Palmer amaranth  pigweed

Resistant weeds demand chemistry rotation, system approach

Cotton growers could have some new technology in 2015 to help them combat glyphosate-resistant weeds.

Palmer amaranth remains by far the No. 1 glyphosate-resistant weed in cotton production. “We haven’t whipped it yet, but we’re getting a better handle on it,” says Alan York, emeritus North Carolina State University weed scientist. “Most growers have stepped up to the plate to address the problem; we’re controlling it much better today than we were a few years ago.”

York says cotton growers are throwing “the kitchen sink” at resistant Palmer amaranth in terms of herbicides. Most of North Carolina’s cotton is grown strip-tilled or no-till, so Valor is commonly applied as part of the burndown. Growers then apply a pre-emerge herbicide, or combination of herbicides, behind the planter and later use residual herbicides such as Dual and Warrant postemerge.

“However, I feel we’re still falling short on layby sprays,” York adds. “We totally got away from them in glyphosate’s heyday and essentially parked our hooded sprayers and layby rigs. After glyphosate resistance hit us, growers returned to directed sprayers for a while, but now we are backsliding again. Our research has demonstrated consistently that making a layby residual application enhances our weed control.”

In addition to glyphosate-resistant Palmer amaranth, North Carolina growers have seen a lot of glyphosate-resistant horseweed or marestail and some common ragweed. “However, if you control Palmer, you’ll control ragweed in the process,” York says.

“Resistant horseweed is a widespread problem in no-till or strip-till, and is fairly easy to deal with. Glyphosate plus Valor plus either 2,4-D or dicamba burndown followed by paraquat or Gramoxone at planting has done a good job. Liberty in the crop is also good if the horseweed is not too big. We still have some growers who let it get away from them because they either don’t realize they have it or they never get around to the right burndown.

“Additionally, we have at least two counties in the southern Piedmont that have glyphosate-resistant ryegrass. That situation has the potential to get ugly. We grow more soybeans and corn than cotton in that part of the world, but ryegrass is as thick as a cover crop in some of those fields.”

Cotton growers could have some new technology in 2015 to help them combat glyphosate-resistant weeds. USDA has deregulated Dow AgroSciences’s Enlist Weed Control System for corn and soybeans; regulatory approvals are pending for cotton. The Enlist cotton will provide tolerance to Enlist Duo herbicide — which includes new 2,4-D choline and glyphosate, as well as glufosinate. Also pending regulatory approval is Monsanto’s Roundup Ready Xtend Crop System for cotton that combines three herbicide-tolerant technologies: glyphosate stacked with dicamba and glufosinate. Growers can apply multiple combinations of these herbicides pre- and/or post-plant.

System approach

“We’re excited about the potential of these new technologies, but they must be used correctly,” York says. “That is, we must use these tools in a system approach. For example, we can first apply a pre-emerge, and then make one or two Enlist Duo postemerge applications, maybe even adding Warrant or Dual, and follow with a layby. Using such a system approach, we have had phenomenal weed control.”

Additionally, in 2014, Bayer launched GlyTol LibertyLink TwinLink cotton varieties that contain full tolerance to both glufosinate and glyphosate herbicides. “Liberty is a good tool to control glyphosate-resistant Palmer amaranth,” York adds.

The North Carolina weed scientist stresses the importance of following stewardship guidelines for the new technologies. For example, use the correct nozzle, and keep the boom at the correct height. “Drift can be reduced dramatically, but it will remain a concern, probably more so in North Carolina than in the Mid-South,” York says. “We have a very diverse agriculture; you can pull up to a crossroads and find four different crops on each corner owned by four different growers.

“Another concern I have is how much we’re using PPO inhibitors, such as Valor and Reflex. We use that chemistry class in every crop including cotton, soybeans, peanuts, tobacco and sweet potatoes. Fortunately, we haven’t seen any resistance yet. We cannot afford resistance to other herbicides. We need to follow sound resistance management practices to protect all chemistry classes so we won’t put the selection pressure on them that we did on glyphosate.”

Timing is critical

University of Missouri weed scientist Jim Heiser says application timing is critical to controlling resistant pigweed. “My recommendation is, if you see it, spray it,” says Heiser, who is based at the Fisher Delta Research Center in Portageville. “Most area cotton growers do a really good job controlling resistant pigweed (Palmer amaranth). Failures occur when weather keeps growers out of the field, preventing them from applying residuals on time. That’s when pigweed takes off and is more difficult to control.”

Southeast Missouri cotton farmers still use cold steel along with herbicides to control glyphosate-resistant pigweed and other weeds. “Our area never really got completely away from tillage,” Heiser says. “We’re fortunate we can do that, unlike west Tennessee which farms mainly no-till because of its rolling land.”

The Missouri Bootheel saw fewer hand crews in the field in 2014 than in the previous two seasons. Weather permitting, farmers are more proactive and spray pigweeds early before the problem gets out of hand.

Heiser says, “We tend to see more chopping crews in fields planted just before rains because the planter can get across the field in more adverse conditions, such as high winds, than a sprayer.” To make sure every acre receives a soil-applied residual, some fields may need to receive a preplant application while weather is favorable and let the planters catch up to the sprayers and not the other way around.

Like York, Heiser is excited about the potential role of new technologies in controlling resistant weeds and managing resistance, pending regulatory approval. Also like York, Heiser emphasizes the importance of educational training for applicators and growers.

“There are some stipulations with both the Enlist and Xtend technologies that need to be followed,” he says. “For example, both require certain nozzles to reduce drift. Both have tank clean-out procedures to eliminate herbicide residue in the sprayer hoses and nozzles. There will be other criteria to follow as we use these new technologies in a system approach with our other herbicides, rotating classes of chemistry to effectively control weeds while avoiding herbicide resistance.

“We need to change up our modes of action as often as possible, continue using soil residual herbicides, make applications with at least 10 gallons per acre carrier volume and target small, actively growing weeds. It’s a system approach.”

Resistance in Southwest

Glyphosate-resistant Palmer amaranth was bad in the Texas High Plains and Rolling Plains in 2014. It could be worse in 2015, especially with adequate rainfall.

Wayne Keeling, Texas AgriLife Extension in Lubbock, says the job growers did in 2014 will also play a role in what they face next year. “If growers had escapes in October, they can expect to see problems next spring unless they take precautions — before resistant seeds germinate,” Keeling explains. “They will need to apply residual herbicides at as high a rate as the label allows.”

To control resistant pigweed, Southwest growers need a system approach that includes preplant and pre-emerge herbicides, residual herbicides applied after planting or with glyphosate applications. Additionally, some old technology, including cultivation, hooded sprayers and rope-wick applicators, may be necessary.

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