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Wheat Growers Rotate Both Crops and Herbicides to Knock Back Wild Oats Two North Dakota wheat growers take a tactical approach to weed control, combining strategic crop rotation with herbicide tankmixing and reduced tillage in a long-term battle against wild oats and other noxious weeds. Dennis Johnsrud of Epping, N.D., and Ron Kohl of Lidgerwood, N.D., are managing weed seedbanks on a long term-basis rather than focusing on weeds during a single season. The distinction is important, as it affects major decisions such as crop selection and tillage scheme.
Dennis Johnsrud of Epping, N.D., and Ron Kohl of Lidgerwood, N.D., are managing weeds on a proactive long term-basis rather than being reactive and focusing on weeds during a single season. The distinction is important, as it affects major decisions such as crop selection, tillage method, and herbicide strategy. Crop rotation is key to both growers’ strategies. Instead of making decisions entirely dictated by short-term crop prices or compliance with government programs, they’re also looking at long-term agronomic effects – including weed control. “I’ve been preaching this approach for 20 years,” maintains Dwayne Beck, manager of the Dakota Lakes Research Center in Pierre, S.D. Beck has studied the issue as part of his work at the center’s 1,200-acre research farm, which is affiliated with South Dakota State University. “There are other reasons to rotate crops, of course,” he says. “But weed control is certainly an important objective. I like to say that Mother Nature is an opportunist. When you have a weed problem, it’s because that weed has an opportunity to grow. A certain set of circumstances will favor its growth. If you provide enough of that opportunity – and if you are predictable and give it at a high enough level – weed populations will build. But if you change the environment with crop rotation, you limit that opportunity.” Every crop grown allows the selection of weeds with similar growth requirements. For example, growing a cool-season grass crop like wheat provides an ideal environment for cool-season weeds such as wild oats and mustard. A warm season crop like corn favors a weed spectrum that includes foxtails and nightshade. Crop rotation can help limit how weed populations grow. Fighting Wild Oats with Crop Rotation “I inherited the problem on my farm,” Kohl adds. “And over the years it’s been one of the harder weeds to control.” Wild oats like cool weather and moist soils. The weed can grow to four feet in height, and just 10 wild oat plants per square yard can reduce wheat yields by 10%. Heavier populations can cut yields up to 40%. The weed is also prolific, with one plant producing up to 250 seeds. These typically germinate within two years, but can lay dormant for up to seven or eight years. Aside from yield loss, growers are also docked at the elevator. In an effort to reduce the seedbank of particularly troublesome weeds, Beck advocates at least three crop types in a typical rotation. The overriding principle is that rotation introduces diversity, which prevents weed populations from building. “It’s also important how you orient those crops,” he says. Beck adds that in places of the world with the most weed problems – such as western Australia and the U.S. Corn Belt – growers favor alternate-year crop rotations. “You have a lot of one weed the first year, you have somewhat less of it the next year, but then you reintroduce the first crop and the weed population stays up,” he says. Then, there’s the common notion in more arid areas of the Northern Plains that scarce water makes it impractical to use intensive cropping, which is often associated with a complex crop rotation. But both Kohl and Johnsrud say times are changing. Johnsrud has completely removed summer fallow from his rotation. “We now grow durum wheat for a couple of years, then rotate to safflower, then to field peas. And we chemical fallow as necessary.” Meanwhile, Kohl uses a rotation that involves soybeans, wheat, corn and barley. Herbicide Strategies For example, in wheat Kohl likes to treat for wild oats three to five weeks after crop emergence. Last year he used Discover (clodinafop + safener) on two-thirds of his wheat. The herbicide is an ACCase inhibitor, which means that it blocks the development of the ACCase enzyme within the wild oat plant from developing. The herbi-cide’s label recommends treatment anytime between the crop’s one- to-six leaf stages. “I like the fact that I have a wide window of treatment, and it’s very gentle on the crop,” he says. Johnsrud likes to get a herbicide on early to pick up other weeds like kochia in a one-pass weed control program. He’s also using Discover, and tankmixing it with broadleaf herbicides. Kohl has used the herbicide in a tankmix with either Bronate, Curtail or the premix herbicide Starane plus Sword for complete broadleaf and grass control in one pass. In other crops, Kohl looks for another application window and mode of action for wild oat control. “I’ve been using more (glyphosate-tolerant) soybeans in my rotation because it helps with long-term control of wild oats,” Kohl says. Glyphosate herbicides can be applied in glyphosate-resistant soybeans either in a preplant burndown, or in-season. Burn-down is another effective application period for wild oat control. Furthermore, glyphosate can be rotated with Gramoxone Max (paraquat) as a burndown herbicide, providing a different mode of action, and yet more diversity. Tillage in the Mix “We refer to this as rotational intensity,” he explains. “There are two primary factors to consider – intensity and diversity. And intensity is based on balancing water use. One of the mistakes in the past with no-till was that growers didn’t use the water they saved. Then the field was too wet, which people often associated with no-till as a bad thing. But once you save the water, you have to figure out how to use it in your rotation. You might have to crop more often, or use a high water-use/high value crop like corn.” He adds that successfully managing a weed seed bank involves low disturbance tillage. “And even more than just no-till, but low-disturbance no-till. We don’t want to dig up dormant weed seeds from depth. If they’re left at enough depth, you won’t have as much of a problem with them.” “As for types of no-tillage that are the least disturbing, obviously a disk opener is less disturbing than a narrow shank, which is less disturbing than a sweep.” In the end, it’s all about diversity, both in the challenges that can arise in a crop production system, and in the methods used to manage those challenges. “Weed problems are a symptom of a farming system that does not contain sufficient diversity,” Beck says. “The weed is Mother Nature’s way to try to add diversity.”
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Additional Resources: Weed Resistance Management |
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