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When to nix winter weeds

No-till and reduced tillage have lots of perks, Moisture, soil, and time savings are a few. Unfortunately, these systems have one nagging drawback: They create a hospitable environment for winter annuals, biennials, and perennials to emerge in the fall and to plague next spring's crop.

In some cases, these infestations can make your field take on a whole new look. During the soggy spring of 2009, butterweed (with its yellow flowers) made some fields in east-central Illinois look like canola fields.

Butterweed, also known as cressleaf groundsel, is a winter annual that germinates and emerges from September through November. In spring, growth continues until the life cycle is completed in early June when butterweed produces seed.

“Glyphosate is very effective on this species when small, but it is much less effective on the near-mature plants,” says Aaron Hager, University of Illinois Extension weed specialist.

Big yield impact

Come spring, uncontrolled infestations from this winter annual and other weeds that make it through winter end up clipping more yields than you might think.

Iowa State University (ISU) research shows weed completion can clip yields by .5 bushels per day over a 16-day window from VE (emergence) to V2 (second fully extended leaf). Yield losses worsen the longer weeds remain. From V4 to V5 (fourth to fifth visible leaf collar), daily losses tallied 17.2 bushels over a seven-day time frame – a 120.4-bushel-per-acre yield loss. Fall weed control enables you to get a jump on winter annuals and others before they stifle next year's crops.

“Usually, we see an opportunity for fall-applied herbicides when farmers in conservation tillage have perennials like dandelion or winter annuals like henbit,” says Mike Owen, ISU weed specialist. “These weeds are more easily controlled with fall applications than spring applications.”

Fall herbicide application also does the following:

● Creates a better seedbed. “The field can be a mess with a thick mat of weeds if people have winter annuals and don't make a fall herbicide application,” says Carroll Moseley, Syngenta herbicide brand manager. “That (weed) mat can delay planting.” Thick flushes of winter annuals are also compounded by lack of residual herbicides the preceding spring and summer, he says.

● Controls insects. A salad bowl of weeds provides a lush environment for insects to thrive. “By controlling weeds, you prevent an overwintering site for nematodes and insects,” says Frank Carey, Valent field market development specialist.

● Conserves fertilizer. “Winter annuals use nitrogen (N) fertilizer,” says Carey. Nixing winter annuals before they start keeps fall-applied N intact for your crops next spring.

● Aids timely planting. “Fall application gets the product down and controls weeds prior to planting,” says Matt Foster, Authority XL product manager for FMC. “If you have a wet spring, you can have problems applying herbicide. When this happens, you lose yields that you can't get back.” It also helps ensure timely planting into a clean field during the spring time crunch, he adds.

● Thins the weed seed bank. “Winter annuals repopulate the seed bank when they seed in the spring,” says Jeff Carpenter, DuPont corn herbicide product manager. Fall weed control stops future infestations in their tracks.

● Adds modes of action. “Applying a preemergence herbicide is also good for resistant management,” says Foster. Mixing up herbicide modes of action helps forestall herbicide-resistant weeds from developing.

Will anything be left come spring?

There's a problem on the other side of the winter, though.

“Because herbicides begin degrading when you apply them in the fall, you will have less of them remaining at planting and less weed control of early season weeds,” says Bob Hartzler, ISU Extension weed specialist.

Thus, another herbicide burndown application will be needed to control weeds that break through the fall application.

“If you choose to go with a fall herbicide application, it must replace the need for the spring burndown,” says Kevin Bradley, University of Missouri Extension weed specialist. “Otherwise, a fall herbicide application is just an added cost in the overall weed-management program.”

There are cases, though, where winter annuals, biennials, and perennials can be so thick that a fall application is needed.

“If you don't kill marestail (which can follow a winter annual life cycle) early, it can develop a thick root mass and a big taproot that make it difficult to kill,” says Carey. “There are a variety of products you can use in the fall that will let you plant in the spring. You just have to pay attention to the replant intervals of the chemical you apply in the fall.”

Successful weed control will take more than glyphosate, for it must have a long-lasting residual component. The price also must be right, says Mark Loux, Ohio State University (OSU) Extension weed specialist.

Be careful, though. Most of your herbicide dollar is best spent in spring preemergence and postemergence applications. Loux advises budgeting between $4 and $12 per acre for a fall application and targeting problem fields with weeds like marestail and giant ragweed infestations.

Fall herbicide applications also tend to be suited best to certain regions. “Fall applications have their place like in the South, where many weeds tend to germinate in the fall,” says Rick Cole, Monsanto technology development manager. “Palmer amaranth is a great example. It germinates before and during the season, and then continues to germinate in the fall. Fall herbicide applications are not a one-size-fits-all solution.”

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