High incidence of glyphosate resistance in Arkansas pigweeds

A few weeks ago, Ryan Doherty entered a greenhouse where rows of Palmer amaranth-filled trays stretched out. Having earlier applied glyphosate to the pigweeds, the University of Arkansas graduate student expected to find the greenhouse largely a dead zone. He found just the opposite.

Doherty called his boss, Ken Smith, Arkansas Extension weed specialist. “He said, ‘You won’t believe this, but these weeds aren’t dying,’” recalls Smith. “‘Out of all them, there are two samples we totally controlled.’

“My mind said, ‘Whoaaaaa. This isn’t good.’”

The first time back at the greenhouses, Smith walked into “a bunch of healthy pigweeds, had a look around, and said, ‘My goodness, if we’re not careful this is going to blow up in our face.’”

Later, the pair of researchers — who are based in Monticello, Ark. — checked their counting procedures and testing set-up to see if the results were in error. Everything had been done properly.


Last fall, Smith and colleagues collected 280 seed samples from escaped pigweed in row crop fields across Arkansas. Without sufficient greenhouse space to screen all of them at once, 143 were grown out in the first batch.

“We took the seedheads and thrashed all the seed out of them. Then, we planted greenhouse trays with those seed. Those plants were thinned back to 200 per sample. That means for every seedhead collected, 200 plants were grown.”

The trays of pigweed were spread out over two greenhouses, then sprayed with a full field rate — the equivalent of 22 ounces of Roundup WeatherMax. Of the 143 seedheads screened to date, only two were killed completely. In the other 141 samples, there was a range of resistance.

“Resistance showed up at 2 or 3 percent all the way up to 97 percent. That means, after spraying, some trays had four to eight plants that lived. In other trays, 97 plants out of 100 lived.”

Smith can’t definitively say “resistance favors one area over another. But the Phillips County and Lee County area seems to have the highest percentage of high-population resistance. That said, resistance is alarming from Newport to Crittenden County to Poinsett County to White County. Resistant pigweed is all over the state, now.”

Smith, who has worked the resistant-weed beat for years, wasn’t surprised the greenhouse work uncovered high resistance levels. However, he was “very surprised” at the magnitude.

Understandably, pigweed is currently on farmers’ collective backburner. But Smith hopes they won’t forget the need to keep glyphosate resistance tamped down.

“Right now, farmers have to deal with floods, high fuel prices and high fertilizer prices, soybean seed is scarce, wheat contracts need to be filled and all the rest. I think it’ll be difficult to move weed resistance up the priority list until later. It’s hard to think about pigweeds when floodwaters are lapping at your ears.

“But I’m telling you: this resistance is more severe and coming on a lot quicker than we thought even a few months ago. If we don’t do something to control resistant pigweed, it’ll be a long, long year. And all this flooding hasn’t helped our resistant weed situation. By now, the water has successfully mixed up and moved all kinds of (weed) seed.”


There are several ways a plant can become resistant to a herbicide. For example, one plant might not take the herbicide in at all. Another plant might take the chemistry in but is able to metabolize it quickly. Another plant might transport the chemistry to leaf margins so overall health isn’t affected.

There are at least two mechanisms of glyphosate resistance in Arkansas pigweed.

“The pigweed patterns in a field are often very distinct. One pattern shows up very scattered — escapes will happen throughout the field. Maybe there’s an escape every 10 steps, or so.

“I can take those ‘scattered resistance’ plants back to the greenhouse and allow them to cross-pollinate. A percentage of the seed collected and grown out will be susceptible to glyphosate at field rates. The other plants may not die, but they’ll get sick. Glyphosate still has some effect and if I continue increasing rates, more plants will be killed.”

A second resistance mechanism is typically found in smaller spots within fields — usually no more than 1,000 to 1,500 square feet.

“Pigweeds in such areas will be very thick — 100, or more, per square yard. That points to all those plants coming from seed of a single female plant the preceding year.”

In such spots, no matter the glyphosate rate, the pigweeds show no ill effects. Even an 8X rate does nothing. Until the surfactant level is high enough to burn the plant, there is no herbicide damage on the plants.

“They’re extremely resistant and it occurs due to a single mutation in the mother plant. And there’s no way to control the offspring. If I cross-pollinate those plants in the greenhouse, every one of the offspring is just as resistant as the parent.”

While Smith worries about all resistant pigweeds, the “spot resistance” is especially troubling. “We must control those areas using whatever measures are necessary. If a farmer or consultant finds one of those spots, he should do what he must to get rid of those pigweeds.”

Last year, a farmer called Smith to a cotton field with such a spot. “I dug up some plants for the greenhouse and was putting them in the truck. When I’d finished, he immediately called his son and said to bring the tractor and disk over. That tractor went right out in the middle of the cotton and disked the weedy area up. They kept that spot clean all year long because they understood what it would mean if that pigweed spread.”

More research

The second batch of Palmer amaranth seedheads is now being tested in the Monticello greenhouses. “Honestly, there’s no reason to believe the results will be much different. The samples we ran first were random, just like these.”

Smith reminds farmers there is no new chemistry coming down to deal with the growing resistance problem. “Some of the old products like Reflex, Valor, Flexstar, Dual and others are available. And if no new chemistries are introduced, eventually the pigweeds will become resistant to those too. The hope now is that we’ll find new chemistries or techniques before that happens.”

If there’s a silver lining to the situation, “it’s that crop rotation will help us. Pigweeds don’t live in the soil for a decade.

“We can severely impact the pigweed soil seedbank by just keeping a crop clean for a couple of years. There’s very few pigweed seed that’ll live more than two years. The bad news is that pigweed plants are capable of producing 250,000-plus seed.”

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