Are those weeds in your field herbicide resistant?

Are those weeds in your field herbicide resistant?

Your crop is up and growing, so are weeds. Are they resistant to the herbicide you applied?

Eric Andersen first noticed several years ago he wasn't getting good control of waterhemp in his soybean fields. "Even with higher application rates of Roundup, we had more and more waterhemp," says Andersen, who farms in the area around Dike in Grundy County in eastern Iowa.

Andersen was facing what has now become synonymous with the Midwest: weed resistance to glyphosate. It's the outcome of many years of widespread reliance solely on one product and the weeds adapting to its chemistry. So much so that the Environmental Protect Agency has recently announced plans to limit the use rate of glyphosate in future applications.

WATERHEMP: Seeing more weeds in a field, you wonder if those weeds are resistant to the herbicide that's been applied. It's a question more farmers are asking these days.

These days, Andersen has a weed management program in place where he uses at least one, and sometimes up to three, modes of action for a preplant residual. Then when he comes back about 20 days after planting, the goal is for a one-pass postemergence herbicide application to "clean anything up." He offers this observation: "The sooner you can get things sprayed, the better off you are. You're also going to do less damage to the soybean crop, the earlier you spray."

First step to weed control success is to know your weeds
You need to know the weed before you try to kill the weed. Scout your fields, advises Iowa State University Extension weed scientist Mike Owen. It's important to correctly identify what you have out there. Waterhemp, for example, is a huge threat to soybean acres because it can germinate throughout the growing season. Other weeds, like giant ragweed, can all sprout up at one time. Then there's lambsquarters, which emerges early on in the season and is usually not seen for the rest of the year.

Dawn Refsell, field development manager of the Midwest Commercial Unit for Valent U.S.A. Corp., says to pinpoint why a particular weed is showing up involves taking into account its biology. Valent makes and distributes Cobra herbicide, SelectMax herbicide with Inside Technology and Resource herbicide, all postemergence herbicides.

"Your first assessment should always be to consider the field's history," Refsell says. "Pairing field history with the biology of the weed or weeds present is very important to understanding why they are there."

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Good reason to rotate herbicides, use multiple modes of action
Growers trying to discern whether or not they truly have weed resistance in their fields should ask themselves the following questions:
* What herbicide products have I been using on this field?
* Where has my equipment been? Am I bringing weed seed in from other fields?
* Did I apply the herbicide at the correct rate and timing, as per the label?
* Have I been repeatedly using the same mode of action?

"If the grower has been using glyphosate for the last 10 years, even just on their soybean fields or only as a burn down treatment, they have established a recurring pattern of herbicide use, and that grower can suspect that they likely have weed resistance to the herbicide," says Refsell. Don't rely on just one mode of action, rotate herbicides, use multiple modes.

To test for weed resistance, submit a sample to university lab
The Weed Science Society of America defines herbicide resistance as "the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type."

Refsell says the best way to know if you have resistance with absolute certainty is to submit a seed sample to your state's land grant university. The paper, "Criteria for Confirmation of Herbicide-Resistant Weeds" explains that "resistance must be confirmed by an unbiased scientist through comparison of resistant and susceptible plants of the same species in a replicated and scientifically sound trial."

There are various screens available, depending on type of resistance (ALS vs. PPO, for example) and also the type of testing being performed. Tests can range from examining weed tissue in a lab to a more comprehensive screening in a greenhouse throughout the plant's growth cycle. "There's no one process that can test for everything," Refsell adds. "It depends on what herbicide mode of action you are screening resistance for."

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Use more than one tool, don't rely on just one practice
With weed control, it's best to use all the tools at your disposal. Don't just rely on one tool, such as glyphosate. In addition to his herbicide program, Andersen employs an assortment of management practices to keep the Roundup-resistant waterhemp at bay. For one, all of his acres are rotated among corn and soybean crops, which is crucial for weed control. "That helps you get more herbicide modes of action over a two-year period by going to corn," says Anderson. "We use mulch tillage, so we're relying heavily on herbicides."

Scouting is also key to having an effective, successful weed management process. As soon as the soybean is planted, Andersen is scouting "to see what weeds are coming through and if we have any kind of escapes." He adds, "We need to identify what weeds they are, and we need to see what size they are because if we're going to kill them, we need to get them as quickly as possible. You have a much better success rate when you spray waterhemp earlier rather than later."

Apply postemergence herbicide treatment early enough
Refsell recommends growers get started on postemergence weed applications by Father's Day weekend or sooner. The most effective control is a two-spray pass that's made before July 1.

Above all else in the fight against weed resistance is to start clean. This is especially important when dealing with horseweed, as some biotypes have less sensitivity to 2, 4-D, making the burndown control of this weed even more difficult.

"The worst case scenarios are the ones where the soybeans have been planted into weedy fields that were not clean or were not effectively burned down with the burndown herbicide treatment," Refsell says. "Once the soybeans are planted, the options for weed control, let alone good control, diminish greatly."

Summing up: 1) Scout your fields to identify the types of weeds present before choosing a herbicide; 2)  Don't rely on just one mode of action, rotate herbicides and use multiple modes of action; 3)  Use an assortment of management practices to keep tough weeds like waterhemp under control.

TAGS: Soybean
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