Rotation crucial for weed resistance

Weeds resistant to herbicides, especially glyphosate, have not become as big a problem for Southwest growers as they have for farmers in the Southeast and Mid-South — yet. But farmers can’t let their guard down, says Paul Baumann, Texas AgriLife Extension weed specialist.

Baumann, speaking at the Blackland Income Growth conference in Waco, discussed potential for weed resistance as well as new options on the horizon for weed control in grain sorghum.

“We have not been hammered by resistant weeds like the Southeast has,” Bauman said. “We’re still using rotation and alternate herbicides and where farmers are planting cotton on cotton, they’re using soil applied herbicides.”

Still, a few problems are developing. He said common waterhemp “is spreading rapidly from the Gulf Coast. It looks a lot like a regular careless weed, but it is harder to control. We have to get it earlier.”

He said farmers are identifying waterhemp resistant to Roundup and ALS herbicides, which include many of our new low use-rate products. “We need to change herbicides and select a different mode of action,” he said. “And rotate now, before you see resistance show up.”

Baumann said one waterhemp plant can produce more than 200,000 seeds. “That can supply seed for years.”

He said the Southeast has been hard hit by weed resistance. “Hundreds of thousands of acres have glyphosate resistant Palmer amaranth.” That should concern Southwest farmers but not cause panic, he said. “Palmer amaranth pigweed is the most common weed in Texas crops.”

The economic advantages of using Roundup Ready varieties and the convenience of using only one herbicide for control of most weeds could create trouble. “Use other products, too,” he said.

Baumann said weed management in grain sorghum has been a thorny issue for decades. Farmers had to rely on pre-emergence herbicides or hooded sprayers for in-season control of grass weeds. “Residual herbicide options for broadleaf weeds have been limited.”

Grain sorghum hybrids with herbicide resistant traits also have not been available. That situation is changing with new technology coming out of Kansas State University and Pioneer /DuPont, Baumann said.

KSU weed scientist Kasim Al Khatib screened wild sorghums to identify natural resistance to herbicides, particularly ALS and ACCase herbicides.

“He’s found resistant genotypes,” Baumann said. He said the resistant varieties may allow growers to use ALS herbicides over the top of grain sorghums that contain the resistance trait. “There are a lot of ALS inhibitor products available,” he said. Those include Accent, Basis, Resolve, Steadfast, Beacon, Pursuit, Ally, Glean, Finesse, Classic and others. Most of these are effective at low rates on several weed species.

Baumann said the materials do not work the same as contact materials and may take two to three weeks for results.

Al Khatib also has identified wild sorghums with ACCase inhibitor herbicide tolerance. These herbicides include the so-called “fop” herbicides Fusilade (fluazifop), and Assure II (quizalofop), which are postemergence grass herbicides. Johnsongrass would be a prime target.

“The bottom line is that if farmers are not growing grain sorghum because of trouble controlling Johnsongrass, these hybrids will allow them to plant sorghum and control grasses with these products,” Baumann said.

He said a systems approach, including a pre-emergence treatment, may be the best option, depending on the weed species present. “Farmers will be able to control troublesome grass weeds post emergence and selectively,” he said. “This provides a reasonable alternative to other in-season grass control options including cultivation and direct spray with Roundup and a hooded sprayer.”

The new herbicide resistant grain sorghum hybrids also provide another tool in managing weeds resistant to glyphosate.

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TAGS: Management
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