Plan Your Weed Control Strategy

February 23, 2010 09:35 AM

Devising a plan of attack to control winter annual weeds should be as top of mind as seed selection and nitrogen management. Don't wait until post-harvest and apply a herbicide or pull out the harrow as an afterthought.

"Experienced strip-till and no-till farmers know that the longer you go without tillage, the greater a problem winter annual weeds are likely to become—if they are not managed,” says Farm Journal Field Agronomist Ken Ferrie. "Those farmers have a control plan in place.”

Often, depending on location, that plan begins with a fall herbicide application, says University of Illinois Extension weed science specialist Aaron Hager. Strategies include applying a herbicide with residual soil activity before most of the winter annuals germinate; applying a nonresidual, burndown-type herbicide, such as glyphosate, 2,4-D or paraquat, to emerged weeds (including winter annual, biennial and perennial species) while they are still small; or a combination of the two.

"The goal is to reduce the amount of vegetation you have to deal with in the spring prior to planting, and possibly eliminate the need for a spring burndown herbicide application,” Hager says. "But remember that a burndown herbicide may still be needed before you plant.”

If your fall treatment program—tillage for conventional farmers or herbicides for no-tillers and strip-tillers—went according to plan, you're off to a good start. Unfortunately for many farmers, last fall was anything but according-to-plan; much tillage did not get completed and many fall herbicides did not get applied.

"Many areas have experienced two years of wet falls, followed by wet spring weather and late planting,”

Ferrie says. "If you were unable to do timely spring tillage and your winter annuals went to seed, the [weed] population is likely to take off.”

This past spring, many winter annuals did go to seed before they finally were controlled, Ferrie adds. Farmers had to till or plant through heavy mats of weeds, such as henbit and chickweed, and the weed seed bank in the soil was increased.

Effects of weeds. Uncontrolled winter annual weeds create many problems. "In no-till and strip-till, and to some extent in vertical tillage, a heavy mat of weeds keeps soil from drying out,” Ferrie says. "That leads to delayed planting—and planting into wet soil leads to sidewall compaction and open slots.”

Lush weed growth provides a habitat for seed corn maggots and cutworms. "You typically think of cutworms being a problem in no-till or strip-till because they have a lot of cover,”

Ferrie says. "But if you have a heavy flight of cutworm moths, the insects will lay eggs in the weeds before you till and incorporate them.”

Heavy ground cover and wet soil creates conditions for seedling blight of both corn and soybeans. Conversely, if the weather turns hot and dry, heavy weed growth can sap the moisture you need to germinate your crop.

"Many weed species serve as hosts for corn and/or soybean nematodes,” points out University of Illinois nematologist Terry Niblack. "Most of the corn-parasitic nematodes are generalists, which will feed on just about any monocot. If you rotate with soybeans, grassy weeds in the beans will help corn nematodes survive and even build up between the corn crops. 

"Several weed species, especially the winter annuals purple deadnettle and henbit, are moderate to good hosts of soybean cyst nematodes [SCN],” Niblack adds. "Buildup of SCN on winter annuals is more likely to be a problem in the southern range of these weed species, because SCN
activity is temperature-dependent.”

Even after they are tilled into the soil, winter annual weeds can still create management problems. "Conventional-till farmers who incorporate a large amount of residue—whether it is from the previous crop or from heavy weed growth—and do not apply nitrogen to the surface or place it in the row may experience slow growth of corn,” Ferrie points out.

This "carbon penalty” occurs because soil microbe populations increase to decompose the residue. As a side effect, they tie up soil nitrogen and make it temporarily unavailable to plants. "You may be able to avoid the problem by moving 20 lb. per acre of nitrogen from sidedress to preplant application,” Ferrie says.

"You may also see slow growth of soybeans,” he continues. "They will make their own nitrogen at the third or fourth trifoliate stage, but until then, growth may be slowed.”

What to do. Conventional-tillage farmers who could not get their tillage completed last fall may need to manage weeds the way no-till and strip-till farmers do—with a burndown herbicide application early this spring, consistent with the timing on the product label.

"Many conventional farmers entered spring of 2009 with no tillage completed the previous fall,” Ferrie says. "In the spring, the weather turned wet again. By the time they could get into their fields, many had a foot of green growth to deal with. It wouldn't flow through tillage tools, and soil finishers balled it up like big beaver huts.”

Farmers who use vertical-tillage tools in the spring may want to consider using a burndown herbicide application before they till. "Vertical tillage is a very good practice,” Ferrie says. "The harrows do a good job of preparing and leveling the seedbed, but they were not designed to be very effective on weeds.

"Vertical-till harrows get some broadleaf weeds and grasses, but winter annuals are too well rooted,” he adds.

"Harrows damage weeds and make them shut down for a while. But in that condition, they are difficult to kill with a herbicide after planting,” he says.

Weed-control tips. Your Extension agronomist, crop consultant or fertilizer dealer can help you prepare a weed management plan, tailored to the weed problems in each field, to keep weed pests from getting away this spring. Here are some tips to keep in mind as you plan your strategy.

"With soybeans, get weeds under control before you plant,” Hager emphasizes. "Don't always expect that every species will be controlled with one product, so plan to use appropriate tank mixes.

"If weeds get tall, don't skimp on rates,” he continues. "You may only have one chance to spray. Use the correct herbicides and additives to suit the weed species and conditions.

"Once weeds are actively growing, don't pass up opportunities to spray [in case the weather turns wet],” Hager concludes. "If you spray early and have another flush of weeds before you plant, scout fields to see what weeds are present and then spray or till.”

Ohio State University weed control specialist Mark Loux agrees. "Watch for the first warm, dry period you have [consistent with product labels], and spray,” he advises. "Burn weeds down, and don't let a field turn into a jungle. Be aware you may have to make a second application.”

"If you use custom application, rather than applying your own herbicide, talk to your applicator ahead of time and make sure he's prepared to respond when the weather becomes favorable for spraying,” Ferrie says. (See the sidebar "Strategies for Vertical-Till and Conventional-Till Situations.”)

"There are a wide range of products available for almost any situation,” says Mark Baer of Sun Ag Supply in Tremont, Ill. "If a grower understands his weed populations, he can tailor the correct products to control them. I believe strongly in residual products, to help with weed resistance management and delay the in-crop application until the appropriate time. This strategy also helps if the weather fails to cooperate with timely applications.”


Conventional-Till Farmers May See Cutworms

No-till and strip-till farmers are used to scouting for cutworms, because they know the surface residue in their fields provides an egg-laying habitat for moths. But if conventional-till farmers are unable to till and incorporate a heavy growth of weeds before the moths begin to fly, they'll have to scout for cutworms just like no-tillers do.

"If even part of a field was weedy, moths will lay eggs in that area,” says Farm Journal Staff Agronomist Ken Ferrie. "Remember that even if you plant rootworm-resistant hybrids, not all of those hybrids resist cutworms.”


Strategies for Vertical-Till and Conventional-Till Situations

The type of spring tillage you employ—vertical-tillage harrows versus full-width horizontal tillage, such as soil finishers—affects your late winter/early spring weed management, says Farm Journal Field Agronomist Ken Ferrie. In both cases, your goal is to keep weeds from getting away before you plant.

For vertical-tillage farmers, Ferrie offers this advice: "If you make a weed-and-feed application in late winter or early spring, consistent with the conditions on the herbicide label, include a grass-control product along with your burndown herbicide.”

One grass-control option is Degree or Degree Extra from Monsanto Company. It uses time-release technology to make atrazine available immediately upon application, but releases acetochlor and safener based on soil temperature.

"There won't be any grasses growing at this point,” Ferrie says. "But grass herbicide, maybe applied at a reduced rate, will keep grass under control until you can do your vertical tillage and plant. If you are able to plant on time, you probably won't need to apply a preplant burndown herbicide. However, you may want to move your postemergence herbicide application up a bit.”

Timing of such applications, and whether they are permitted by the herbicide label, depends on your latitude, Ferrie points out. As a rule of thumb, if conditions in your area are right for topdressing wheat, they may also be right for safely applying a herbicide.

If you can make this weed-and-feed application early enough, you may be able to omit the glyphosate and apply only 2,4-D and atrazine, adding some diversity to your herbicide program, Ferrie notes. "Weeds will be very small at this point, so you will need to scout fields before choosing your treatment,” he says. "If you don't make this application, three weeks later you will be wondering where the marestail and dandelion [a perennial weed, not a winter annual] came from.”

Ferrie emphasizes: "The purpose of this late winter/early spring application is to keep the field clean until you plant. You're not looking for season-long control, because you will be applying a postemergence treatment for grasses and broadleaf weeds.”

Weeds delay spring dryout. Conventional-tillage farmers control weeds with horizontal tillage, so their goal is to keep weeds from getting too tall before they work the soil. "When the opportunity arises, conventional farmers may want to go out with 2,4-D, or maybe glyphosate, depending on the weather,” Ferrie says. "A little 2,4-D or glyphosate is cheap insurance against the kind of problems we saw last spring in many areas.

"This is especially important for conventional-till fields that have high populations of winter annual weeds because weather conditions delayed field work and allowed weeds to go to seed the past couple seasons. With heavy winter annual weed populations, you might add a little Princep or atrazine to the 2,4-D, consistent with product labels.

"Do this as soon as you can get in the field, consistent with the herbicide label. You want to take out, or at least slow down, those winter annual weeds before they get much size to them. The soil probably will be dry enough to run a floater applicator weeks before it is dry enough to do tillage without causing too much compaction. Floater tracks are less of a concern with conventional tillage, because you'll take them out when you work the field.

"If you are planning to do aggressive tillage, such as chisel plowing in continuous corn or to fill in ruts, do not apply a residual weed control product at this time,” he continues. "Aggressive tillage will streak herbicide and lead to poor control. You can apply your residual herbicide when you do your final tillage before planting.

"However, if you just plan to run a field cultivator in soybean stubble and then plant, you can apply a grass-control product with your burndown herbicide.

"You may get your final tillage completed before the grass even germinates. You just want to hold the winter annuals so they don't complicate your tillage.”


Plan Your Marestail Strategy

Among winter annual weeds, mares-tail (horseweed)—which actually germinates in the fall and in the spring—merits special consideration in your weed-control plan. If the weed gets away from your control, it can become a nightmare, especially in non-GMO soybeans.

Near Clinton, Ill., rainy spring weather in 2009 made it impossible for Don Schlesinger's custom applicator to apply glyphosate while the weeds were still small.

"By the time we were able to spray, the weeds were too tall to control, even though we used a higher rate of 2,4-D in one of the fields,” Schlesinger says. (One field was sprayed with Valor and 2,4-D, while the other field was sprayed with Roundup and 2,4-D).

"The higher rate of 2,4-D just made the big marestail mad—it knocked out the growing point, but the plant sent out new branches from the root, so I had four to six marestail shoots instead of one.”

Because Schlesinger had planted non-GMO soybeans, there were no postemergence options for fall-emerged marestail. Ultimately, he had to spray a harvest-aid herbicide in one of the fields to kill the weeds and permit him to harvest.

No-till non-GMO soybeans is probably the most challenging situation for controlling marestail, as well as other tough weeds such as waterhemp and lambsquarters, says Mark Baer of Sun Ag Supply in Tremont, Ill. "For a successful non-GMO soybean program, you must apply a burndown herbicide, with a residual herbicide, within one week of planting,” he says. "That's in addition to a burndown herbicide applied the previous fall.”

Preventive measures pay. "Herbicide programs must include a burndown product to ensure the field is free of marestail at the time of soybean planting, along with residual herbicides to control marestail for another six to eight weeks,” agrees Ohio State University weed control specialist Mark Loux.

"The herbicides should always be applied before plant height exceeds 4". Larger plants are difficult to control,” he adds.

With marestail, there is also the issue of glyphosate-resistant plants, Loux points out.

"Resistance developed because we sprayed with only glyphosate,” he says. "Always use glyphosate and 2,4-D, or glyphosate and Sharpen [a new product]. We have found that glyphosate/2,4-D and glyphosate/Sharpen work on weeds up to 6" to 8" tall.

"With corn, planting a non-GMO [variety] is not as much of a concern,” Loux adds. "Some herbicide will probably work.”

You can e-mail Darrell Smith at

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