Fall weed growth driven by an early harvest or wet conditions may have you considering a herbicide application to make life easier next spring.
When to apply herbicide is critical, and time can be condensed given a late harvest. To get a good burndown, it’s best to wait for more weeds to emerge, but not let them grow beyond two inches in height.
Corn Belt agronomists and weed scientists also recommend a spraying cutoff date before a freeze. “Once that occurs, weeds like dandelion and thistle become less sensitive to herbicides, although the winter annual weeds are still susceptible” says Mark Loux, Ohio State University weed scientist.
What is the ideal time to apply fall herbicides? When the morning temperature is above freezing combined with a few days of 50 degrees or higher daytime temperatures, apply herbicides during the middle of this period.
Warmer temperatures help systemic products like 2,4-D and glyphosate achieve better foliar control. Be sure to check herbicide labels for exact recommendations, rates and timing. Weed control is reduced when temperatures drop below 40 degrees.
If spring weather blew up your original weed control plans, as confirmed from the combine windshield, applying a herbicide treatment in the fall can buy you time next spring.
Walking fields after harvest, especially in weedy areas you noted from the combine, often involves pulling up corn residue to check for new weed growth. Weed seeds that lack dormancy, like marestail and some winter annuals, can drop seeds that can grow immediately to increase fall weed problems.
Depending on your geography, weeds that can drive a beneficial fall application are herbicide-resistant marestail (horseweed) and common winter annual weeds like downy brome, henbit, field pennycress, prickly lettuce, shepherd’s purse and tansy mustard.
“In addition, we have seen an increase in dandelion, wild carrot, Canada thistle and annual bluegrass in Ohio, which are good to control in the fall,” Loux says. “We’ve also seen a relatively new weed, roughstalk bluegrass, which is about impossible to control in the spring.”
If dandelion is left unchecked, it can grow large taproots making spring burndown difficult. “If sprayed in the fall, even a slight frost can help take herbicide down into the root to aid control,” Loux adds. Leaving marestail until spring can result in taller plants that can more easily evade the control by auxin herbicides like 2,4-D and dicamba.
Once scouting identifies your weed mix, match it with the right tank-mix. Typical fall burndown herbicide options include 2,4-D, glyphosate, dicamba and a number of herbicides that contain ALS-inhibiting herbicides—tribenuron, rimsulfuron, thifensulfuron, and chlorimuron (prior to soybeans only). If an early harvest occurs, the residual control from metribuzin can help control herbicide-resistant marestail and late-emerging weeds until the ground freezes. And if you plan to plant corn, simazine can also be applied in the fall.
Don’t expect fall-applied residual herbicides to control weeds in the spring. One exception noted by researchers in the eastern Corn Belt is chlorimuron. An effective fall treatment using chlorimuron can keep weeds at bay through late April, as long as they are not ALS-resistant.
When examining your overall weed control program, Loux believes using a strategy of a fall application every other year, especially before rotating to soybeans, can help reduce weed pressure. In addition, a basic tank-mix is relatively inexpensive to get good fall weed control, as long as harvest and weather cooperate.
Spraying in the fall spreads out the spring workload to increase flexibility. Buying more time in the spring can optimize weed control by spraying smaller, more uniform-sized weeds. And decreased competition from weeds can maximize yield potential.
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