Perry Galloway stands in one of his two soybean yield contest fields outside Gregory, Ark., and points out a treeline to the northwest. “The first field with established PPO-resistant pigweeds in Arkansas – which I didn’t farm last year but am in 2016 -- is about 1.5 miles up the road from here. I’ve been farming this field for six or seven years. So, sometime recently, the PPO resistance jumped the road or has just been building over the years.”
It wasn’t supposed to work this way. Last year, Galloway’s 108.76 bushels per acre yield set the bar for Arkansas soybean producers. This 70-acre field of Pioneer 47T36s, he hoped, would set the bar even higher. Instead, Galloway is set to destroy them.
From the road, the rows appear uniform and the field looks deceptively healthy. A few rows in, though, and the problems become apparent. The soybean plants are ragged, the leaves splotchy. Kneel down and look a bit closer and a plethora of small pigweeds is easy to see.
“We’re also looking at another 120 acres that may need to be taken out,” says Galloway. “We’d prepared for this season by upgrading our spraying equipment. Knowing we were facing some potential issues, we went with twin-jet nozzles, bumped our volume up to 15 gallons per acre, just did everything we could to make sure applications would be the best possible.”
In addition, “the field is drip irrigated so we can spoon feed all the fertilizers. We wanted to break our previous record yield there. When planting, we used an in-furrow fertilizer, soil amendments, premium seed inoculants, premium seed treatments. We had a lot of money invested – probably $100 per acre just going across the field with the planter.”
None of the planning, none of the field prep and equipment upgrades mattered.
“I tried to squeeze one more year out of Roundup Ready and figured if we could establish a canopy early we’d outrun the pigweeds. I was wrong – we’re out of bullets except for LibertyLink. Now, we’re hoping for a 70-bushel yield and that’s a heavy price to pay for this education.”
Galloway, says Bob Scott, Arkansas Extension weed specialist, isn’t alone. “Unfortunately, Perry’s situation isn’t isolated. I don’t know how many bean fields are actually being replanted but I’ve sure been getting a bunch of calls and texts. It’s painful to do, but when you’re replanting you need to take out the old crop before it starts acting like a weed.
“This PPO resistance seems to have spread very quickly. I sent out a tweet and got a lot of responses. A lot of guys are seeing pigweed failures with PPOs and they’re wondering what to do. The truth is, we’re in a bad spot -- there’s just not a lot that can be done.”
Samples and consequences
Last fall, Scott and colleagues took many pigweed samples and tested them. “That was done not only through greenhouse work but also through help from University of Illinois researchers who have developed a bioassay. So, we were able to assay a bunch of populations. Everyone had a pretty good warning to be on the lookout for PPO-resistant pigweeds going into this season.
“We determined in the greenhouse that population of pigweed is resistant not only to Flexstar – the PPO chemistry – but also glyphosate, ALS chemistry as well as Prowl and Treflan. That’s a four-way resistance and it severely limits control options. … When you find resistance to so many things it’s alarming. We’re down to one mode of action that works when applied post.”
That, in turn, led to a large acreage of LibertyLink soybeans being planted this year. “Also, metribuzin (formerly the active ingredient in Sencor) as Boundary or in various combinations of products are getting renewed interest,” says Scott. “Metribuzin is an old chemistry that had largely gone by the wayside for, probably, the last 15 or 20 years. Now, though, all the sudden it’s the most popular pre-emerge system.
“So, there’s been a good response from growers to our pigweed warnings. But they don’t always know where the problem fields are, they have to discover them. Some may suspect a problem but believe ‘hey, I can get one more year out of a Roundup Ready variety.’ If they get by with it, then great. But a lot of times that ‘one last year’ approach can really prove to be costly. Replanting is a significant expense. Unfortunately for many growers, this feels like a repeat of when pigweed first became resistant to glyphosate.”
Galloway planted the contest field on April 23, “and before emergence it got metolachlor, metribuzin and paraquat,” he says. “Then, on May 2, we had a breakthrough with Palmer pigweed and decided to spray with 24 ounces of fomesafen products, a pint of metolachlor, a quart of glyphosate, and an 80/20 surfactant.
“We kept a close watch on it and got some kill. But cotyledons popped up – when they’re so thick I call it ‘purple haze’ because of the color tint. They weren’t dying like they should have.”
By the time Scott came to the field several days later “we were having some serious worries. He said to give it a few more days. After four or five days, there were too many escapes and had to make a decision to go with UltraBlazer or something else. We decided to go ahead and bit the bullet with the plant-back restrictions for grain sorghum and corn for next year and applied another round of (the previous mix of products). For that one, we used an AMS adjuvant.”
Once again, Galloway got “about 75 percent control with virtually no residual activity. That was apparent after four or five days because the cotyledon pigweeds kept coming. The lower leaves were kind of speckled or bronzed.”
On May 23, Galloway’s consultant recommended the crop be destroyed and replanted in LibertyLink soybeans. “It’s been wet since then so we haven’t had a chance to do that yet. The obvious choice to take the crop out is paraquat. When it dries out we’ll see if we can do it with paraquat alone, go with a different product or make a mix.”
Jason Norsworthy, University of Arkansas weed scientist, says “at all the winter meetings, we told growers if you farm north of I-40 don’t be surprised if you have PPO-resistant pigweeds. Sure enough, this week I’m getting a lot of calls on that very thing.”
Galloway heard that message. “When the first pigweeds came up in the crop it wasn’t like we went ‘uh-oh!’ We have tremendous pigweed pressure here so it was always going to be an uphill battle. But, man, I had no idea PPO resistance would make the mountain this steep.
“We got alarmed about 24 hours after that first post-emergence application. The pigweeds didn’t appear to be dying as they had in the past. Usually, when the pigweeds are a quarter-inch tall, you can put a herbicide out and they’ll be wilted and gone within a day. That isn’t what happened.”