Fighting a superweed before you plant

Just over a decade ago, a farmer in South Georgia noticed something odd in his fields. Palmer amaranth was growing among the rows of his high-value cotton. The cotton was resistant to glyphosate; the pigweed wasn’t.

At least it wasn’t supposed to be. Farmers had found enormous success pairing glyphosateresistant cotton with glyphosate to free them of the trouble of mechanically controlling weeds. Now, researchers and farmers across the country are fighting the superweed that seems to be able to adapt to different classes of chemicals just as it did to glyphosate.

A survey of more than 700 cotton growers in Georgia found that more than 80 percent of them were hoeing by hand to control pigweed. That’s an expensive proposition, but possible with a plant like cotton. Hand-hoeing stops seed emergence. If a pigweed plant manages to sprout, these farmers cut it, burn it and spray. They have found that tilling can be an effective way to stop the troublesome plant on the front end.

“Tillage is an effective tool, but it has to be used wisely,” said University of Georgia researcher Stanley Culpepper, who has been studying pigweed control for more than a decade. Plowing of the land that gets the seed at least 4 inches deep is 50 to 70 percent effective at keeping pigweed from taking over. Growers in some parts of the country fight weeds by delaying planting until the ground has warmed enough for the weeds to germinate, then using secondary tillage to chop them down and incorporate. That’s an iffy strategy with pigweed or waterhemp.

“We wouldn’t be here if we’d kept just spraying,” Culpepper said. But, he adds, “There’s not a plant in the world that’s resistant to cultivation.”

“You have to be careful with a stale seedbed,” Culpepper said. “Once the plant is growing, it is hard to control that year. Through tillage and chemicals and cover crop, you want to never see the plant.” A two-year study in Arkansas showed that deep tillage combined with a cover crop provided the greatest control of Palmer amaranth in cotton, with an 85 percent reduction in emergence at the end of a 2009 trial. In 2010, plots that had a deep till the previous year, followed by a rye cover crop, had 68 percent less Palmer amaranth.

Cover crops and plowing will not eliminate glyphosate-resistant soil-applied and post-emergence herbicides. Additional efforts should focus on the integration of the practices evaluated in this research with the use of residual herbicides.

“As pigweed and its cousin waterhemp have spread, some plant experts are calling the plants invasive species. That term usually applies to exotic pests brought into the country, but the definition works here,” said Kevin Bradley, a weed science expert at the University of Missouri.

Researchers from ten universities across the country are working together to test different programs to fight pigweed, most of them working with a combination of tillage, cover crops and chemicals. Culpepper is among them and has traveled to 24 states to show his research into how a deep tillage pass every few years can be an effective part of controlling the invasive weed. If the plant gets nasty enough, it may even be time to break out the moldboard plow to bury the pigweed seed deep, experts say.

Public Enemy No. 1

Pigweed is quickly becoming farmers’ Public Enemy No. 1 as it develops resistance to chemicals and defeats herbicide-alone programs. In some places, different variations of the plant have adapted to three or four herbicide classifications, including glyphosate, PPO inhibitors and ALS inhibitors. How do you deal with it?

Attack it. A single pigweed plant can produce 300,000 seeds, but they die quickly in the soil. Tests show that half of waterhemp seeds remain viable after a year, but at five years, only 10 percent can germinate.

Shade it. Narrow rows provide quicker canopy and deprive pigweed of the sunlight it needs to grow. In testing soybean seeding rates of 130,000, 160,000, 190,000 and 220,000 seeds per acre, late-season weed density dropped each time the seeding rate rose.

Cover it. In tests, a cereal rye cover crop dramatically reduced pigweed emergence.

Control it. Farmers can keep the upper hand. The key is a multipronged approach that involves tillage and chemicals.

Bury it. Judicious plowing can keep pigweed seed from emerging. Every three to five years, plowing (more than three inches) can drastically reduce spring emergence of weeds. In one test, a pigweed population of six to 10 plants per meter in a minimum-till system was cut to almost zero after a plowing pass.

Repeat it. Till every three years. While the first attack can be rewarding, repeating plowing every year will defeat the purpose. Returning the seed to the surface too soon will allow it to germinate.