I'll be there the next day.’ We're very concerned and want to head any such problem off early.”
And it will likely happen, says Smith. “We're fairly comfortable in saying that it isn't really a question of ‘if’ it will occur in Arkansas, but ‘when.’ Of course, there's also no way to know what weed species resistance will show up in.”
Smith says because of the mode of action of the chemicals, it isn't as easy to develop resistance to glyphosate as it is to a product like Staple. But resistance — to whatever product — can happen, as has been seen in Arkansas pigweed.
Currently, about 50 percent of the pigweeds in northeast Arkansas are resistant to Staple. It didn't take long to get that resistance — about four years. Once pigweed pressure was high, producers selected those with resistance. As a result, Staple is no longer the product of choice for pigweed, having given way to glyphosate.
“We've gone from 10 to 12 percent Roundup Ready cotton in 1999 to about 75 percent this year. Pigweed is a reason for that switch,” says Smith.
Pigweed is a weeder's nightmare. Producing 500,000 seed per plant, it would take just one escaped specimen with a glyphosate-resistant mutation to cause untold headaches. That one plant, depositing 500,000 seed annually in the soil bank for a couple of years, could produce acres and acres of resistant pigweed. Thus the need to find any resistant weeds early.
“We have to catch it early; once they're out there, it's over. Once a number of seed are out, we can't go back and eliminate them. The seed may sit in the ground for 10 or 12 years before germinating.”
Whole cropping systems have been affected by pigweed.
“Buctril is a really good morning-glory product, but it doesn't have much activity on pigweed. BXN cotton in northeast Arkansas has dropped incredibly — from something like 30 percent to 10 percent. Pigweed is driving the whole scene.”
So Smith's antennae are up. His colleagues, he says, are very aware of pigweed's adaptability.
“I told a group of farmers recently that I'm dreading the day when I pick up the phone and someone says, ‘Ken, I've got pigweed I can't control with Roundup.’ If that happens, it will be a dark day. But, like I said, it may turn out the resistance shows up in some other weed species entirely. But knowing the genetics of pigweed — how rapidly it changes, crosses and the genetic diversity in the weed — that's the one most of us suspect first.”
So it's not inconceivable that glyphosate-resistant weeds are already in the state? “It's not inconceivable that there's a pigweed out there tolerant to Roundup. We don't know that for sure and haven't found it. But it's possible.”
What would be the consequences of glyphosate-tolerant pigweed?
Such a weed would impact particularly in the northern two-thirds of Arkansas, says Smith. If resistance to pigweed emerged there, it would likely “completely eliminate Roundup Ready crops. That's how serious it is. The same thing has been seen with Buctril. It isn't because pigweed developed resistance to Buctril, but because that product just isn't effective on it.”
Remember, says Smith, pigweed is a relatively new problem. Ten years ago, pigweed wasn't even on the state's most troublesome weed list. It wasn't on the radar screen, he says. Today, though, pigweed is the number one problem — particularly in cotton.
A lot of emphasis is being placed on pigweed management. “Management is a better word than control when speaking of pigweed.” Researchers are doing a lot of work on pigweed, devoting a lot of resources, says Smith.
“Right now, one of the most effective treatments for pigweed is glyphosate. As long as resistance doesn't show, glyphosate will do the trick.
“But God help us if it does. If resistance is found, the cost of raising a cotton or soybean crop will jump tremendously. The cost of herbicide inputs would more than double.”
What about general burndown recommendations in the Arkansas?
“It depends on the weeds you have. If you just have poa annua and henbit, farmers will likely just use glyphosate some 21 days out and give it time to burn on down. Or they might also use Gramoxone Max if the window is seven to 14 days. If the poa is big and tall, it's probably better to go with glyphosate.”
When cutleaf eveningprimrose is thrown into the mix, farmers must look to other products because glyphosate isn't that effective. Farmers will usually go with 2,4-D on primrose. If growers use it 21 days out, it should be pretty safe, says Smith.
“The label is a bit vague — 90 days prior to planting or until the herbicide has sufficiently dissipated not to cause crop injury. Well, that encompasses a lot. But we're fairly confident that 2,4-D can be applied 21 days out, and it'll be safe to plant cotton.”
Smith suspects that Gramoxone Max, glyphosate or glyphosate/2,4-D covers about 80 percent of the state's burndown. On some soybean ground, Harmony Extra is used, particularly if there are onions in the field.
What about any new products farmers might be interested in?
Valor, made by Valent, has a label for burndown, and Dow has brought back Delta Goal.
“Both products should see a lot of publicity. Valor is being touted as having some residual control, which it does. But Valor may not have enough residual to last far into the season. Valor has a 30-day prior-to-plant label. At the rates used for burndown, it probably has close to a 30-day residual. That means it probably won't last into the cropping season as long as we'd like. We haven't looked at Valor close enough to make any definite statements on residuals, but the product does work.”
Cotton and soybean farmers are cutting corners wherever they can. In past years, they might not have accepted a little green when they were planting. This year, many will let it slide. No one will spend any more money than he deems absolutely necessary, says Smith.
“I hear this at every meeting: ‘I can't afford a bunch of inputs in my cotton. Can I go in with two shots of glyphosate and then just leave it?’ My answer is no. Doing what the farmer suggests would probably give 50 percent weed control. Control at that level may mean a 75-percent yield reduction.
“In our normal cropping system, two shots of glyphosate just won't do the job. If farmers feel that's the only option, it would probably be better to be doing something other than planting cotton.”
This will be a tough year for input costs, says Smith. But farmers can probably do more to cut costs by being timely with applications than anything else.
“I tell growers that all the time. If my research plot needs spraying, it gets that spraying today or tomorrow. I don't wait until next week. Because of that, I can do things in my field that I otherwise couldn't. Timeliness is the most cost-effective thing producers can do.”
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