pigweed resistance conservation tillage

CONSERVATION TILLAGE ACRES in the U.S. are at risk of being converted to higher-tillage systems due to the prevalence of glypohsate-resistant weeds. 

Conservation tillage systems threatened by herbicide-resistant weeds

Glyphosate-resistant weeds are now present throughout the Southeast. Hundreds of thousands of conservation tillage acres are at risk of being converted to higher-intensity tillage systems.

Southeastern growers know all too well the path of destruction left by glyphosate-resistant weeds. Now this scourge of farmers everywhere is threatening the advancement of conservation tillage.

This trend could have an even greater impact in states like Alabama, where conservation tillage systems are being used to help bolster anemic soils.

“In Alabama, we have a lot of highly erodible soils with organic matter of about ½ percent on most of them,” says Andrew Price, weed scientist with the USDA-ARS National Soil Dynamics Laboratory in Auburn, Ala. “Conservation systems are intended to increase that organic matter which in turn impacts productivity on those soils.”

But glyphosate-resistant weeds are forcing many growers to re-think their tillage strategies. Glyphosate-resistant weeds are now present throughout the Southeast, and hundreds of thousands of conservation tillage acres are at risk of being converted to higher-intensity tillage systems, says Price.

The shift to higher-intensity tillage facilitates the burial of small weed seed as well as the use of preplant incorporated herbicides for the control of problematic weeds, especially in dryland cotton production.

Problems with glyphosate-resistant Palmer amaranth pigweed have been well documented, says Price, recalling the story of an Alabama farmer who asked for permission to clear his neighbor’s field of the weed pest to prevent its spread to his own fields.

“And Palmer amaranth pigweed is not the only weed challenge in conservation systems. We’ve also been battling glyphosate-resistant marestail for quite a few years in north Alabama. In addition, Italian ryegrass is becoming a problem, and we’re keeping an eye on it in Mississippi and northwest Alabama,” he says.

Glyphosate use has increased while the use of herbicides with different modes of action has decreased, says Price, and conservation tillage increases mimic this trend because those systems go together hand in hand.

“Glyphosate-tolerant crops and the use of glyphosate as a burndown treatment facilitated the use of conservation tillage. Now we have this reverse problem where we’re battling the loss of conservation tillage acres due to the challenge of controlling resistant weeds,” he says.

The goal of all growers should be to start clean, says Price, and that can include effective burndown treatments, possibly some type of tillage, or irrigation to activate herbicides, allowing crops to emerge and become competitive with any weeds present.

Farmers are being told that tillage is the solution to controlling glyphosate-resistant Palmer amaranth pigweed, but there are other options, he says.

“We’re working with extreme cover crops, residual herbicides, and timely post-emergence herbicides. All of these can be used effectively to help with controlling glyphosate-resistant weeds.”

Searching for solutions

As Palmer amaranth has continued its march across the Southeast, growers have begun removing it from the landscape and tilling, says Price.

“But the flip-side of that is some of the work we’re doing to maximize cover crops, covering the soil and also the weed seeds on the soil surface in our conservation tillage systems.”

In conservation tillage systems, a heavy residue cover crop like rye can help reduce the emergence of glyphosate-resistant weeds by suppressing weed germination and growth, hesays. When the winter cover crop is planted early and managed for maximum growth, a dense mat is formed on the soil surface. In addition, conservation tillage systems that minimize soil disturbance (direct seeding or minimum tillage) can help reduce seed germination.

“Since weed emergence and growth are suppressed by the physical barrier and shading of the residue, more residue results in better weed control. Depending on the severity of the glyphosate-resistant weed infestation, multiple strategies involving the integration of cultural as well as chemical weed control will be needed to overcome this threat.”

Integrating high-residue cover crop systems may help facilitate weed control in row middles, says Price. However, weeds emerging in the crop row remain a threat to crop performance, especially in dryland cotton.

“We looked at three different planting dates in the fall of a rye cover crop. We had an early planting date where we accumulated a lot biomass, and as we went later in the fall we decreased the amount. We also used a conventional tillage system where we didn’t use inverted tillage but used disks and harrows. We also had a winter fallow in the trial.

“The worst pigweed was in winter fallow because we’re not burying it, we’re not using tillage, and we’re not using residue. We reduced it some with disking and field cultivation, but we did our best reduction with higher amounts of residue, regardless of the herbicide system.”

Prices urges growers to use any new weed control technologies with care.

“With some of our new technologies coming down the pike, like dicamba and 2,4-D resistant cotton, we are being very proactive in recommending that our producers keep using soil-applied herbicides that overlap to prevent the emergence of these problematic weeds and to also prolong their use. If we treat them the same way we did glyphosate-resistant technologies, we’ll also develop resistance to these other technologies.”

In addition to increasing the amount of residue on a field, farmers can help prevent resistant weeds by using a combination of soil-applied herbicides with different modes of action, and by intensifying their crop rotations which helps to break up the reproduction cycle of weeds.

“We’re looking at planting high-residue cover crops that will bury weed seed and not give them the stimulation they need to germinate. And then there are some general best management practices, including going out and seeing what’s in the field, mechanically removing weeds, minimizing residue disturbance, and using smaller width strip-till rigs if we’re using that type of system. We also can use shielded sprayers and get back to using post-directed type herbicides.”

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