Weed scientist evaluating new herbicide systems

Olivia McClure, Gould, Frances I., Blanchard, Tobie M.  |  11/9/2017 3:12:26 PM

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LSU AgCenter weed scientist Daniel Stephenson holds a ragweed parthenium plant at the annual field day at the Dean Lee Research and Extension Center in Alexandria. Photo by Olivia McClure

Three new herbicide systems — including one now commercially available and two expected to be released soon — could give Louisiana farmers some much-needed tools to fight the resistant weed populations they have struggled with in recent years.

But LSU AgCenter weed scientists warn that the new products, which they have been evaluating in field trials, will only be helpful in the long run if they are used carefully and in combination with other herbicides and good management practices.

Among the most difficult to control weeds in Louisiana and many other states is Palmer amaranth, a prolific seed-producing plant also called pigweed that often cannot be killed with glyphosate-based herbicides. Glyphosate is an ingredient in some of the most popular herbicides. Farmers have relied too heavily on the chemical, and weeds such as Palmer amaranth, johnsongrass and Italian ryegrass have developed a tolerance for it in the past few years.

Those weeds can be controlled with other herbicides, such as dicamba and 2,4-D. The problem is that herbicides also can kill crops, so they cannot be applied on weeds that pop up during the growing season — unless they are used as part of a system with crops containing genetic traits that safeguard them from a certain herbicide.

Glyphosate is used in the Roundup Ready system. The Xtend system, which includes dicamba and soybeans that can tolerate dicamba, recently came on the market. A similar system, called Enlist, uses 2,4-D and has yet to be commercially released.

AgCenter weed scientist Daniel Stephenson has been studying the effectiveness of Xtend and Enlist, among other technologies. While both show promise, he said, they work best in cases where pre-emergence herbicides had already been applied to the soil before planting.

Eliminating weeds as early as possible is critical, said AgCenter weed scientist Donnie Miller, who has been evaluating Balance GT, another soon-to-be-released technology. That system includes soybeans with a trait allowing pre-emergence applications of Balance Bean and over-the-top applications of glyphosate.

Balance Bean is an isoxaflutole-based herbicide that kills weeds by blocking an enzyme commonly known as HPPD, which stands for 4-hydroxyphenylpyruvate dioxygenase.

“It will essentially get the weeds before they ever emerge, whereas with glyphosate, dicamba and 2,4-D, the weeds have to be sprayed,” Miller said. “When you’re ready to make your application and weeds are at the optimum size, you always have that chance of a rain coming in and delaying you. You’ve gone from spraying optimum-size, 1- to 3-inch weeds to spraying 6- to 7-inch weeds.”

The arrival of additional ways to control weeds is good, but not because any of the new products alone are a perfect solution. Rather, having more options helps farmers combat not only weeds, but also the problem of herbicide resistance.

HPPD-inhibitor herbicides like Balance Bean — a relatively recent development in the weed science field — offer a mode of action different from any others currently available. Incorporating multiple modes of action is “the basis for any good resistance mitigation program,” Miller said.

Using more than one product and making both pre- and post-emergence applications also ensures better results overall, so long as label directions are carefully followed and spraying equipment is thoroughly cleaned to avoid unintended herbicide injury, Stephenson said.

“There’s a false assumption that dicamba, which can be used in the Xtend technology, is essentially going to be a silver bullet for resistant pigweed,” Stephenson said. “It is not.”

He added: “We have to protect this technology. If you ride the dicamba horse and that’s all you’re using, we’re going to follow the same path we did with glyphosate and end up with resistance.”

Farmers also should consider implementing weed control practices following harvest. Josh Copes, AgCenter field crop production specialist, has begun a project to study how post-harvest decisions affect the soil seed bank and weed populations.

He wants to find out if late-season herbicide applications in corn can delay when farmers have to get back in their fields to control weeds to prevent weed seed production after harvest.

“A lot of weed seed is being produced after harvest, especially with corn. Corn comes off as early as mid- to late-July in some cases,” Copes said. “That’s a long period of time for weeds to germinate, go to seed and create big problems for future crops in those fields.”

He also is looking at how the timing of mowing and disking fields after harvest affects weed seed production and how herbicide program intensity influences the seed bank.

Olivia McClure

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