How long will it take Palmer amaranth to become resistant to the new formulations of dicamba and 2,4-D that have been approved to be applied over the top of dicamba- and 2,4-D-tolerant cotton and soybeans?

That depends on what growers do to protect the new technologies, according to Bob Scott, a University of Arkansas Extension weed scientist and a speaker at the Pigposium 3 herbicide resistance meeting in Forrest City, Ark.

“If we follow on the resistance path that we’ve been following, and we just come in here and add dicamba to take care of this problem (PPO inhibitor resistance), what do you think is going to happen?” Dr. Scott asked. “We’re just going to add to our list of resistant weeds. We’re going to add dicamba to the growing list of resistance.”

Dr. Scott traced the history of the development of herbicide resistance in Palmer amaranth or pigweed in soybeans, beginning with Prowl and Treflan in the 1980s, the ALS herbicides such as Scepter in the 1990s, glyphosate in the 2000s and the PPO inhibitor herbicides such as Reflex and Flexstar since 2010.

Extension weed scientists at the University of Arkansas have already demonstrated how quickly resistance could develop to dicamba, the active ingredient in the new Xtendimax, Engenia and FeXapan herbicide formulations.

Three generations

“We proved this in a laboratory where this particular population of pigweed in just three selections using sub-lethal doses was not controlled with 16 ounces of dicamba,” Dr. Scott noted. “So we just proved that it can happen if we don’t do something to address herbicide resistance, and we’re not proactive in managing this from Day one.”

He had some words of warning about glufosinate or Liberty, which is one of the few remaining herbicides that can be applied postemergence to control pigweed – in Liberty Link cotton and soybeans.

“The last herbicide that’s put on the field is where the selection pressure occurs,” he said. “I had somebody ask me the other day about planting Liberty Link beans and putting Prefix or Zidua down and using Liberty post. But that last application they’ve been putting out has always been Liberty post, right?

“So they said ‘Is that a good enough reason to rotate to Xtend beans?’ and I said ‘absolutely.’ It’s a good reason to rotate chemistry. If it’s been working, change it. We have to rotate to change that last selection pressure that goes on that field.”

Farmers in northeast Arkansas have about a 50 percent chance of encountering resistance to PPO inhibitor herbicides in their fields in 2017, according to Jason Norsworthy, professor of weed science at the University of Arkansas and one of the organizers of the Pigposium.

Multiple resistance

But some growers are having to deal with Palmer amaranth populations that are not only resistant to PPO inhibitors but to three other groups of herbicides, as well, said Dr. Norsworthy, who holds the Elms Farming Chair of Weed Science at the U of A.

“In 2015, this field near Gregory in Woodruff County was found to contain resistance not only to the PPO inhibitor or Group 14 herbicides, but also to the ALS chemistries (Group 2), the dinitroanilines, things like Treflan and Prowl (Group 3), and Roundup (glyphosate – Group 9,” he said, referring to a field overgrown with pigweed.

“This was actually a conventional soybean field. Bob Scott did research in this field in 2015. And when you take a look at this population, we’re unable to grow Roundup Ready or conventional soybeans in this field because there is no effective postemergence option for the control of a pigweed population that has PPO resistance as well as glyphosate resistance.”

Weed scientists have now documented resistance in Palmer amaranth to the PPO inhibitor class of herbicides in seven states – Arkansas, Mississippi, Tennessee, Missouri, Illinois, Kentucky and Indiana. Norsworthy said 19 counties in Arkansas have confirmed PPO resistance.

“If you had any Palmer amaranth in one of your fields at harvest in 2016, you have better than a 50 percent chance that you have PPO-resistant Palmer in your fields,” said Dr. Norsworthy. “Folks, this is spreading, and it is spreading no different than what we saw with glyphosate. We are quickly losing one of the mainstays, especially in soybeans, from a weed control standpoint.”

Better than 50 percent

He displayed a slide of a bench top containing plants from about 40 different populations of Palmer amaranth. The plants were sprayed with 1.5 pints of Flexstar when they were about 1-inch tall.

“Anything you see that is still alive on this bench top (40 to 50 percent of the plants) would be resistant to the PPO chemistry,” he said. “Research has been conducted that shows the resistance mechanism is very similar to what you see in waterhemp in the Midwest. However, there are other PPO resistance mechanisms in this population that are much more resistant to the PPO chemistry than in some of the other pigweeds out there like waterhemp.”

Dr. Scott said that once resistance occurs that herbicide is lost to producers whether it’s glyphosate, Treflan or Flexstar.

“There’s no fitness penalty for the most part,” he said. “Some resistances have fitness penalties – the weed is damaged by the herbicide so it’s less competitive – but in pigweed it just seems to make it stronger the more resistant it gets.”

The threat of multiple resistance or resistance to more than one class of chemistry is frightening, Scott says. “I’ve had people calling me wanting to know where they can buy a good hoe, and that’s not my idea of weed science.”

Change it if it’s working

He said overlapping residual herbicides, rotating chemistries – even when the current herbicide is working – and using cultural practices will all be needed to preserve any new chemistries or traits growers may get in the years ahead. And growers need to treat the new traits – Xtend, Enlist and the new Balance trait that is expected to be approved in the near future – as if they were new herbicide chemistries.

Dr., Scott also called on herbicide manufacturers to help growers “do the right thing” economically when it comes to helping growers avoid overusing the currently available herbicide tools.

To read more about herbicide resistance-fighting efforts, click on http://www.deltafarmpress.com/cotton/odds-not-arkansas-growers-favor-ppo-herbicides-2017