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Managing resistant weeds demands systems approach

Resistant weeds are putting farmers out of business. Mississippi specialists have identified as many as eight weeds that are resistant to glyphosate. Overuse of good products exacerbates the problem.  

Resistant weeds are putting farmers out of business.

In some cases, lack of adequate weed control from usual programs results in lost yield, in some cases loss of an entire crop as weeds overwhelm fields and out compete cotton or grains. In other cases, landowners may take farms away from producers who do not manage herbicide-resistant weeds adequately, says Tom Eubank, Mississippi State University weed specialist.

Eubank discussed “a systems approach” to herbicide resistant weed control during the annual Agriculture Technology Conference in Commerce, Texas.

Eubanks, who has had significant experience with herbicide resistance in Mississippi, warned Texas producers to take precautions to prevent severe monetary losses to the devastating weed species.

“Fifteen years ago we witnessed the introduction of the Roundup Ready era,” Eubank said. “Many growers today don’t know anything but glyphosate. And it has significant advantages, including: spray weeds only when needed; no crop injury; increased efficiency allows growers to cover many acres; and it is cheap.”

But there is no perfect herbicide. “With resistance, we are seeing old practices become new again,” as farmers try to find ways to keep crop-limiting weed infestations under control. He said turning plows are showing up in Delta fields. “They try to bury seed too deep to emerge. We’re also seeing more farmers chopping weeds.”

Mississippi specialists have identified as many as eight weeds that are resistant to glyphosate. “Goosegrass is the most recent,” Eubank said. “Within eight years we went from zero weeds resistant to glyphosate to eight resistant.”

Italian ryegrass also creates problems, and not just with glyphosate resistance. “Italian ryegrass causes serious competition problems with grass crops in Mississippi,” he said. “It is extremely competitive in corn, grain sorghum and wheat.”

Overuse of good products exacerbates the problem. “The Department of Transportation, for instance, uses some of the same products that farmers use. That’s one reason we could be seeing resistance develop so fast. A lot of products now offer little control of Italian ryegrass.”

Products such as Hoelon are not as effective as they used to be. “We’ve seen a lot of ALS resistance. We have limited options for control of Italian ryegrass in winter wheat.”

He said product such as Command, Zidua and Dual Magnum offer 90-day control. “Very few offer control that good. Post-emergence, glyphosate is a good option.”

Farmers should develop specific weed control programs for specific crops—both fall and spring applications. For wheat, a fall application of Axiom and a spring treatment with Axial XL seems to be the best option for resistant Italian ryegrass.

Exposed soil offers an avenue for pigweed to take hold. “Ryegrass leaves bare ground,” he said.

Palmer amaranth has attracted significant attention in recent years. “A lot of research money is going into managing this weed,” he said. “We first documented resistant Palmer amaranth in Coahoma County, Miss. Then we saw it in Tunica and in just a few years we had it over nearly half the state. We’ve also identified ALS resistant pigweed.”

Residual herbicides will be the “foundation of control” for resistant Palmer amaranth, Eubank said. “That makes a significant difference in control. Some of the best, most popular herbicides are producing no more than 88 percent control. That’s not good enough.

“It takes a program, including preplant incorporated herbicides, pre-emergence herbicides and postemergence herbicides. Also, we can use Liberty Link seed. We can still manage weeds, but the key is that we have to manage them.”

With Liberty herbicide, “timing is critical. It offers an alternate chemistry, over-the-top. Hit weeds when they are small.”

He said some new technology is coming with 2, 4-D and Banvel tolerant crops. “Use those products on small weeds, too, 2 inches tall. At 6 inches, control is not acceptable.”

Eubank said these new products and new technology are tools, just like the old standards, and will need to be incorporated into a comprehensive weed control strategy. “These are not stand-alone products.”

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