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Corn+Soybean Digest

Big On Burndown

Two years ago, an array of winter annual weeds stretched out across Bob Drake's no-till soybean fields, forming a dense, green carpet that left the southern Ohio farmer feeling frustrated and unsure of what control measures to take.

“Purple deadnettle, henbit, chickweed, marestail — it was like we had continuous (winter) annuals out there,” recalls Drake, of Ross County. “They kept the soil from warming up in the spring and made such a mat in fields that it was tough for us to plant.”

The solution, Drake decided, was a fall herbicide application, a weed control strategy most corn and soybean growers are aware of but that few practice.

“It really worked well for us,” Drake says of the glyphosate-2,4-D combination he still uses each year following harvest.

Fall herbicide applications can be an effective control strategy for Midwest farmers to help manage cool-season perennial and winter annual weeds, particularly in no-till fields, according to Mark Loux, Ohio State University extension specialist.

For instance, Loux notes that spring control of well-established dandelions with 2,4-D and/or glyphosate is inconsistent and extremely slow at times. However, a fall application of 2,4-D, or other systemic herbicide, will move with the available nutrients into the roots of dandelions during the fall, prior to ground freeze, and provide better control.

University of Illinois (U of I) research indicates that by minimizing vegetation in fields from late fall through the following spring, growers can also benefit from reduced insect pressure because pests have no protection from the elements.

Loux says, “If you have a nasty winter annual or dandelion weed problem, fall applications can be a good call for growers.”

He estimates Ohio growers invest at most $6-8 in herbicides for the burndown and residual control needed. He adds that some growers spend as little as $2-3 in herbicide costs and still reap adequate benefits.

Plus, he says spraying an active ingredient with a mode of action different from glyphosate helps minimize any foothold for resistance problems, such as those found with giant ragweed that exhibits ALS resistance in parts of the Midwest.

“With the residual from the fall application, you stand a decent chance of needing only one (post) application of glyphosate next spring in your beans,” Loux says. He estimates that 70% control of weeds in fields is common with residual herbicides, such as Canopy, at the time of growers' second or post herbicide application.

Loux adds that about 10% of Ohio no-till soybean growers now use fall applications. He says more growers, particularly those in Ohio, southern Indiana, southern Illinois and Missouri, could benefit from fall weed control measures if they gave consideration to soil type, geography, weather conditions and temperatures prior to making their applications.

Al Zirk agrees. As a retail manager for UAP Great Lakes in DeForest, WI, Zirk says there are risks associated with fall applications that growers need to consider.

Those risks include frozen ground at time of application, poor weather conditions such as heavy winter rains and snow, and sloping ground, all of which can promote herbicide runoff and minimize a product's effectiveness.

However, he notes, “A fall treatment can work well, and we do recommend it where it fits.”

Unfortunately, many corn and soybean growers across the Midwest experienced more of the negatives instead of the positives when fall weed control measures were first introduced.

“Some soybean growers here tried fall applications when the concept was new a few years ago, but ended up having to use their spring weed control methods anyway,” says Dennis Bowman, U of I extension agronomist at Champaign, IL. “You're putting a lot of pressure on the herbicide to hold down weeds through winter and early spring until you can come in with that post application of glyphosate. In general, growers here in Illinois just found they didn't have clean fields.”

Bottom line, Bowman says, “The question is are you willing to risk your chemical dollars on a fall application that may or may not provide you the weed control you'll need come spring.”

For Drake, the answer was and still is yes. “We always try to get that fall application made after harvest, most any time the ground's not frozen. For us, it's well worth the risk you run of having to make a third chemical application,” says Drake, who farms with his father Robert, brother Pat and son Jason.

Zirk says he is careful to encourage fall herbicide applications for only those farmers he is sure will benefit.

“Fall applications can be good for large farms where you have to cover a lot of ground in a timely manner. It can help you spread the work load so you aren't in a time crunch come spring, and the farmer can usually plant earlier,” says Zirk, who makes fall applications, typically using Valor herbicide, on about 3,000 no-till acres in parts of south-central Wisconsin.

Southern Illinois University extension service recommends three basic approaches for growers who want to try fall herbicide applications:

  1. Apply a herbicide with soil-residual activity before most of the winter annual weed species germinate.

  2. Apply a non-residual herbicide, such as glyphosate, 2,4-D or Gramoxone, to emerged winter annual, biennial and perennial weeds while they're still relatively small or in the rosette stage.

  3. Use a combination of the first two approaches.

All of these approaches strive to reduce the amount of total vegetation that needs to be dealt with in the spring before planting.

Loux adds that his research in Ohio fields has shown good control results with 2,4-D paired with Canopy and Express herbicides.

Drake makes fall applications using 1 pt. of glyphosate and 1 pt. of 2,4-D per acre, along with ammonium sulfate. He likes this combination of products because it gives him planting flexibility on his river bottom ground, which sometimes floods and retains water in the spring.

While fall herbicide applications work for him, he realizes that fall weed control tactics aren't for everyone.

“The best thing you can do is try this on a few acres to see if it works in your situation,” he says. “If it does, you can always increase your acreage the next year.”

Loux agrees. He adds that fall weed control measures are most beneficial to those growers who then scout their fields for early emerging weeds, such as giant ragweed (horseweed), the next spring.

Unfortunately, growers are time-pressed and spend less than ever scouting fields, so Loux offers this advice about post applications as a rule-of-thumb: “You need to spray when the weeds are no more than 6 in. high,” he says. “Beyond that, you're losing money because those weeds are competing with the beans.”

He adds that growers often like to wait until soybeans are close to canopy before spraying glyphosate, which is too late. “Don't let the bean canopy dictate what you do,” he says.

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