PPO-resistant pigweed moves into Mississippi

PPO-resistant pigweed moves into Mississippi

New populations of PPO-resistant pigweeds popping up in Mid-South. Mississippi confirms resistance now in handful of counties.  

It was big news in 2015 when Arkansas and Tennessee producers saw yet another pigweed-fighting chemistry — PPOs this time — sent to the bench. And the problem is spreading to other Mid-South states.

“We started getting calls pretty early in the growing season saying, ‘Hey, these pigweeds are blowing through Valor,’” said Tom Barber, University of Arkansas weed scientist, at the recent Tri-State Soybean Meeting in Stoneville, Miss. “Valor goes out on a lot of our acres.

“In Arkansas, we sent 27 populations to Pat Tranel at the University of Illinois and they ran tests to determine if the PPO gene in waterhemp — a cousin to Palmer pigweed — is the same as the PPO gene in our Palmer pigweed samples. It turned out, 27 populations came back positive for the same resistance gene.”

Since then, Barber and associates have received many samples from several fields and counties across Arkansas. “Between (University of Arkansas weed scientists) Jason Norsworthy, Bob Scott and myself we have three greenhouses full of pigweed screenings trying to determine where all the resistance has spread. Jason Bond (with Mississippi State University) is doing the same thing (in Mississippi).”

Based on the greenhouse tests, Arkansas’ Poinsett County is a new addition to the PPO-resistant list. There are still about 100 populations in the greenhouse being checked for resistance.

“This is a big deal. If you don’t understand how big a deal, you will going forward.”


And Mississippi is now part of the club. On Jan. 7, Bond “texted me and sent pictures. He and (colleague Vijay) Nandula have been running greenhouse tests. Sunflower, Coahoma, Bolivar, Desoto and Tunica counties now have confirmed PPO-resistant pigweed.”

Observations in the field indicate initially “you may go to your fields after a Flexstar application and find a lot of leaves — maybe all of the leaves — desiccated in the lower portion of the pigweed plants. Or they may just be kind of spotted or necrotic. Two or three days later, you’ll see a green bud. Five to seven days later, you’ll (find a thriving plant).”

The spread of resistance in pigweed has been remarkably quick. In 2007, when Barber moved back to Arkansas, there were still a few counties with glyphosate-susceptible pigweed.

“Some had pigweeds that were resistant to half rates and others were completely resistant. By 2008, all the counties had completely resistant populations. There was no difference in level of tolerance.

“By 2010, glyphosate-resistant pigweed had spread across the state. We’d gotten calls from pasture producers, forage growers, non-row crop-related Palmer pigweed. Everyone has experienced it in Arkansas.”

In 2015, in Crittenden County, “which includes West Memphis right next to the river, it looked like 2010 again. It seems we missed the two years of variable susceptibility in the county. As a matter of fact, if you drove much in northeast Arkansas it looked a lot like 2010 in regards to the amount of pigweed escapes in fields.

“Bob Scott found a population in Woodruff County in conventional soybeans. We have an interesting scenario in the state. There are some poultry companies buying conventional beans and paying an extra $1 or $1.50 per bushel.”

Barber showed the crowd a photo of a field overgrown with pigweeds. “Well, this field had Valor followed by Flexstar followed by Blazer followed by Cobra. I don’t think that field was harvested.”


Barber’s first focus was mostly on post-emergence control with various PPOs after soybeans and pigweeds come up. What happens with Valor, Reflex and other PPO herbicides pre?

“These data are not represented as percent control but seedling survival. So, we planted 100 seeds of these sampled populations from Crittenden and Woodruff counties in greenhouse pots and compared them to a known susceptible pigweed population. There were rates of 1/16, 1/8, ¼, ½, 1x rate (16 ounces per acre), and a 2X rate of Reflex pre.

“First thing we noticed is it takes a higher rate to control any of the resistant populations compared to the susceptible.

“For the 1X rate, the Crittenden County population had about 50 percent survival. Going to a 2X didn’t help much. The Woodruff County population was controlled at 80 percent.”

It’s the same story with Valor. “If you go out and spray Valor pre in these fields, you can expect 70 or 80 percent control in a best-case scenario.

“We’re starting to incorporate a lot of Sharpen as a pre-emerge. At a 1X rate — 2 fluid ounces — it has the same level of control. We did see a bump in control from a 2X rate. But if you put 4 ounces of Sharpen on your beans, they’re going to be 80 percent controlled, as well,” said Barber to much laughter.

What about sulfentrazone? “With the Woodruff County population a 1X rate seemed to provide a little better control. For some reason that population seems less tolerant, or resistant, than the Crittenden County population.

“Considering all the chemistries we’ve lost, it’s our conclusion that conventional or Roundup Ready beans can’t be grown in fields with PPO-resistant pigweed. That’s now our recommendation.”

Producers still have Liberty to apply over the top. “Gramoxone still kills pigweeds, although we can’t use it in the beans. Metolachlor/acetochlor (Dual, Warrant), pyroxasulfone (Zidua, Anthem Max) and metribuzin are your residual options now.”


In the offseason Barber has spoken to many growers getting plans together for 2016. “The first thing they want to spray pre is generic Valor. ‘Times are tough and it’s cheap.’ Well, with generic Valor you’ll get 75 percent control in these fields. What we know about Palmer pigweed is with only 75 percent control, in the end, you’re not going to be able to clean them up. You’re not going to have enough money to spend chopping them out.

“And if you go out with just another PPO, the pigweeds that survive will be completely resistant. You’ve selected the plants that will cover your field next year. Right now, we have about a 20 to 30 percent resistant population in most of these fields.

“You might say, ‘I’ll put Valor out and plant LibertyLink beans. Then, I’ll come back with Liberty post.’ That’s sounds like a good plan until you get a rain, or can’t get back in the field in a timely manner.

“Now, you’re spraying large pigweed with Liberty post four times off label and probably still won’t kill them. This puts a lot of pressure on one of the only modes of action we’ve got left and lessons we learned from Roundup tell us that resistance to Liberty in this scenario is only a couple years away.”

Barber’s top recommendation is rotating to corn or rice. “I’d rather you not plant milo considering all the fields I had to walk for Roundup drift last year. If we’re locked into beans, there’s no question in northeast Arkansas they have to be LibertyLink. And you need three modes of action in the field to protect Liberty. In most cases this means using something like Boundary (a mixture of metolachlor and metribuzin) pre and coming back timely with Liberty plus Prefix post.”

Growers can also go with narrow row spacing to help. “Most of y’all plant on beds for irrigation and drainage. That’s a good system. If you can narrow the rows and still keep yields, it’ll help with pigweed control.

“It’s too late to plant cover crops this year, but consider it in the future. At one time, I was a big naysayer on cover crops. I tried to find every reason not to recommend cover crops. I thought it would mean more money to spend, more to clean up, more insects earlier.

“Now, I’ll tell you, the more we’ve looked at cover crops there’s no doubt in my mind that for soybean and cotton we should incorporate cover crops, especially cereal rye. That’s a must for pigweed control.”

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