Heavy residue combats resistant pigweed

While glyphosate-resistant pigweed may threaten the gains made in high-residue conservation-tillage cropping systems within the last couple of decades, many of the nation's crop scientists are not giving up without a fight.

In addition to their brains and expertise, they're adding another formidable weapon: Obstinacy, coupled with an abiding confidence that, come what may, they will be victorious in the end.

Andrew Price, a plant physiologist with the U.S. Department of Agriculture's National Soil Dynamics Laboratory based at Auburn University, says the loss of glyphosate use due to the growing presence of glyphosate-resistant Palmer pigweed is being felt acutely on some farms. Even so, he says this does not call for the abandonment of high-residue conservation-tillage practices.

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Quite the contrary: Based on his own experience in researching high-residue cropping systems he says there is every reason to stick with these approaches — and not only for the advantages they provide in soil moisture retention and enhancing organic matter. High-residue systems also appear to be highly effective in suppressing weeds — and, as it turns out, Palmer amaranth, especially.

In fact, traits that have enabled Palmer amaranth to develop resistance to glyphosate also render it more susceptible to the effects of high-residue crop systems.

"Many small seeds, such as Palmer amaranth seeds, have specific light requirements in order to germinate," Price says. "Putting inches of residue on top of the seed prohibits light as well as other environmental cues required to germinate the weed, he says.

In adopting these high-residue cropping systems as a weed suppression strategy, Price says timeliness is of the essence.

The most successful farmers who have adopted these high-residue systems quickly follow harvest with a cover crop planting using a no-till drill, he says.

"They come in with the drill and within weeks have a cover crop," he says, adding that by getting an early start with a cover crop, farmers better ensure that sufficient biomass will be in place by the time the cash crop is ready to be planted.

Price is also advising producers to fertilize the cover crop at a rate of 30 pounds of nitrogen an acre, ideally in early spring to allow the cover crop to mature to ensure optimal levels of biomass.

"The goal should be not to terminate it (the cover crop) early, but to take it up as close as two to three weeks before the cash-crop planting," he says.

Price says this approach works especially well with peanuts and cotton, which can be planted comparatively later to allow the highest levels of biomass.

Corn, on the other hand, presents a challenge.

"Obviously, with corn, you're planting early, and that's a disadvantage in one respect because you're going to see less biomass."

Clover may be an option in some cases.

"As a legume, it doesn't offer the same advantages as rye as a cover crop, but it does have its merits," he says. "For example, with that system, you do have the advantage of certain effective herbicides, such as atrazine, a very efficacious herbicide in terms of preventing weed germination."

While a growing number of farmers are having success with these high-residue strategies, Price says there will be an added reliance on pre-emergent herbicides — one reason why he's encouraging farmers to adopt irrigation practices.

"Integrating these pre-emergent herbicides within irrigated systems is going to work much better in getting them activated," Price says. "The real problem with this approach comes with dryland systems where we don't get enough activation of pre-emergent herbicides."

Despite the success with these cropping systems, Price says he has turned up one irony: Minimal levels of tillage used in tandem with cover crops have proven effective in weed suppression.

"One of these (minimal-tillage systems) involves going in with fall tillage, burying the Palmer amaranth seed and then planting a high-residue cover crop without disrupting the soil.

"Tillage is something we typically want to see minimized, so in one sense we're presented here with a catch-22."

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