Wallaces Farmer

Winter annual question: To treat or not to treat?

For most growers, weed management is an ever-present part of their operation, but battling weeds is also much more complex than simply spraying in the spring and moving on to something else.

Mark Yontz, Contributing Editor

September 17, 2014

5 Min Read

For most growers, weed management is an ever-present part of their operation, but battling weeds is also much more complex than simply spraying in the spring and moving on to something else.

This is especially the case when evaluating the impact of winter annuals on your fields, and then determining whether or not fall applications are justified. Unfortunately, there’s no one-size-fits-all formula to help make the decision to treat or not, but it’s important to know that winter annuals can have a direct impact on your yields (and bottom line) if left unchecked.


Common chickweed, field pennycress, shepherd’s purse, henbit … the list goes on when it comes to winter annuals that can negatively impact your fields. But a problem in one state or region may be virtually nonexistent in another.

“Winter annuals can be somewhat regional, but a lot of the species we’re seeing are the same ones we’ve seen over the last 15 to 20 years,” says Aaron Hager, an associate professor in the Department of Crop Sciences at the University of Illinois.

However, Hager says an “explosion” of marestail (or horseweed) about two years ago in northern Illinois really got people’s attention, especially after it showed a resistance to herbicides. He says it’s a good example of a situation where fall applications can be more effective than spring spraying in controlling certain species. That’s what they’ve discovered to be the case with marestail, a weed that can take on the characteristics of both a winter and spring annual.

Beating marestail

“Fall applications can be more consistent because the plants will be smaller than what they are in the spring; and you’re not going to treat every acre. You only apply to the trouble spots,” says Hager.

But before you can jump into fall applications, you have to do some work ahead of time to identify not only your problem weeds, but also where the problem areas are in your fields.

“Without a doubt, you have to scout and identify areas of concern,” notes Hager. “The weed history of the field is also important, but it’s important to remember that this doesn’t necessarily account for new weeds in your field.”

Given this, taking field notes and marking problem spots using GPS during harvesttime are good practices when evaluating your fields for winter annuals. But don’t overlook simple, nontechnical solutions either when assessing your fields.

“The best technology to see trouble spots is boots-on-the-ground,” says Mike Owen, a professor and weed science Extension specialist at Iowa State University.

To make this work Owen believes growers need to learn how to identify weeds accurately. He says investing in some type of weed identification app that can be used in the field would go a long ways toward more successful weed control practices, including the use of site-specific management practices in dealing with winter annuals.

“You first have to clearly identify what type of target weeds you’re trying to manage, and then determine how big the problem is and act accordingly,” says Owen. “But growers also need to set realistic expectations for what results their applications will provide.”

Fall strategy

For example, to address winter annual problems, Owen says a fall application should offer some immediate burndown, but also provide residual treatment benefits. However, this is where he believes growers need to be realistic about what fall applications will do over the long term.

“Don’t think your fall applications are going to take care of your spring weeds,” cautions Owen, who says growers should look at using both chemical application and tillage practices to deal with weeds throughout the year.

In the end, deciding whether or not to treat a winter annual problem is dependent upon a number of variables, many of which a grower has no control over. However, you also have to remember that both action and inaction come with a cost attached. Either way there will be an impact on your bottom line.

“No one can predict what conditions will be like next spring, which is an argument for taking care of winter annuals now,” says Hager. “So if you invest now, and do not allow these plants to add to the seed bank, it may really pay off for you in the future.”

Being forward-thinking and having a willingness to adapt to changing conditions are key aspects of addressing any winter annual problem.

“Be flexible and think proactively about the alternatives so you’re prepared from both a tactical and economic standpoint,” recommends Owen.

8 tips for fall herbicide application

Aaron Hager, an associate professor of crop sciences at the University of Illinois, offers these tips before making that fall herbicide application:

1. Scout fields to determine what weeds are present.

2. Check the label to see if the herbicide you want to use is labeled for fall use; not all are.

3. Check timing restrictions to determine if there are limits to a herbicide, even if it is labeled for fall use; and you’ll want to maximize the control you get based on the herbicide you choose.

4. Combining two or more herbicides can boost the weed control spectrum, but be sure to use the
appropriate spray additives, too.

5. Geography plays a role in winter weed control; work with your local agronomist to determine the best approach.

6. Fall applications of soil residual products don’t always mean a clean field next season. Delays in spring field work may allow fields to green up before you plant.

7. Don’t use fall herbicide applications as a strategy for tackling summer annual weeds. Control summer annual weeds — like waterhemp — with products used closer to planting.

8. The rise of some resistant weed species may show application in fall to be more efficacious than in the spring.

-Yontz writes from Urbandale, Iowa.

Solution Center is independently produced by Penton Farm Progress through support from SureStart® II herbicide. For more information, visit GetMoreTime.com.

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