Mix it up

Rotate control methods and crops to slow development of PPO resistance in Palmer amaranth.

Palmer amaranth
Season-long competition by Palmer amaranth at 2.5 plants per foot of row can reduce soybean yield by as much as 79 percent, according to information from the United Soybean Board—Photo courtesy United Soybean Board

By Vicky Boyd

Although two new herbicide-tolerant crop technologies have recently hit the market and a few others are in the pipeline, weed experts say growers should still practice integrated weed management to slow herbicide resistance from developing.

“We have new technology coming out, but really, things haven’t changed,” says Bob Scott, University of Arkansas Extension weed scientist. “We’re trading Liberty for dicamba. You still need to follow all of the other recommendations to be successful.”

His words take on even more importance as Palmer amaranth, also known as Palmer pigweed, has developed resistance not only to glyphosate but also to a few other herbicides, though on a lesser scale.

Palmer pigweed’s resistance to the PPO-inhibitor family of herbicides—often referred to as Weed Science Society of America Group 14—is the latest mutation to challenge growers.

Glyphosate is a Group 9 herbicide. Surveys conducted by University of Arkansas weed scientist Jason Norsworthy found the heaviest populations of PPO-resistant Palmer pigweed in northeast Arkansas.

In that area, Scott says growers have a 50-50 chance of having PPO resistance in their fields.

In addition, Norsworthy’s surveys identified pockets of PPO resistance bordering the Mississippi River to the south.

Tennessee also has PPO-resistant populations in more than a half dozen western counties. Altogether, PPO-resistant Palmer amaranth has been confirmed in seven states: Arkansas, Mississippi, Tennessee, the Bootheel of Missouri, Illinois, Kentucky and Indiana.

Pat Roberts, who runs Monticello, Ark.-based AGvice Crop Consulting with his two sons, says PPO-resistant Palmer amaranth has not been confirmed among his grower clients yet. But he has heard of a few other area growers suspected of having PPO resistance in their fields.

“We’re thinking we’re right on the verge of being hit because we have been hammering (Palmer pigweed) pretty hard,” Roberts says.

He and his sons already have begun meeting with soybean producers to map out herbicide programs for the 2017 season. Part of that will include multiple modes of action, starting with a residual pre.

“We’re trying to get them to run something right before they plant or right behind their planters,” Roberts says.

If a grower has grass problems, for example, he says Fierce—a premix of a Group 14 and Group 15 from Valent U.S.A.—put out just ahead of the planter is one of his go-to products.

Palmer amaranth
Hand rogue stray Palmer pigweed in
fields before they go to seed. A single
female plant can produce more than
500,000 seeds in a season—Photo courtesy United Soybean Board

How to deal with resistance

The problem has worsened as growers relied more heavily on PPO herbicides after Palmer amaranth developed widespread resistance to glyphosate, Scott says.

“Obviously, the problem is not just the PPO resistance, but there is pigweed resistant to glyphosate, ALS and in some cases, even to the family that includes Prowl and the DNA (dinitroaniline) chemistries,” he says. “This really has the biggest impact on cotton and soybeans, and particularly soybeans, because it leaves us with very limited options. One we are recommending is rotation to corn or rice if possible.”

By rotating to a monocot, or grass crop such as corn or rice, producers are able to use different modes of broadleaf herbicides they couldn’t in soybeans.

Roberts agrees. “We have guys who have tried rotating corn, and they’ve done a good job (with weed control) in the past.”

A rice rotation also brings in water as weed control, since pigweed can’t germinate through flooded fields. But rice producers still have to pay attention to pigweed control on levees and headlands that are above water, Scott says.

Outside of crop rotation, growers with glyphosate-resistant Palmer amaranth have two options if they want to stick with herbicide-tolerant crops: LibertyLink and XtendiMax.

In cotton, producers are able to use glufosinate and dicamba during the same season on XtendFlex varieties, he says. But soybean producers must decide before planting whether they want to go with glufosinate-tolerant
LibertyLink or dicamba-tolerant XtendiMax soybean varieties.

Always overlap residuals

Regardless of the technology, Scott always recommends starting with clean fields and overlapping residual herbicides beginning with a pre-emerge application. Because some populations of Palmer amaranth have developed resistance to multiple herbicides, he encourages tankmixes with two different
modes of action.

His recommended pre application is a tankmix of a Group 5, such as metribuzin or atrizine, and a Group 15, such as Outlook, Warrant, Dual or Zidua.

Mid-season, growers should return with a tankmix that includes a Group 5. Scott also has been recommending row spacings as narrow as 15 inches because of faster canopy closure.

“That way we shade out the ground and prevent late-season emergence of pigweed,” he says.

Intensive rotations slow glyphosate resistance

In Louisiana, soybean producers are seeing increased glyphosate resistance in Palmer amaranth but not to the extent as in other states, says Ronnie Levy, Louisiana State University AgCenter state soybean specialist in Alexandria. So far, PPO-resistant Palmer pigweed has not been confirmed in the state.

“That doesn’t mean we don’t have any,” he says. “We haven’t had the level of problems with (glyphosate) resistant pigweed in Louisiana, but we’re starting to see more and more of it in the last couple of years.”

Levy attributes the lower amount of resistance to aggressive rotation of crops and modes of action. Depending on the region and the market, soybeans typically are rotated with rice, corn or grain sorghum, or cotton to a lesser extent.

LSU AgCenter Extension also urges growers to apply a pre-emerge residual herbicide followed by a post-emerge residual product as part of an integrated weed control program.

Where in-season use of glyphosate has lost its effectiveness in Roundup Ready soybeans, Levy says many growers have switched to the LibertyLink system. And he says he expects some growers may consider planting dicamba-tolerant soybeans this season.

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