Herbicide-resistant weeds getting worse

No matter what crop you are growing, nor which weed infests it, the chance of having to deal with herbicide resistance is getting better and better each year.

If you haven’t dealt with herbicide resistant weeds, stay tuned — they are coming.

Cotton acreage is up across the Southeast — good news for farmers and the entire cotton industry. Likewise, good news for weeds looking to develop a little resistance to the vast array of old and new chemistry used to control weeds in cotton.

Cotton farmers in Alabama’s Tennessee Valley are learning first hand how severe glyphosate resistant Palmer amaranth, commonly called pigweed, can be. Despite the economic advantages of planting cotton this year, many farmers don’t have the extra acreage to increase production because of the risk involved in managing herbicide resistant weeds.

Wheat acreage is down in the Southeast, but herbicide resistant weeds continue to be a limiting factor as growers plan for the 2010-2011 crop. The primary culprit is multiple-resistant Italian ryegrass.

Peanuts are making a little comeback in 2010, some growers suspect a long-time weed nemesis — nutsedge — may have developed resistance to herbicides that have kept it in check for nearly a decade.

Any land that will grow a crop will grow a weed. After all, the adage goes that a weed is just a plant out of place. Keeping these out of place plants in check has gotten considerably more complicated over the past few years.

The big culprit in the Southeast, especially for cotton and soybeans, has been glyphosate resistant Palmer amaranth, almost universally known as a pigweed.

The New York Times in a recent article referred to it as ‘super weed’ and indicated it was immune to herbicides. That’s a good stretch from reality, but Palmer pigweed is clearly a threat to the resurgence in cotton acreage this year.

Glyphosate resistant Palmer pigweed is far from being the only resistance problem for growers in the upper Southeast.

Take the case of chickweed. For many, many years it wasn’t even considered a major weed pest because it was routinely and indirectly managed by herbicides used to control other weed pests.

Last year it popped up in some scattered fields in Virginia. Virginia Tech weed scientist Scott Hagood says, “In one field in New Kent County, Va., Harmony, a standard herbicide control for chickweed at 32 times the label rate had no visible effect.” Of more concern, he says, samples from this field were sprayed with a number of other sulfonylurea herbicides, with a similar lack of control.

“We also tested these resistant plants to other ALS herbicides other than the sulfonylurea herbicides. The New Kent site was resistant to both Pursuit and Arsenal. Arsenal in particular is a very persistent and efficient herbicide, and resistance to this material should give growers some insights as to how difficult multiple-herbicide-resistant chickweed will be to manage,” Hagood says.

Chickweed isn’t likely to decimate grain crops in Virginia. Combine it with resistant horse nettle, Palmer amaranth, giant ragweed, cocklebur, Italian ryegrass and so on and so on and the combination becomes a big problem.

First documented in 2005 in Missouri, giant ragweed resistance to glyphosate is slowly moving south and east. Suspect cases in Virginia have been cited by a number of growers. Both common ragweed and giant ragweed resistant to glyphosate have been reported in Tennessee.

In 2006 North Carolina State University researchers documented giant ragweed resistance to ALS-inhibiting herbicides. Dual resistance to ALS and glyphosate-based herbicides would be double trouble for no-till growers on a number of cropping rotations.

Alan York, retired North Carolina State weed scientist and arguably the guru of weed resistance in the Southeast says weed resistance to herbicides is a big threat, but nothing new, and he has a chronology of North Carolina weed facts to back that up.

The first documented case of weed resistance came in 1973 with dinitroaniline resistant goosegrass in cotton. In 1980, smooth pigweed resistant to atrazine in corn was documented, followed that same year by atrazine resistant lambsquarters. By that time both chemistries were being replaced by new, more powerful, less toxic to human materials, in particular ACCase inhibitors.

By 1990, Italian ryegrass was documented as having resistance to ACCase inhibitors sethoxydim and diclofop-methyl. By that time another family of chemistry ALS inhibitors was on the scene and by 1995 Palmer amaranth had developed resistance to these materials.

Cotton growers in the state got the first of what has become a series of herbicide resistance challenges when DSMA and MSMA, two widely used organoarsenicals, were shown to have resistance issues with common cocklebur.

In the 1990s glyphosate tolerant cotton and soybean plants hit the market and by 2003 cotton growers in North Carolina were faced with the reality of glyphosate resistant horse weed. Relatively easy to manage with other herbicides, horse weed was a proverbial piece of cake compared to the 2005 revelation of documented proof of glyphosate resistant Palmer amaranth.

Documented proof means scientifically segregating, growing and spraying weeds suspected of having tolerance to a specific herbicide. Documented proof of herbicide-resistant weeds is one thing, veteran farmers spraying weeds, knowing the weeds were treated with what should be a lethal dose of a herbicide and not managing it is much more widespread.

Though Italian ryegrass was documented as having a resistance problem nearly 20 years ago, upper Southeast wheat producers have little problem finding ways to manage it — until it developed resistance to glyphosate. Without a reliable burndown herbicide, planting wheat for the 2010-2011 season is significantly more risky.

Glyphosate has never been overly active on Italian ryegrass and resistance was often attributed to this factor. However, recent widespread problems in the Delta and more sporadic problems throughout the Southeast led researchers to take a closer look at the level of resistance.

When Italian ryegrass emerges with wheat, it chokes out the crop and lowers yield. Italian ryegrass is a prolific seed producer, and those seeds germinate rapidly at rates between 71 and 80 percent in a two-week span. If Italian ryegrass is not controlled early, seeds can be further dispersed across the field during harvest.

In response to the increase in Italian ryegrass resistance problems, Syngenta researchers developed an in-season resistance test call RISQ, or Resistance In-Season Quick test.

“The in-season resistance test can determine whether or not a particular weed biotype is resistant to a herbicide within 10 days,” says Don Porter, herbicide technical brand manager for Syngenta. “We are currently working to conduct additional experiments before making this resistance test available on a larger scale and over an expanded geography, Porter adds.”

Growers in the 2010 cropping season should pay special attention to any weed-herbicide combination that seems to work differently than in past years. Rotating herbicide chemistry and crops is a good hedge against resistant weeds. Knowing your enemy and knowing how to deal with it is a better option.

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