Corn in a Deep South crop rotation remains one of the best weed management tools or decisions a grower can make — when he can make it. A corn crop squeezed into a field at least every three years in a corn-cotton-peanut cycle is most effective.

“There is an inherent value to a good crop rotation that is likely priceless, especially in the long-term weed management of a farm,” says Eric Prostko, weed specialist with the University of Georgia Cooperative Extension.

For corn particularly, its arsenal of herbicides is a welcomed addition to fields, he says, noting that most common field corn herbicide programs farmers use all give similar weed control results.

“But for one big reason, corn is the only major crop we grow where we don’t have to use a PPO (protoporphyrinogen oxidase) herbicide. Atrazine is carrying the load for us with corn” he says.

That herbicide’s economic, broad-spectrum weed control is certainly a plus, but the biggest benefit it brings to fields in the Deep South is its control of pigweed — a problem that isn’t going away.

For south Georgia farmer Philip Grimes, the atrazine-glyphosate one-two punch that his corn rotation provides is essential to his management of herbicide-resistant pigweed that showed up on his farm a couple of years ago. The resistant weed has been confirmed in the state for six years, but likely was present for closer to a decade.

“Corn works well in my rotation to clean up pigweed,” says Grimes, who grows 1,800 acres of peanuts, watermelons, cantaloupes, cotton and corn, and has a reputation for hating weeds. “Rotation is the most important thing you can do for yield and weed management, too.”

Georgia does have confirmed atrazine resistance in one location in the middle of the state, Prostko says, and it is suspected in other counties, particularly in heavy dairy regions with histories of corn and sorghum rotations.

“The only downside to atrazine is that is doesn’t last as long in the wet, warm and humid southern climate, and in soils where it has been used repeatedly,” he says.

But for corn, it isn’t pigweed that’s the problem, Prostko says — it’s morningglory, which caused big problems in some fields during the 2013 growing season, which saw record to near-record rainfall over much of the region. “Morningglory control will always be a challenge for southeastern field corn growers.”

There is no herbicide on the market that will control morningglory season-long, because the region’s climate favors late-season emergence of morningglory plants as the crop matures and the canopy dries down, which can be as early as June for Deep South corn fields.

Another benefit of field corn in a rotation is that it sets the stage to knock out fall-emerging Palmer amaranth populations and helps get the following planting season off to a better and cleaner start. 

Corn harvest winds down in July or early August in the Deep South. Frost doesn’t hit until late October, or even later the farther south you go, plenty of time for pigweed to grow and deposit a monster seed load into a field for a tough surprise the following planting season when temps heat up.

Farmers must knock back late-season pigweed flushes, or they will regret it, Prostko says. “If they don’t, a year or two worth of in-crop control has gone down the drain.”

Some Tennessee farmers show interest in double-crop soybeans behind wheat and corn. Some get 30 bushels per acre behind corn if they can get the beans planted in July.

But threading the needle on which herbicides to use in the corn to allow that early plant-back in soybeans — well, that can be tricky, says Larry Steckel, University of Tennessee weed specialist, who notes that a quick turnaround on crops eliminates corn’s main go-to herbicide.

A farmer who wants to double-crop soybeans must start off clean, then use a program that includes a preplant or preemerge herbicide followed by something postemergence, Steckel says. And a good wheat stand will shade out emerging weeds, a good situation to follow with beans to reduce the Palmer amaranth seed bank.

“On the other hand, in thin stands of wheat or in drowned-out areas, there is often considerable weed pressure. The two weeds most common in these situations are horseweed and Palmer amaranth.”

If summer turns dry, Steckel says, Tennessee growers will want to be conservative on how much to invest in herbicide for a soybean crop that may or may not set good yields in dryland.

It is unrealistic at this point in the herbicide-resistant weed conflict increasing across the Deep South to expect to win the struggle against the weeds with herbicides only. Growers must start off with clean fields, use a residual, and spray postemergence herbicides early.

Both specialists agree southern growers have come a long way in recent years in becoming good weed managers. Farmers had to do so — resistance was about to eat them up.

“One reason we’ve started seeing some success, some improvement, is because growers now have re-found, so to speak, their appreciation for residual herbicides,” Prostko says, noting they had lost that appreciation for a time.

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