Weed escapes could signal herbicide resistance

Genetically engineered crops take a lot of the work out of producing a crop, but the technology also requires that farmers closely monitor their crops for signs of resistance.

For Extension weed scientist Andy Kendig, with the University of Missouri's Delta Center in Portageville, weed resistance in herbicide-tolerant soybeans are at the top of his list of concerns, although he hasn't seen any problems to date.

“I don't worry as much about herbicide-resistant cotton because I think there is some need for residual herbicides,” Kendig said. “So there is a natural mechanism to get some other herbicides into the program. In herbicide-resistant corn, we still have atrazine and it's a little cheaper than Roundup.”

But in soybeans, many growers have gone exclusively to Roundup Ready technology, which to a weed scientist means heavy selection pressure and a higher probability that weeds will develop resistance.

Kendig has been impressed with grower awareness of resistance issues in the Bootheel, but he's worried that there's not enough attention paid to rotating chemistry. And there's good reason.

“Right now the economic situation is so bad that growers may be aware of resistance but can't afford to rotate to another crop with a more expensive weed control program.”

Another option, and arguably a cheaper one, is to go back to traditional chemistry and tillage in soybeans when and where possible. “We used to be able to control weeds before we had Roundup,” Kendig said. “And most of the time, if we do the arithmetic, conventional beans come out a tiny bit cheaper basically because of the seed costs.”

Kendig admits it's tough for grower to “bite the bullet” in that situation. “With glyphosate, the benefit is convenience more than weed control. We've also talked a lot about the benefits of no-till, but if there's ever an opportunity to go back to planting beans in rows and cultivating, we could consider that.”

Kendig suggests that to manage resistance in soybeans, “work with a consultant or agronomist to make sure you've got all bases covered. Tank mixes are good, but just because you're mixing another herbicide with glyphosate doesn't mean you're doing a whole lot to prevent resistance.”

Kendig explained that tank mixes or pre-emerge herbicides “have to target the weeds they need to target. If not, you're not doing anything to prevent resistance.”

Resistance management extends to conventional weed control programs in crop production, too, notes Kendig. “Since we've been talking about the possibility of resistance in glyphosate, we've become aware of Facet-resistant barnyardgrass in Arkansas. So overuse of any product can lead to resistance. We need to keep our eyes open.”

How does resistance set in? “Past experience on Scepter-resistant cocklebur tells us that the resistant plants can be out there in a really small number,” Kendig said.

“If you happen to have a field that had no Scepter-resistant cocklebur, you could probably use Scepter forever and probably never get resistance.

“But if you have the resistant types out there, the first year you use the chemical you've at least started the ball rolling. What we found out with Scepter was it took about three years for those plants to multiply into a problem.”

Kendig suggest that growers “keep your eyes open for any patch of weeds that should have been controlled. If it's there two years in a row or seems to have spread, call somebody to have a look at it.

Clearfield rice, which confers resistance to an imidazolinone herbicide, also belongs in the resistance discussion although the mechanism for resistance is cross pollination with closely related red rice plants rather than selection pressure.

“We've got a lot of red rice up here and we have a lot of interest in the Clearfield rice. It wouldn't surprise me if there was some out-crossing.

“Manage the technology exactly according to directions. Watch and rogue any escapes. Strive for 100 percent weed control. And after you've grown one crop of Clearfield, you should rotate that field to soybeans for at least one year and use a red rice control program in the soybeans.”

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