Weed Sense

ROCKVILLE, Md. (DTN) -- You're not the only one relieved to see those empty crop fields. Winter annual weed species are stretching out their roots and enjoying the newfound space of your harvested acres.

Fall herbicide applications will be more important than ever this year, after resistant weeds made serious headway in the Midwest in 2016. But don't expect a spring miracle from fall applications, even if they contain a soil-residual herbicide, weed scientists warn. Fall residuals can control winter annual species like marestail, but they won't hang around long enough to fully control waterhemp and Palmer amaranth in the spring -- no matter what the label says.

"We are aware some products have [labeling] recommendations that suggest the product will control certain summer annual weed species following application in the fall," said University of Illinois Extension weed scientist Aaron Hager in a university Pest Bulletin article. "Particularly concerning to us is that 'pigweed species' are listed on at least one product label."

"From a biological standpoint, it doesn't make much sense to expect spring annual weed control from a fall herbicide application," added Purdue weed scientist Bill Johnson. "The highest dose occurs when weeds are germinating in the fall, not in the spring, because the herbicide will break down over the winter."

WINTER WEEDS

Marestail is thriving in the Midwest, Johnson said. Populations with two-way resistance to glyphosate and ALS herbicides are common in the Eastern Corn Belt. "There's no official confirmation of 2,4-D resistance yet, but I wouldn't be surprised if we see it in the next couple years," he added.

Other winter annuals that can cause problems in this region are henbit, chickweed, field pennycress, annual bluegrass and cressleaf groundsel (or butterweed). Perennials such as dandelion and Canada thistle are also the targets of fall herbicide applications.

For the increasingly aggressive populations of marestail, fall herbicides are becoming essential, Hager noted.

"Targeting emerged marestail with higher application rates of products such as 2,4-D in the fall almost always results in better control at planting compared with targeting overwintered and often larger plants with lower rates of 2,4-D in the spring," he wrote.

Farmers in the Eastern Corn Belt saw a "train wreck" with marestail infestations this year, thanks to missed or skipped fall herbicide applications, Johnson said. "The ones that make it through the winter are resilient, with bigger root systems," he warned.

Soil residual herbicides are a good idea early in the fall, but by early- to mid-November their usefulness wanes, Hager added. At this time of year, most winter annuals have emerged already, and the residual herbicide's availability in the spring is a giant unknown.

"Applying a soil-residual herbicide late in the fall in hopes of having a clean field prior to planting is akin to gambling on the weather," Hager said. A mild winter or early spring will almost certainly degrade any fall residual chemistries. While a long, cold winter can increase the persistence of a residual, the remaining concentrations could actually hurt your spring crop and won't control spring annual weeds sufficiently, he added.

PALMER AND WATERHEMP

Palmer amaranth made serious inroads in the Midwest this year, and waterhemp went wild in places, Johnson noted. "We had a real wake-up call on waterhemp this year, because combines spread it the previous year," he said of Indiana farmers.

With many populations of Palmer and waterhemp toting along glyphosate, ALS and PPO resistance, farmers should plan on spring residual applications to control those spring-emerging annual weeds, both Johnson and Hager said.

Without spring control, the small concentrations of fall residual herbicides in the spring can actually contribute to herbicide resistance, Hager warned.

"Soil-applied herbicides are not immune from selection for herbicide-resistant biotypes," he said. "Following a fall application, the concentration of herbicide remaining in the spring when Amaranthus species begin to germinate will be much lower compared with the same product rate applied closer to planting."

For more information, see Hager's university Pest Bulletin article here: http://bit.ly/….

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

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

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