There are over 20 ALS-inhibiting herbicides currently on the market, including the most widely used broad spectrum material in peanuts, Cadre.

ALS is an acronym for acetolactate synthase, which is an enzyme that is 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, effectively control a broad spectrum of weeds.

Four weed species have been documented to have resistance to ALS-inhibitors or glyphosate. These include rigid ryegrass, horseweed, common ragweed, and Palmer amaranth pigweed.

In peanuts, a widely used program consists of Prowl, followed by Cadre. Prior to availability of ALS-inhibitors, that same weed management program would have typically consisted of five herbicides, each with a different mode of action.

Pigweed is 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 different weed species have been documented to have varying degrees of ALS-resistance. In the U.S., 38 species have been documented to have ALS-resistance.

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

Prostko notes pigweed is always going to be one of the most competitive plants in a field. In tests in 2006, he says Palmer amaranth pigweed grew two inches per day. In peanuts, it creates major problems by robbing nutrients during the growing season and by clogging up diggers at harvest time.

In Georgia, Prostko’s research team treated 2,900 plants from fields suspected of having resistance problems. Using a Cadre rate 10 times the lethal amount in the field, 2,400 of the plants survived in the greenhouse. The bottom line, Prostko says is those weeds should have died, and they didn’t.

If small areas of weeds continue to grow in fields in which the grower is certain there has been good control on other weeds, there is 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 is high that there is a resistance problem.

“It used to be that we looked at resistance as a last resort reason for lack of control, now it’s one of the first things we look at,” Prostko notes. “It would be nice to be able to walk out into a field and say with some confidence — you’ve got resistance problems. Unfortunately, it’s not that easy to diagnose. To be sure of weed resistance to a herbicide requires harvested seed from the suspect plant, growing these seed in the greenhouse, apply varying rates of herbicide under controlled conditions. All this takes time,” the Georgia weed scientist stresses.

e-mail: rroberson@farmpress.com