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Managing Weed Resistance
IS IT WORKING? “Farmers need to scout their fields before and after applying herbicide to gauge how effective the treatment is,” says ISU’s Bob Hartzler. Using a multiyear plan and herbicides with different modes of action limits weed control issues.

Time to get serious about controlling weeds

ISU specialist urges farmers to commit to weed control despite added expense.

Farmers faced with tight profit margins may consider cutting back on weed control efforts this growing season, but an Iowa State University Extension weed management specialist says doing so may cost farmers money in the long term.

ISU’s Bob Hartzler says low crop prices in recent years may lead farmers to tolerate a low population of weeds that might not affect yields. But doing so may allow enough weeds to go to seed in fields and give rise to the spread of herbicide-resistant weeds in the future, a growing concern for corn and soybean producers across the state.

“Farmers might be tempted to do just enough to protect yields but still allow some weeds to survive,” says Hartzler. “They might be unwilling to spend the extra money to get to the next level of weed control, and in the short term, you can rationalize that. But in the long term, that’s going to lead to further evolution of resistance.”

Weed resistance widespread in Iowa
For years, the herbicide glyphosate made weed control a one-size-fits-all proposition for farmers, but Hartzler notes that most Iowa fields now experience pockets of weeds that are resistant to glyphosate. The resistance occurs when weeds that possess the right genetic mutations to survive the herbicide treatment are passed on to subsequent generations of weeds.

Weeds are showing resistance to nearly all herbicides used in Iowa, he says. Populations of waterhemp, for instance, have demonstrated resistance to five different groups of herbicides.

Hartzler recommends farmers round out their weed control strategies by using a variety of tactics and tailor decisions to individual fields.

Rotating crops that have markedly different growth cycles compared to corn and soybeans interrupts the life cycle of weeds as well. Putting winter wheat into the crop rotation, for example, presents an opportunity to combat weeds without the use of herbicides, though he acknowledges this strategy imposes expenses that may not work for many farmers.

Success requires multi-pronged strategy
Narrower rows also fight weeds by placing crops closer together, creating a tightly spaced canopy of plants that block sunlight from reaching weeds, Hartzler says. But that option requires a planter capable of planting corn and soybeans in narrow rows.

Judicious use of tillage also can prevent weeds by burying seeds deeper into the soil where they can’t grow.

Hartzler recommends farmers scout their fields before and after applying herbicide to gauge how effective the treatment is. “We’re in an era where you really have to pay attention to the details if you want an effective weed control strategy,” he says.

To accomplish weed control success, you must make timely herbicide applications, notes Hartzler. Use a burndown or tillage to start with a clean field. Use a preemergence application of effective residual herbicides. And make timely postemergence applications of products featuring multiple herbicide modes of action.

He emphasizes that using multiple modes of action is vital to control current weed pressure while also limiting the development of herbicide resistance that can cause heavier pressure in future years.

Other advice from agronomists follows:

• Value of using residual herbicide. Residual herbicides go a long way in helping the crop establish a good stand, says Steve Snyder, a field agronomist for Dow AgroSciences. “Waterhemp and glyphosate-resistant giant ragweeds are two of the most challenging weeds. Growers are using preemergence products to control these weeds in soybeans before applying postemergence herbicides.” A good preemergence herbicide program delays weed emergence and expands the window for effective postemergence application, says Snyder.

• Timing postemergence treatments. Keep in mind that no matter how good a postemergence herbicide is, it’ll work better if applied before weeds get too big. Timing is everything with post treatments. “Weeds are more of a challenge when they get above 6 to 8 inches tall,” says David Hillger, another Dow agronomist. “Ideally, you want to treat them at 3 or 4 inches, especially marestail and Palmer amaranth, which are two of the biggest concerns in the Corn Belt.”

Weed resistance is forcing farmers to learn to be more flexible. “You have to be willing to rotate modes of action to maintain the efficacy of our herbicide tools,” Snyder says. “You need to take a multiyear approach on herbicides. You understand the value of rotating crops; you also need to realize the value of rotating herbicide modes of action.”

This requires reading and following the herbicide labels, he adds. You may have two products with different brand names, one you use on corn one year and the other on soybeans the next year. But if they offer the same mode of action, you’re taking a chance on weeds developing resistance to that mode of action.

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