ROCKVILLE, Md. (DTN) -- The weeds under the pivot in April spoke volumes to Kansas farmer Karl Jacobson.
A carpet of flowering henbit sliced a slim, purple path through an otherwise pristine soybean field, marking precisely where the center pivot had blocked the Kansas farmer's herbicide pass the previous fall.
"I should have moved the pivot!" Jacobson told DTN. "We've been doing fall burndown for at least the past 12 years. It really pays."
Jacobson, who farms with his son, Ron, near Concordia, Kansas, will aim to spray most of his harvested acres by early November. He uses a combination of glyphosate and 2,4-D and adds metribuzin for its residual action, along with the flexibility to plant either corn or soybeans the following year.
His main targets? Henbit and marestail, two aggressive winter annual weeds that start growing in the fall and can quickly infest fields in the springtime.
"It does cost you a few dollars per acre," he said. "But henbit is a real nutrient and moisture robber, so it really pays off."
Purdue weed scientist Bill Johnson urges growers to think like this, even as they search for ways to lower their input costs.
"We had a statewide train wreck when it comes to marestail control in Indiana last year," he warned. "We have such a difficult time trying to burn down marestail in the cool, wet weather of the spring in the Eastern Corn Belt. Fall-applied herbicides are such an easier solution for these guys."
For those worried about costs, generic versions of dicamba and 2,4-D are the best bet, Johnson added.
Fall herbicide applications are aimed at winter annual weed populations. Marestail, henbit, chickweed, purple deadnettle and the perennial weed dandelion are the primary targets for most growers, Johnson said.
Winter weather will kill some of these populations, but not all.
"Depending on how severe a winter we have, we see that anywhere from 20% to 50% of what emerges in the fall makes it to the spring," Johnson said.
For marestail, which has shown resistance to a number of chemicals, including glyphosate and ALS herbicides, this can be a disaster.
"The ones that make it through the winter have a good root system and are pretty hardy plants in general," Johnson warned.
Speedy spring control is not always possible, which is what motivates Jacobson to get an autumnal jump on marestail populations in north-central Kansas.
"Marestail is a tricky little weed," he said. "You've got to get it when it's in the rosette stage. If it starts bolting up, it's often too late."
If growers add residual products to their fall burndown mix, they could see some residual control in the early spring, particularly in more northern regions of the country, Johnson said.
But never rely on fall herbicides to control spring-emerging weeds like Palmer amaranth and waterhemp, he added.
The ideal fall burndown spray window is often a moving target in October and November. Most growers need to finish up harvest before turning to spraying, but if they wait too long, cold temperatures could slam the window shut.
"The key thing to keep in mind is you want actively growing plants," Johnson explained. "If you have good soil moisture, daytime air temperatures consistently above 50 degrees and nighttime temperatures above freezing, the weeds will be actively growing and pretty sensitive to herbicides."
"Keep in mind that you can't spray herbicides on frozen ground -- that's a label violation," he added.
But if growers spray too early, winter annual weeds could re-emerge and require a second herbicide pass before the winter weather hits, Johnson said.
He also recommends giving no-till cornfields a rest period after harvest before spraying, to allow thick residue to settle.
"If you have really heavy residue, you need to increase carrier volume so you get good spray coverage," he said.
Jacobson aims to finish fall spraying and winterizing his sprayer before Thanksgiving. It can be hard to work in during the post-harvest rush, but he makes it a priority.
"One time we missed a field in the fall, and we had to disk the whole thing in the spring," he said. "It's a nuisance. We had to spend time and fuel to work the whole field. So now I just make it a point to do it in the fall."
Consult your local Extension office for fall burndown recommendations for your region. For example, here is Purdue's herbicide recommendations for growers in the Eastern Corn Belt: http://bit.ly/….
Keep plantback restrictions in mind when picking your burndown mix. See a breakdown of the common herbicides from the University of Arkansas here: http://bit.ly/….
Emily Unglesbee can be reached at Emily.firstname.lastname@example.org
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