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Marestail Worries? How About A Fall Herbicide Application?

As if fall wasn't busy enough...

Putting down a herbicide application this fall could help you keep weeds knocked down better next spring in a lot of circumstances in no-till production systems, 2 Purdue University specialists say in a report released this week. The utility of fall herbicide applications isn't the most widely accepted practice, but in the right situation, applying something like glyphosate, 2,4-D or dicamba starting in a couple of weeks could go a long way to keeping down winter annual emergence prior to corn and soybean planting time next spring, say Travis Legleiter, weed scientist, and Bill Johnson, plant pathologist, both of Purdue University Extension.

"The necessity of a residual herbicide in the fall is always in debate amongst producers and weed scientists. A residual herbicide applied later in the fall can keep fields cleaner longer in the spring, and can in some years provide enough activity to keep fields clean up to planting. With the cold, harsh winter we experienced this past fall, residual herbicides persisted well into the spring planting season. There were several cases this year where residuals persisted too long and soybean injury occurred because of additive effects from the remnant fall residual and a spring residual that was applied," according to a university report from Legleiter and Johnson. :The success of this past years fall residual herbicides will not occur every year, it all depends on the weather and we all know it’s improbable to predict what the winter and next spring will bring."

There are a few specific circumstances under which a fall application is critical; if you're no-tilling soybeans and have a marestail problem, for example, Legleiter and Johnson say a fall application's "a must." More generally, though, think immediate control first, not residual, and target what you can see now, not what you think you might have a problem with later on.

"The recommendation from Purdue has been and will remain to be that fall applications should consist of products that will control the weeds that are present and to save the use of a residual herbicide until as close to planting as possible in the spring. This eliminates the guessing game of what the winter and spring will bring and whether of not an additional residual application will be needed. A planned fall burndown without residual followed by a spring burndown with residual assures that the residual will still be present into the growing season. However, given our continual struggle to control marestail throughout much of the state, we are revising this recommendation in areas where additional horsepower is needed for marestail control," according to Legleiter and Johnson's university report. "The use of a fall application, regardless of whether or not it includes a residual, is a must if you are trying to control marestail in no-till soybean. The emergence pattern of marestail in fall as well as in the spring and summer means that multiple herbicide applications are needed and these applications need to start in the fall. Again the fall application needs to focus primarily on controlling the marestail rosettes that emerged in the fall and we will like to see a low-cost residual component added to the foliar product. The residual component should not be expected to provide residual control of marestail in the spring for more than 2 weeks. A program like this will make the spring burndown more effective as there will be less marestail plants to control and the plants present will be the smaller spring emerged rosettes, rather than large bolting fall-emerged plants."

Though it's not the only weed that's problematic this time of year and holds the potential to jump into next spring easily without treatment now, marestail is just one weed that you need to watch for and treat in a fall application if early-spring weed pressures concern you. Just make sure you're selecting what you put down carefully and how you approach your whole herbicide program with attention to what your specific fields need.

"The treatments that included 2 burndown applications with a residual included in at least one of those applications not only had the highest control of marestail, but also the most consistent. The treatments that included fall residuals were the highest and most consistent, again this is due to the delayed spring and extended persistence that will vary from year to year," according to Legleiter and Johnson. "The treatment that did not include any residual herbicides had the lowest amount of control and was the least consistent. This again solidifies the need not only for multiple burndown applications, but also for the use of residual herbicides to manage marestail in no-till soybeans."

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