Researchers find herbicide resistant ragweed

The dramatic increase in no-till and strip till peanuts generated a research project at North Carolina State University to find some options to wheat as a winter cover crop. North Carolina State University researchers Barbara Shew, David Jordan and Bridget Lassiter found some good cover crop options, but made a surprising discovery that offers some interesting challenges to peanut growers in the Southeast.

Speaking at the recent American Peanut Research and Education Society meeting, Lassiter says, “the obvious reason for using a winter cover crop is to reduce soil erosion, but there is also some evidence they may reduce weed competition in subsequent crops. So, the main objective of our study was to determine the impact of different cover crops on weeds common to peanut production in North Carolina.”

As part of the study, Lassiter says the researchers noted a significant drop in ragweed control from 2005 to 2006. In the second year of the test, the more economical herbicide only produced 76 percent control of common ragweed.

“We saw very healthy looking ragweed in otherwise pristine plots. We couldn’t figure out what was going on with these weeds,” Lassiter says.

Upon review of the field history they began to understand the problem. Peanuts had been planted in the field every year since 1999. ALS-inhibiting herbicides are commonly used in peanut production, and the researchers were using the same family of herbicides in their tests.

Herbicides that target the enzyme acetolactate synthase (ALS) are among the most widely used in the world.

Unfortunately, these herbicides are also notorious for their ability to select resistant weed populations. Now, there are more weed species that are resistant to ALS-inhibiting herbicides than to any other herbicide group.

In most cases, resistance to ALS-inhibiting herbicides is caused by an altered ALS enzyme. The frequent occurrence of weed populations resistant to ALS inhibitors can be attributed to the widespread usage of these herbicides, how they have been used, the strong selection pressure they exert, and the resistance mechanism.

In several cropping systems, ALS-inhibiting herbicides were used repeatedly as the primary mechanism of weed control. These herbicides exert strong selection pressure because of their high activity on sensitive biotypes at the rates used and because of their soil residual activity.

“We were doing exactly what we encourage farmers not to do. Though it seemed obvious we had ALS-resistant ragweed, we had to collect seed, take it to the lab, grow it out and test it with ALS-containing herbicides to be sure,” Lassiter adds.

Both resistant and non-resistant ragweed plants were planted, three to a pot, in a greenhouse and sprayed when the plants were 2-3 inches tall with ALS-containing herbicides at rates from 1X to 32X.

At label rates the resistant ragweed were comparable to untreated check plants. At 16X and 32X rates the resistant ragweed were severely stunted, but not killed. The greenhouse demonstrated that ragweed has developed resistance to ALS-inhibiting herbicides, which may present some new problems for peanut growers in the Southeast.

Though glyphosate is not commonly used in peanut production, giant ragweed has been added to the list of resistant weeds.

North Carolina State Weed Specialist Alan York has already demonstrated dual resistance to ALS-containing herbicides and glyphosate-containing herbicides in Palmer amaranth weeds. Palmer pigweed, because of their prolific seed production capabilities, are usually among the first to develop resistance to herbicides.

If multiple weeds continue to show up with multiple herbicide resistance capabilities, this could mean more challenges for peanut growers and accentuate the need for growers to know the mode of action of all herbicides they use and to rotate these chemicals, regardless of the crop.

Resistance in one weed, in this case ragweed, to both ALS and glyphosate-containing herbicides may be significant if peanut growers continue the trend toward no-till or strip-till systems that include cover crops that require herbicide burndown.

In these systems growers should be careful in choosing burndown herbicides to avoid further complicating resistance issues with ragweed and a number of weeds proven to be resistant to both ALS-inhibiting weeds and glyphosate resistant weeds.

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