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Weed resistance: Alternate chemistries best weapon

Weeds resistant to various herbicides will continue to pop up in Texas cropland but they need not cause producers to panic, says a Texas A&M weed specialist.

Paul Baumann, speaking recently at the annual Ag Tech Conference in Commerce, said herbicide resistant weeds have been showing up since the 1970s but recent technology, specifically herbicide resistant crops, which support use of fewer products, may increase the likelihood.

In some cases, “field crop economics support use of only Roundup Ready crops,” Baumann said. “It's an economical, highly effective weed control strategy but presents pressure to select for resistance.”

He said thousands of acres in the Southeastern United States already have problems with glyphosate resistant Palmer Amaranth (pigweed), which is the most commonly occurring weed pest for cotton growers in many parts of Texas.

Potential exists for that and other resistant weeds to spread. The case for spread includes the way we use glyphosate, he said. “It is highly effective and used several times in a season. We also have slow development of new alternative chemistry from industry.”

Multiple crops with glyphosate resistance also may encourage weed resistance.

But Baumann makes a good case against widespread resistance. “Roundup has no residual soil activity,” he said. “Also, a good rotation program to non Roundup Ready crops, wheat, for instance, discourages weed resistance. And we have effective, alternative herbicides available. Our best weapon is to alternate chemistries with different sites of action or add them to our program.”

Baumann said farmers should develop production programs to prevent or at least delay weed resistance. That program should start with chemistry rotation. “We also need to consider adding soil-applied herbicides in Roundup Ready crops.”

He said chemicals with a different site of action include Ignite and Liberty Link cotton. “Other options depend on the crop, but Treflan, Prowl, atrazine, Caparol, Direx, Dual, Staple, Envoke, Permit, Aim, Suprend and Valor are possibilities.

“Consider tillage, especially before or after planting. It may pay to do some tillage if resistant weeds are present. Also consider switching to conventional varieties in fields with resistant weed populations.

“Eliminate suspected resistant weeds immediately. One surviving weed in 10 acres produces a lot of seed. Kill it! Stop additional applications of a susceptible herbicide if you suspect resistance.”

Baumann said growers who think they have a resistance should clean all tillage and harvest equipment thoroughly to prevent spreading seed from one field to another.

“Also, make certain suspected resistance is not a performance problem. Evaluate the field to see if other weeds normally controlled are present. If you see pigweed and also annual grasses, it's probably a performance problem.”

He suggested looking for patterns of poor control. Skips may indicate a stopped up spray nozzle instead of weed resistance.

“Also, make certain the material was applied at the appropriate rate and at the correct time. That's especially important with glyphosate. Read the label. Controlling pigweed, for instance, is much easier when the plant is less than 10 inches high. Timing depends on the weed species. Optimum control height may vary with different species. For example, controlling waterhemp (which looks like your average pigweed) is more difficult to control than other amaranthus species.

“Climatic conditions also affect efficacy. Lush, fast-growing weeds in June will kill easier than a weed under stress later in the season.”

Baumann said volunteer Roundup Ready corn and cotton have become new weed problems. He said a number of products will control volunteer Roundup Ready corn but not at minimum rates. “It's tougher to control volunteer corn that sprouts from cobs,” he said.

A lot of cotton in South Texas was “zeroed-out last spring during a prolonged drought. When rains came, cotton seed sprouted.” He said controlling the volunteer cotton chemically was necessary to conserve limited moisture.

“We have a number of products that provided a quick burn down, including Gramoxone Max, Valor and Ignite.”

Baumann said volunteer Roundup Ready corn also presents control problems in Roundup Ready cotton ad will require use of additional herbicides.

“We have several herbicides that work well and are economical,” he said.

Baumann said farmers may see resistant weed species in different crops. “But we have tools to fight resistant weeds, including chemical, mechanical and cultural methods. Growers may need to alter some production practices instead of using the same herbicide over and over.”

Baumann said growers should be “aware of the potential for resistance but not scared of it.”

Resistance potential is relative to the practices producers employ now to prevent its appearance in their fields, he said. “If they do nothing, resistant weeds can show up quickly and have an immediate impact.”

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