Fitting in Fall Herbicides

Post-Harvest Spraying Decisions Loom

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Growers have a tight window to spray herbicides this fall, but keeping fields clean this winter could be worth the effort. (DTN photo by Pam Smith)

ST. LOUIS (DTN) -- Harvest is lagging far behind normal on Scott Meadows' no-till farm in northwestern Missouri.

Ideally, the Mound City, Mo., farmer likes to spray herbicides after harvest to clean up newly sprouting winter annuals, but Mother Nature has had different ideas this fall.

"With resistant weeds, we've had to become more proactive and get to weeds earlier when they're smaller," he told DTN. "But it's wet and late, and it all depends on timing."

With only one-tenth of his acres harvested and more wet weather on the way, Meadows said he'll have to pick and choose which fields get a fall application this year.

As harvest slowly advances, it's time to check fields for weed levels and consider squeezing in a fall herbicide application, weed scientists told DTN. However, when weighing the decision to spray, growers should keep in mind what fall herbicide programs can and cannot do and when they are most effective.

A NARROW SPRAY WINDOW

Much of the Midwest has seen abundant moisture this fall, which could provide weed seeds with a great growing environment, Iowa State weed scientist Bob Hartzler pointed out. Winter annuals such as marestail, henbit, pennycress, shepherd's purse, and chickweed can infest fields in the fall and become major headaches in the springtime.

"I suspect we'll see more winter annuals than the last two years," Hartzler told DTN. "I think it will be worthwhile for no-till growers to scout fields and see how many winter annuals are out there. They might have heavier infestations than they're accustomed to that would warrant an application."

If you still have crops standing, they could suppress weed emergence for now, Meadows noted.

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"So far our fields are pretty clean, due to the late harvest," he said. "We've had cover on the ground and the lack of sunlight helps."

However, the delayed harvest will also tighten the window for spraying, as temperatures dip ever lower in the coming months, Hartzler noted.

"You do want to [spray] while conditions are still warm enough for plants to be growing," he explained. "The weed has to be growing for the herbicide to disrupt something."

Fortunately, winter annuals are adapted to grow in 50 and 40 degree temperatures, he added. Moreover, even a late-fall herbicide pass in cold temperatures can be better than nothing.

"Fall applications are generally very effective because even if the herbicide might not completely kill the weed, it weakens it enough that it can't survive the winter," Hartzler explained. "You can get by spraying in colder temperatures in the fall better than you can in the spring, because of the added stress of the coming winter."

KNOW YOUR HERBICIDES' LIMITS

Fall herbicide applications don't have residual superpowers, University of Arkansas weed scientist Tom Barber warned growers in a university newsletter.

Many growers are under the misconception that fall herbicides will continue to work for them well into spring planting season. "This is simply not true," Barber wrote. "Residual activity of herbicides applied in the fall begins to decrease rapidly in early March, when soil temperatures begin to increase."

As a result, you can't rely on fall herbicides to control summer annuals like waterhemp and Palmer amaranth, and spring applications may still be required.

Also, less expensive fall herbicide options like glyphosate and 2,4-D can burn down existing weeds but will not supply any residual activity, Hartzler noted. "The farther south you go, you can have weeds that continually emerge into the fall, so having something with residual activity may make some sense," he said. "But again, don't apply it at a high rate thinking it will still be there in the spring."

Stay focused on what your fall herbicide program can do for your operation. Spraying in the fall will allow you to kill persistent winter annuals when they are still small and easy to control, Hartzler said.

Marestail is especially well worth targeting. Soggy spring weather often keeps farmers out of the field, and the prolific weed can grow very quickly, Hartzler noted. With the spread of many herbicide-resistant populations, controlling marestail in the spring can be quite challenging, he added.

Fall weed programs can eliminate insect overwintering sites and reduce the number of egg laying spots next spring for insects such as black cutworms. Research also suggests certain annuals like henbit and purple deadnettle can act as especially good hosts for soybean cyst nematode, University of Missouri weed scientist Kevin Bradley noted in a university newsletter.

Fall herbicides may also help conserve valuable resources like nitrogen and soil moisture, Bradley added.

For example, a recent study from Kansas State showed winter annuals stole an average of 16 pounds of nitrogen per acre from fields where they overwintered.

For more details on how fall herbicides can help your operation, see Bradley's article here: http://goo.gl/….

For more details on fall herbicide options for different crops and weeds, see this Ohio State University publication: http://goo.gl/….

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee.

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Emily Unglesbee