Though pigweed resistance to glyphosate in cotton has been the big newsmaker recently, resistance of several weeds, including pigweed, to the ALS-inhibiting family of herbicides could be equally devastating for peanut farmers.

There are more than 20 ALS-inhibiting herbicides currently on the market, including the most widely used broad-spectrum material, Cadre. ALS is an acronym for acetolactate synthase, an enzyme essential for plants to grow. By inhibiting the channels of transfer of ALS herbicides in the same family as Cadre, Classic, Ally, Beacon, Strongarm and 20 or so more, these materials effectively control a broad spectrum of weeds.

Peanut farmers often use Prowl plus a half rate of Cadre, or other ALS-inhibitor. Prior to ALS-inhibitors, that program would have typically consisted of five herbicides, each with a different mode of action.

Herbicide resistance is not new. The first documented cases came about the same time broad-spectrum herbicides were introduced into the market — in the 1960s. For 20 years or so little happened with resistance.

Rotating cotton with peanuts – often at the exclusion of corn – could have been a positive factor in reducing ALS resistance, because of widespread use of glyphosate in Roundup Ready cotton. Using ALS inhibitors in cotton to avoid glyphosate resistance may be the kiss of death for growers who rotate cotton with peanuts, because resistance to weeds – especially pigweeds – speeds up.

Jay Ferrell, weed scientist at the University of Florida, speaking at the recent Southern Peanut Farmers Federation annual meeting in Panama City, Fla., says herbicide resistance in pigweed is both difficult and easy to explain.

On one hand, he says it is a numbers game. One Palmer Amaranth pigweed produces up to 500,000 seeds. Two mature female Palmer Amaranth pigweeds provide a one in a million chance for mutation to occur and resistance to follow. Twenty mature weeds provide a one in a billion chance for resistance. By comparison, common cocklebur produce less than 10,000 seed and a peanut plant less than 200.

On the other hand, a complex set of biological changes must occur at the precise time in the weed’s growth for resistance to develop. The most common causes for resistance are changes that occur biologically in the weed and alter the site where herbicides attach. Ferrell compares such an altered site to using a Phillips head screwdriver to attach a standard head screw. When such an altered site occurs, no amount of herbicide will attach to the weed’s growth mechanism and it becomes immune to the chemical.

For cotton growers the great fear should be that Palmer pigweed is just the first weed to play the numbers game and win. For peanut growers that fear should be ALS chemistry. For both, the great fear is that pigweed has developed cross resistance to both herbicides.

Pigweed is perhaps the most prolific seed-producing weed that peanut or cotton farmers have to face, but it is not the only one to develop resistance to herbicides. Worldwide, 95 species have been documented with varying degrees of resistance. In the United States, 38 species show herbicide resistance.

“In the past lack of pigweed control was usually due to poor timing of application. Applying a herbicide to a 2-inch versus a 2-foot high pigweed are different things,” says University of Georgia Weed Scientist Eric Prostko. “Now, those 2-feet weeds are likely to be resistant to the herbicide of choice.”

In Georgia, Prostko’s research team treated 2,900 peanut plants from fields suspected of resistance. Using a rate two to three times the lethal amount in the field, 2,400 plants survived in the greenhouse.

Growers who suspect ALS or glyphosate resistance should look at the history of herbicide use in these fields. A consistent use of either family of herbicide across multiple crops over a period of years is a first red flag.

If small areas of weeds continue to grow in fields in which the grower is certain of good control on other weeds, he has reason to be concerned. If these plants are treated with labeled or higher rates of a herbicide and the expected result doesn’t happen, the likelihood of a resistance problem is high.

Several species common to peanut and cotton fields have been documented to have resistance to ALS-inhibitors and/or glyphosate. These include rigid ryegrass, horseweed, common ragweed and Palmer amaranth pigweed. Common lambsquarter likely will be added to that list.

It is critical for farmers to understand that resistance starts slowly. It may occur in a field four or five years before numbers get high enough to cause significant problems. By that time, it’s essentially too late to do anything other than avoid using the target herbicide.

Palmer amaranth resistance, for example, likely will start with a few plants that survive in a cotton or peanut field sprayed with glyphosate or an ALS-inhibitor. There are hundreds of reasons, other than herbicide resistance, for these escapes. However, with resistant pigweed in the fourth or fifth year, the number of resistant plants will explode and literally take over the field.

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