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Arkansas fields of glyphosate-resistant pigweed

As Roundup Ready crops have come to dominate Mid-South agriculture, so have glyphosate-resistant genes come to increasingly dominate a handful of problem Mid-South weeds. Currently, Palmer amaranth, a pigweed, tops the list of worries.

It was mostly that weed, along with the cropping systems and treatments around it that Christopher Preston, an Australian, was in Arkansas to tour in late August. Preston, a weed specialist and associate professor at the University of Adelaide, made the trip at the invitation of the University of Arkansas.

Preston is “well-known and entirely respected in the world for his work in weed resistance issues,” said Bob Scott, Arkansas Extension weed specialist and Delta Farm Press contributor. “We hope he can provide some insights into what’s happening here that can lead to things that’ll help our growers.”

After a trip through Arkansas row-crop land, Preston recently spoke with Delta Farm Press from Fayetteville. Among his comments:

Your initial impressions of the Arkansas weed situation?

“In terms of glyphosate-resistance, Palmer amaranth is clearly the worst issue farmers will have to deal with. And that’s out of all the weeds that have so far gained some sort of resistance. That’s simply because there’s so much of it and the control options are reasonably, although not totally, limited.

“The worst case I’ve seen of the problem was the original infestation in Georgia. I visited there in 2005 and that situation had been allowed to get out of control.

“What I’ve seen in Arkansas is a really good attempt to control the problem once it had arisen. It hasn’t been totally successful, but at least a farmer can put a combine through his soybeans.”

On resistance issues in Australia…

“We don’t have Palmer amaranth.

“The big glyphosate-resistant weed in Australia is annual ryegrass. We also have glyphosate-resistant barnyardgrass — in your terms it might be called ‘jungle rice.’

“The ryegrass was originally planted across the country for sheep feed. When cropping intensified, farmers wanted to get rid of the ryegrass and used herbicides to do so.

“Of course, when you treat large numbers of weeds across a large area with a lot of herbicides, you’ll end up with herbicide resistance. And that’s almost the same story with Palmer amaranth here.”

On parallels between the two nations’ resistance problems…

“The most important parallel is where we saw the glyphosate-resistant ryegrass — and where it still occurs there — are situations where the only thing being used to control the weed is glyphosate. The producers aren’t using tillage, not using other herbicides. Only glyphosate is used.

“Well, do that for 10 years, or so, and you’ll end up with glyphosate resistance as we saw with ryegrass.

“That parallel is almost identical with the U.S. pigweed. Farmers here have used Roundup Ready cropping with glyphosate as the only herbicide while pulling back on tillage. In that system, the only thing that’s really controlling the weed is glyphosate. Do that for a decade and you’ll have problems.”

What weed research is going on in Australia that might translate here?

“We have a number of situations. Where we’re controlling the problems best, we’re using competitive crops and alternative herbicides. We are still using glyphosate, but residual chemistries, as well.

“We’re also employing seed-set control as part of the rotation. Producers actually stop plants from setting seed.

“For us, those are the keys.

“What can you guys learn from us? I think competitive crops can help. If producers have a problem with glyphosate-resistant pigweed in a crop that is relatively uncompetitive like cotton, one of their options is to go with a crop with more herbicide options.

“As for seed-set control, I don’t know it will be quite as easy here in the States. I don’t know the tools are available that we have.”

Please expand on that a bit — what tools?

“Some Australian producers might normally grow wheat. But instead, of that they’ll grow peas or lentils. Then, they’ll use a paraquat-based herbicide to stop seed-set of the ryegrass late in the season.

“While that might cause a bit of crop damage, they accept that as part of the weed management strategy.

“Another thing, some producers — those with only a small area of glyphosate resistance — are killing crops and weeds just to stop the ryegrass from setting seed. They manage the rest of the field normally. But where the problem is, they run the seed bank down to reduce the weed.”

So ryegrass growth isn’t staggered? You can get it in one swoop?

“That is an advantage we have. And that’s one reason I’m not sure it would work here. Most of the ryegrass in Australia does go to flower at around the same time. If you control the first flush of flowering, there is very little seed produced by later-growing weeds.

“From what I’ve seen of palmer amaranth, though, I don’t know how you’d employ that technique here.

“The other things we’re doing — crop competition, residual chemistries, alternative chemistries — could certainly be used here.”

On general crop conditions in Australia…

“A lot of the country has been in a long-term drought. It seems to be breaking, although we probably won’t know until next year whether it’s truly broken. We’ve had some reasonable rains across the drought-stricken (parts of the country), although some areas have missed out.

“Some areas are fine for moisture, some only need a rain or two and some are so parched a crop wasn’t even planted. This hasn’t been the best year for cropping. Fortunately, grain prices remain high so producers that have crops can stay afloat.”

On research conducted by Preston…

“There are two research programs I’m involved with. (Among the question we’re trying to answer are) what are the various things we can do to control these weeds? What is causing the resistance? How can we change our system to reduce the incidence of glyphosate resistance?

“In some parts of our cropping system, that’s relatively easy and straightforward. Other parts have proven to be a lot more difficult. I’d liken some of those things to your Palmer amaranth problem: a limited number of options and getting producers to accept those options can be difficult.

“Other research we’ve been doing is with pre-emergent chemistry in ryegrass management. Again, there are probably some lessons you might learn in dealing with pigweeds. We’re finding that — especially once post-emergent chemistry is lost — pre-emergent chemistry becomes really valuable in managing weed populations.

“We’re trying to figure out how best to use those chemistries to maximize benefit and minimize problems.”

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