Fewer herbicide options combined with more herbicide-resistant weeds could equal disaster on your farm. That makes knowing how to manage your fields for weeds – and prevent herbicide resistance – critical. Weed scientists often see the same mistakes while working with farmers.
Here are the most common ones and how to avoid them.
Mistake: Applying herbicides when weeds are too tall
When weeds get tall, they are extremely hard to kill, says Dallas Peterson, Kansas State University weed scientist. “Most postemergence herbicides recommend application before weeds exceed 3 to 4 inches tall,” he says. “Deciding to wait until later is almost always a mistake, especially since some weeds, like Palmer amaranth, can grow 1 to 2 inches a day.”
Solution: Spray weeds when they’re shorter than 4 inches. Don’t wait.
Mistake: Not using residual herbicides or not using adequate rates of residual herbicides in pre-plant/pre-emergence programs
Overlapping residuals – pre-plant, pre-emergence and postemergence herbicides – provides the most consistent and persistent weed control. Residual herbicides lessen yield loss from weed competition, provide a longer time for the crop to establish and reduce the likelihood of resistance to postemergence herbicides.
Solution: Include residual herbicides that are effective on target weeds prior to germination.
Mistake: Applying herbicides at rates lower than the labeled minimum
Full-rates kill weeds with low-level resistance, and dead plants cannot produce resistant progeny, says Richard Zollinger, North Dakota State University weed scientist. “Reduced rates allow plants with low-level resistance to survive, hybridize and produce progeny with elevated resistance,” he says.
Solution: Apply a herbicide with the right site(s) of action at the right rate for the right size at the right time.
Mistake: Using ineffective herbicides for weeds that are in the field
“If you know what weeds are in your field, you can do a better job managing them,” says Christy Sprague, Michigan State University weed scientist. Apply a herbicide with the right site(s) of action at the right rate for the right size at the right time. Use the Take Action Herbicide Classification Chart to compare herbicide sites of action.
Solution: Scout your fields and be knowledgeable about weed identification.
Mistake: Failure to control winter annuals in a timely manner
Without a growing crop to protect, farmers might overlook the impact of winter annual weeds in their fields. However, dense mats of winter annual weeds can result in delayed soil warming, competition for water and nutrients, and other difficulties during planting. Plus, winter annual weed species can act as alternative hosts for pests.
Solution: Incorporate fall and/or spring burndown herbicide applications or other forms of winter-weed control, such as tillage.
Mistake: Relying on one herbicide to get the job done
A number of factors determine the frequency of resistance in weed populations. But the No. 1 factor is overreliance on a single herbicide (or group of herbicides with the same site of action) – without using other weed management options.
Solution: Diversify your weed-management plan by combining herbicides with different sites of action and incorporating nonchemical control tools, such as tillage and cover crops.