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Resistant pigweed serious, so far not tragic in West Texas

Resistant pigweed serious, so far not tragic in West Texas
Palmer amaranth, or pigweed, has been the number one weed in Texas cropland for years. Herbicide resistant pigweed was discovered last summer in West Texas. Farmers have strategies to deal with the problem.

The situation is serious but “it’s not the end of the world,” says Wayne Keeling, Texas AgriLife professor, agronomy systems and weed science, in reference to discovery last summer of glyphosate resistant Palmer amaranth in several West Texas counties.

Keeling, speaking at a Bayer CropScience Southwest Cotton Technology and Innovation meeting in San Antonio, said Palmer amaranth, or pigweed, “has been the number one weed in Texas cropland for years, at least as far back as 1994, the beginning of the Roundup Ready era.

“In 1994, we still had a lot of acreage treated with products for pigweed,” he said. “Roundup was used on very little acreage. Also, 90 percent of acreage still had a pre-plant incorporated herbicide applied and 20 percent had a pre-emergence herbicide. Only 1 percent used post-emergence treatments but about 40 percent was spot treated.”

Keeling said farmers were still heavily reliant on cultivation. “Some 98 percent of the acreage was cultivated 3.1 times per season,” he said.

“We were using a lot of Atrazine but silver leaf nightshade was a problem. With Roundup, we cleaned that up.”

Keeling said discovery of resistant Palmer amaranth is particularly disturbing because it infests so many acres throughout Texas. “It’s in all cultivated cropland,” he said.

Keeling explained that widespread use of glyphosate has not created a super plant that’s resistant to the chemical. Resistant weeds, he said, are “naturally occurring plants with an inherited ability to tolerate herbicide.”

Widespread use of a particular herbicide will kill the susceptible weeds but leave the resistant ones to go to seed and spread. And Palmer amaranth can spread rapidly.

Resistance found

“We had no documented evidence of Roundup-resistant Palmer amaranth until we saw it last summer,” Keeling said. “We identified pigweed that was unaffected by Roundup applications.”

Keeling and others collected seed from the suspected resistant plants, planted them in a greenhouse and sprayed emerged weeds with various rates of Roundup, some at high rates. “We had survivors. We sent some samples to Monsanto for further analyses and third-party confirmation,” he said.

Monsanto technicians grew the seeds, treated the plants and compared them to known susceptible and known resistant checks.

“Terry County samples were similar to resistant checks from Georgia,” Keeling said.

The team collected more samples last fall from 13 locations in Terry, Hockley, Hale, Briscoe and Swisher counties. “Of 12 samples, 8 had some level of resistance,” Keeling said.

As scientists and farmers look into 2012, Keeling said they may wonder if they are looking “at the tip of the iceberg or about all the problem we’re going to have.”

He said most of the resistant samples came from fields where producers had over-used glyphosate and did not use a residual herbicide. “But in some fields farmers had done the right things.

“This is bad news,” Keeling said, “but it’s not the end of the world.”

Now that resistance has been detected, Keeling recommends farmers take even more preventive measures. He said farmers have several ways to prevent (delay/minimize) development of glyphosate resistant weeds and improve weed management.”

Using herbicides with multiple modes of action is a key element in preventing herbicide resistance, he said. He also recommends using a soil-applied herbicide, post-emergence tank mixes, sequential systems and some cultivation. He also expects new transgenic technology to help manage herbicide resistance.

Keeling said the dinitroanaline pre-plant incorporated herbicides such as Trifluralin and Prowl are good options to control annual grasses and small-seeded broadleaf weeds. “Incorporate with tillage or irrigation. Rates vary with soil type.”

He said it makes sense to control most weeds “before they come up, and use the highest rate allowed.”

Pre-emergence herbicides also have a place, Keeling said, but use has decreased since Roundup Ready technology was introduced. “Pre-emergence use dropped from 20 percent to almost zero,” he said. “But some of these materials have a lot of value.”

He said such herbicides as Caparol, Direx, Cotoran, Dual Magnum and Staple LX may need more attention. He said the pre-emergence materials have a broader spectrum of control than the dinitroanalines. Different products may have a different niche. Dual Magnum is good on yellow nutsedge and grasses, for instance. He cautioned cotton farmers about Staple LX. “You may have potential residue and rotation problems with sorghum or corn.”

Post-applied residual herbicides also could play a role in weed control strategies. He said post-emergence topical applications of Staple LX, Dual Magnum, Prowl H2O, Warrant or Envoke are options. Tank-mixes with glyphosate may help control pigweeds. Staple LX and Envoke “improved post and residual morningglory control,” Keeling said.

Residual herbicides such as Caparol, Direx, Cotoran and Layby Pro may be applied alone or tank-mixed with glyphosate for residual pigweed or morningglory control or post-emergence control of morningglory.

GlyTol and Liberty Link cotton also offers potential for improved herbicide resistance management, Keeling said. GlyTol cotton is resistant to both glyphosate and glufosinate herbicides.

Keeling cautioned farmers about using Liberty and Roundup together. The combination, he said, has an antagonistic effect and actually reduces control. A better option is to use them in sequential applications, depending on which weeds emerge first.

“If morningglory is the first problem, apply Liberty first and then get pigweed with Roundup.”

Keeling said a lot of questions remain about glyphosate-resistant pigweed on the Texas Plains. “How widespread is it?”

He said no one knows yet. “But we impress on farmers to get back to different modes of action and don’t rely solely on Roundup. Combine pre-plant incorporated, pre-emergence and post-emergence chemistry and consider some cultivation.”

He said zero-tolerance for glyphosate resistant Palmer amaranth escapes is also a reasonable goal.

“This is a problem, but we have the systems and the strategy to deal with it.”

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