For farmers preparing to plant, weed management may not be a top-of-the-mind item, but this is the exact time to implement a weed management program, Dekalb Asgrow agronomist Randy Lloyd said.
“Pre-planting is when weeds need to be controlled,” he said. “Most herbicides need to be applied before planting.”
A great rule of thumb to use with herbicide applications, Lloyd said, is “the earlier, the better.” If you are held out of the field for any reason, a pre-emergence herbicide is already out there working for you. It pays off in the long run, he said.
“The goal of weed management is to minimize the effect weeds have on your crop,” Lloyd said. “Once it emerges, a weed is competing with and taking resources from your crop.”
In addition, some actively growzing weeds are harder to manage. Residuals don’t do well against emerged weeds if they get established, he said.
The efficacy of a residual herbicide depends on a few factors, Lloyd said, weather activity being key among those factors.
If the ground is too dry, that is bad for the herbicide’s absorbency. If the ground receives too much moisture the herbicide will move too far into the soil, he said. The type of soil and the pH level of the soil can have an impact, as well.
“The key to a weed management plan is multiple modes of action,” Lloyd said. “Herbicides dissipate. When they do, you will have weed emergence.”
Lloyd said that the best plan will include getting another residual product active before the previous application runs out. But, he cautions, do not use the same product over and over.
“Diseases can become resistant to drug treatments,” he said. “Likewise weeds can become resistant to certain herbicides.”
Established populations of weeds can become tolerant or resistant to a type of herbicide. It is not a genetic mutation, just a selection, Lloyd said.
For example: A producer hits a stand of Palmer amaranth with herbicide A. It kills 95% of the pigweed. The remainder may be more tolerant to that product. Those plants produce seeds. The next year, the same product may kill only 80% of that group of weeds.
The best plans would be overlapping one herbicide with another, each of them employing different modes of action. According to professor Lynn M. Sosnoskie, of Cornell University’s School of Integrative Plant Science, the mode of action is the method by which the herbicide disrupts a weed’s biological processes.
One herbicide may inhibit a plant’s ability to produce lipids, another may stop the production of critical amino acids and a third may prevent mitosis. Used in a combination, these applications would kill just about any weed.
Weeds are invasive, insidious and intransigent. It takes diligence to defeat them – diligence, and a plan, Lloyd said.
“Failing to plan is planning to fail,” he said.