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Herbicide-Resistant Crops May Lead to Weed Resistance

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AgriLife agronomist: Mix it up when it comes to weed control

By Kay Ledbetter

Texas A&M

AMARILLO, Texas (February 12, 2010)  – Too much of a good thing might be a bad thing when it comes to controlling weeds, a Texas AgriLife Extension Service specialist said.

Prior to herbicide-resistant crops, producers used a variety of methods to control weeds, including herbicides, cultivation and hoeing crews, said Dr. Peter Dotray, an agronomist with AgriLife Extension and Texas AgriLife Research in Lubbock.

Dotray, speaking at the Texas High Plains Grain Elevator Workshop in Amarillo recently, told the crowd, “As we’ve changed the things we plant, we’ve changed the way we try to control weeds.”

Across the U.S., he said, agriculture as a whole is controlling weeds differently using some of this “very good technology.” For example, 90 percent of the cotton planted last year had resistance to glyphosate (Roundup) in it.

That has resulted in some areas of resistant weeds, especially in the states where the technology was adopted the quickest, Dotray said.

There are many benefits to Roundup Ready systems, including broad-spectrum control, convenience and simplification of weed control, he said.

Dotray said another study looked at conventional tillage, no-till and strip-till systems, along with 16 different pre-plant treatments and a variety of different application timings of residual herbicides. It showed that while tillage is a very effective tool to control weeds in conjunction with herbicides, the Roundup-only system had effective control without the use of residual herbicides.

“And the Roundup system was as profitable as any,” he said.

In another study, he worked with three growers who planted 60 acres of dryland cotton each and controlled the weeds with a Roundup Ready system. A side-by-side comparison was made on each farm which had conventional fields on which cultivation and hand-hoeing crews were used to control weeds.

“Roundup Ready was every bit as good, if not better, and the inputs were the same or less, so it was more profitable,” Dotray said. “That’s why the technology is being adopted.”

But this intense focus on one herbicide means less plowing, less cultivation, bigger application equipment and less use of herbicides with different modes of action, he said.

“We are relying more on one herbicide and we are starting to see resistant weeds developing and population shifts,” Dotray said. “Weeds that prefer less tillage are becoming more problematic. It doesn’t mean they’ve developed resistance, but it tells me we are doing something different in controlling weeds.”

Resistance in weeds is not a new concern, but it is a big concern because many thought this herbicide was the answer – a silver bullet of sorts, Dotray said.

He said there are more than 340 different resistant biotypes of weeds found in more than 300,000 fields on a worldwide basis, so this is a problem worldwide. The resistance to Roundup began developing in 1998, first in rigid ryegrass and then in 2000 in marestail and later ragweeds and Palmer amaranth.

“The problem was most apparent in states that had put the cultivator away and had early adoption of Roundup,” Dotray said.

“So what’s next?” he asked. “We have to try to make sure producers are aware so that they know the problem is real.”

To determine if weed resistance is becoming a problem, Dotray advised producers to look at the weed patch of concern and see if there is a whole spectrum of weeds that was not controlled, or just one type of weed.

“I like to collect seed and do my own tests,” he said. “We want to make sure we have thorough coverage on that area and that it wasn’t a result of poor coverage or some other operator error.”

Dotray said some strategies producers with resistant weeds are already using are more cover crops, deep tillage, herbicides at planting and other technologies like Liberty Link cotton.

While Texas was not one of the first states to adopt widespread use of Roundup Ready crops and was not one of the early states to see the resistant weeds, it is beginning to see reduced tillage and less use of dinitroaniline herbicides such as Treflan and Prowl, he said.

Dotray said herbicide resistance can be somewhat predictable. The weeds are generally annuals that produce a lot of seeds and have several germination flushes a year. Also, if there are a lot of different biotypes of that particular weed, it is more likely for one to develop a resistance.

He said it is possible to delay – not prevent – the development of herbicide-resistant weeds by not relying on glyphosate-only systems and incorporating tillage into the system, as well as other modes of action.

“As the trend toward herbicide resistance begins to occur here, I would like to see other means of weed control in our weed-management programs and not just relying on one herbicide during the course of the year or for several years in a row,” Dotray said.