After the Fallow

Watch for Weeds, Nutrient Problems on Empty Acres

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Flooding caused millions of acres to go unplanted in 2015, and growers will face tough weed populations and possible nutrient deficiencies there in 2016. (DTN photo by Elaine Shein)

ROCKVILLE, Md. (DTN) -- Fallow acres never stay empty long -- just ask Jim Crawford, superintendent of the University of Missouri's Graves-Chapple research farm.

As record rainfalls in May and June kept frustrated farmers out of the field last year, Crawford watched the low-lying river bottom acres of northwest Missouri grow up into dense, waist-high fields of weeds.

"It was your standard mix," he recalled. "Marestail, waterhemp, lambsquarter, sunflowers. Without any burndown or pre-emergence applications, and no crop to shade out the weeds, they just went nuts."

By the time fields dried out enough for farmers to venture in with equipment in early July, the only option was to till the weeds into the soil. "They knew they were creating 10 years worth of seedbank," Crawford said. "But there was nothing to spray on it strong enough at label rates to kill it."

In Missouri alone last year, farmers were forced to declare prevented planting on more than a million acres of soybeans and half a million acres of corn. Across the country, 2.4 million acres of corn and 2.2 million acres of soybeans were left unplanted in 2015, according to the USDA's Farm Service Agency.

Spring will serve as a reckoning time for the farmers of those fields, where massive weed seedbanks and possible nutrient deficiencies await growers, crop experts told DTN.


Weed control is a standard spring concern, but in fallow acres that problem will be terribly magnified, University of Missouri weed scientist Kevin Bradley said.

"If you had a prevent plant situation last year, there is going to be more seed out there than you've ever experienced," he said. "Most of those prevent plant acres were just monocultures of waterhemp."

The traditional weed control mantras -- aggressive pre-emergence and burndown applications, spraying when weeds are 2 to 4 inches tall, and mixing up your modes of action -- will be more important than ever.

The sheer volume of weeds coming up this spring will make the possibility of herbicide-resistant weed escapes more likely, Crawford added. "You just cannot wait until they are a foot tall out there, because it will be such a dense a mat of weeds that they will all survive."

Keep in mind that pre-emergence applications will need to be followed up by another residual herbicide around V3 or V4 in both corn and soybean fields, Bradley said.

This heightened need for weed control comes at an unfortunate time given the farm economy, but Bradley is urging growers not to cut corners in this area of their operation. "I hear a lot of people talking about cutting something and I'm just hoping they've learned from the past on weed control mistakes," he said.


Fallow field syndrome, which surfaced in fields after the record 1993 floods, should be on growers' radar once again, University of Missouri agronomist Bill Wiebold told DTN. Each year, beneficial fungi infect the roots of growing crops and help the plants absorb important nutrients, like phosphorus. These fungi need a host in order to live and multiply, however. Fallow fields are at risk of lower populations, which will make any existing phosphorus deficiencies especially problematic in the spring, particularly for corn crops, Wiebold said.

Corn growers should also be on the lookout for nitrogen problems, given the potential for leaching and denitrification last spring, Wiebold added. "Just watch your corn," he advised. "If when it's knee high, it's telling you it needs some N, some later application is probably appropriate."

Don't assume your fallow acres necessarily require soybean innoculants, however. "The rhizobia bacteria are free living bacteria that can live in the soil without soybean plants, so they only decline if fields are flooded for a long period of time and they have no oxygen," Wiebold explained. "In the drought that followed the flooding this summer, oxygen was not a problem, and I think populations wouldn't decrease any more than they would from a corn crop being there."

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Emily Unglesbee