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Timely herbicide applications, management limit weed escapes

An unusually wet spring and soggy growing season could have spelled weed control problems for West Texas cotton farmers in 2007, but timely use of old standby herbicides, plus success of the Flex and Liberty Link systems prevented severe infestations.

“In general, wet years promote numerous and potentially heavy weed flushes,” says Peter Dotray, a weed scientist with a joint appointment with Texas A&M and Texas Tech, in Lubbock. “We saw some early weed problems but most growers still take advantage of the opportunity to use “yellow” herbicides, including Prowl and Treflan, in cotton. If growers did not use a pre-emergence herbicide prior to planting or at planting, they likely had some trouble keeping fields clean with post-emergence applications only.”

Dotray polled growers during several recent crop tours this fall and found that only a small number have gotten away from using yellow herbicides, even if they switched to a Flex or Liberty Link system.

“For the most part, growers used residual herbicides and that helped tremendously, in a wet year like 2007.”

Dotray says Palmer amaranth (careless weed) still poses the biggest weed control concern for West Texas cotton growers. “Fortunately, we have good tools to control it. Annual morning glory is another concern and it seems to be developing into a more serious problem year after year.”

He says weeds such as Russian thistle (tumbleweed), kochia and marestail (horseweed) were less trouble than expected, especially with a wet spring. He says woollyleaf bursage (lakeweed) and silverleaf nightshade (whiteweed) have become less of a problem in the last few years. “We still see them but we are gaining the upper hand.

“The last few years farmers have done a good job controlling perennial weeds,” he says. “They've been able to use Roundup (glyphosate) in-season. A fall application followed by some tillage and then Roundup in-season has become a good weed management program for perennial weeds.

Dotray says Roundup Ready Flex and LibertyLink cotton have proven useful tools in weed management strategies.

“We've looked at LibertyLink now for several years and recommend farmers follow several key application techniques:

  • They have to target weeds when they are small (see label for restrictions).

  • They have to assure thorough coverage and use the full-label rates.

  • Temperature and humidity are important and it's best to apply Ignite either early in the morning or late in the afternoon when humidity is higher and temperatures are not quite so hot.

“If farmers do not follow the recommendations they may not be happy with the results,” Dotray says. “Growers who use Ignite as they would Roundup may not be satisfied.”

He says the LibertyLink system provides an excellent tool for growers and offers season-long application options but may be “less forgiving than Roundup Ready Flex.”

Carrier volume also makes a difference. “We recommend 15 gallons of water per acre, minimum, with Ignite,” he says. “Some growers who have used 18 to 22 have reported improved weed control compared to 15 gallons. If growers accept that and follow that recommendation they'll be much more satisfied than if they use only 10 or 12 gallons. They have to follow the specifications.”

Dotray says 2007 was the first full-scale test for the Flex system. “It worked beautifully. Growers made the first application of Roundup early, somewhere around the four to six leaf stage.”

They used several different systems to make a second application. Most likely they sprayed over the top, but some used a direct spray technique — shielded sprayers, hooded sprayers or drop nozzles.

“They wanted to put the spray under the canopy to get the next weed flush,” Dotray says. “And some added other herbicides (such as Direx) to Roundup to get residual activity. Either way, weed control was excellent. The way growers applied Roundup to Flex cotton may have been a little different and we're still learning how to use herbicides in a Flex system.”

He says his crop tour polls indicate growers are aware of the potential for weed resistance to occur. “They hear about it, read about it and are thinking about it. If weed control is a bit off they wonder if it's resistance.

“For the most part, we do not see resistance to glyphosate, but it's good that growers are aware of the potential. They are following good herbicide resistance prevention strategies, using multiple chemistries with different modes of action. Many continue to use preplant herbicides (2,4-D, Valor, Treflan, Prowl) and some are adding other materials at planting (Caparol, Staple, Direx, Cotoran). They also have options at the postemergence application timing (Staple, Dual), and at layby (Layby Pro, Caparol, Direx). They are aware of resistance and are trying to do what they can to prevent it. We know it has occurred elsewhere and keep watching for it.”

Dotray says Kansas has identified herbicide resistant common ragweed and California has confirmed resistant hairy fleabane. “We know resistance is happening,” he says.

Isolated pockets of marestail that may be resistant to glyphosate have been identified north of Lubbock. “But marestail has an apparent natural tolerance to Roundup. An application of 2,4-D in the spring will prevent a mess at and after planting.”

He says folks in South Texas are watching water hemp that “is not responding to Roundup. They have not confirmed resistance, but are concerned and are working to identify the problem.

“We don't see much water hemp in West Texas and we have not seen resistance yet. But we are committed to doing the best we can to prevent it.'

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