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Corn+Soybean Digest

Weeds Could Fill Void Created By Departing Crops

Winter annual weeds could be plentiful, says Bill Johnson, Purdue University Cooperative Extension Service weed specialist. He urges farmers to inspect their fields after harvest for signs of the unwanted vegetation, and take appropriate action to stop the intruders.

"An abundance of moisture in the early part of September has led to a number of weeds emerging," Johnson says. "I've observed henbit, chickweed, purple deadnettle, marestail and a few others in the last couple of weeks. Because there's been so much rain it's diluted corn herbicides, so we're not going to get weed suppression into the fall from our soil-applied herbicides. If we have a warm, moist fall we could potentially have an overabundance of winter annual weeds."

Winter annual weeds germinate in the fall, survive vegetatively through the winter, resume growth in early spring and then flower and set seed mid- to late spring. The weeds usually die in the summer.

Farmers might be inclined to relax once crops are harvested, but they shouldn't, Johnson says.

"One of the best things you can do within a couple of weeks after harvest is look at the field, pull the residue back and see if weeds are starting to come up," he says. "Many times we pull the combines off the field and we might do some tillage, but we don't think a lot about the field until spring. But if you look closely – and these fields are relatively easy to scout in the fall – you can see some winter annual weeds starting to come up. If they're coming up and they're relatively dense, you may want to consider making some kind of herbicide application."

Herbicide treatments vary by crop and tillage system, Johnson says. For instance, fall-applied herbicides aren't just for no-till acres. Winter annual weeds can cause problems in conventional till production, as well, especially when harvest and post-harvest tillage is done early. Farmers in those situations should carefully weigh their weed management options, Johnson says.

Producers intending to plant winter wheat this fall also should factor weeds into their production strategies. Johnson recommends starting with a clean seedbed, which can be achieved with tillage or by applying Gramoxone or a glyphosate-based herbicide. Growers should not apply 2,4-D either before or after planting wheat, Johnson says.

Herbicide practices are different for fields that will sit idle until being planted to corn or soybeans next spring.

"There's a couple of things to consider when deciding how to manage winter annual weeds before growing corn or soybeans," Johnson says. "The first thing is to make sure the product you choose allows some rotational flexibility. There are a number of different products that allow you to grow either corn or beans, and there's just a couple of products that tie you into growing either corn or beans."

Fields treated with Canopy XL or Backdraft herbicides must be planted to soybeans the following year, while those sprayed with Princep or a simazine-based product must next be planted to corn, Johnson says.

"Another thing to consider are those products that have residual activity in the soil," Johnson says. "Typically, you can make those applications as soon as you get the crop off the field in the fall. Those applications can be done as early as the middle of October. If you're using products that don't have residual activity in the soil, you should delay those applications as long as possible but try to make them when you have at least a couple of days in a row where daytime air temperatures are at 50 degrees or greater. That will maximize the activity of those products."

Additional recommendations for controlling winter annual weeds and a herbicide chart can be found in the Purdue Extension information sheet, "Fall Applied Herbicides," by Johnson and Purdue Extension weed scientists Tom Bauman and Glenn Nice. The information sheet can be downloaded online at

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