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Halt resistant weeds

Gil Gullickson 01/26/2011 @ 2:10pm Crops Technology Editor for Successful Farming magazine/Agriculture.com

Herbicides are a cornerstone of weed control. Sometimes, though, they’re too good. In some cases, repeated use of a successful herbicide selects for resistant weed biotypes.

There are ways to prevent or forestall this from happening, says Jason Norsworthy, University of Arkansas weed scientist. These eight steps can help reduce problems with herbicide-resistant or herbicide-tolerant weed biotypes.

1. Know Your Weeds.

Properly identifying weed species will help pinpoint which herbicide program works best for each acre. Be sure to closely monitor problem areas with difficult-to-control weeds or dense weed populations.

2. Rotate For Diversity.

Rotating crops and herbicide modes of action reduces the selection pressure of a single mode of action. It keeps weed populations off balance, much as a baseball pitcher does to baffle batters with a diverse pitch selection.

3. Rotate Crops Right.

Crop rotation helps forestall herbicide-resistant weeds. But rotate right.

Norsworthy gives the example of a farmer who planted rice following soybeans. “The field flooded, but he failed to control the Palmer amaranth on the levees,” he says. “When he rotated back to soybeans the next year, the Palmer amaranth was worse than ever.”

4. Start Clean.

Proper tillage or using a burndown herbicide prevents early-season weed competition. Early-emerging weeds can rob yields even when it’s not apparent.

5. Use Full Herbicide Rates.

 “We used to cut rates of glyphosate when it was expensive,” says Norsworthy. “Farmers not only cut rates, but also delayed applications to save on herbicide rates. So, they got tall weeds controlled by fewer applications and lower-than-recommended rates. Eventually, weeds developed resistance to glyphosate.”

6. Don’t Delay Application.

Apply postemergence herbicides early according to the label. This is particularly true for Ignite, used in LibertyLink systems.

“Ignite works great on small pigweed, but not on 12-inch-high weeds,” he says.

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