Pigweed – When Do Post-Harvest Herbicide Strategies Fit?


To reduce future populations of Palmer amaranth (pigweed), hit them where it hurts with a post-harvest herbicide program.

US Map Backup copy 20151214The program can be used behind corn, soybeans, cotton or any crop where there is time for pigweeds to emerge and make seed after harvest. Varying approaches also fit into tactics aimed at marestail (horseweed), particularly in the Midwest.

“If farmers have a really clean crop, they can put out fall residuals and have an entire year where little or no pigweed is going to seed,” says Bob Scott, Extension weed scientist with the University of Arkansas. “That can significantly impact pigweed’s soil seed bank.”

Scott says it might take two or three years of a post-harvest program for a farmer “to visually see depletion in the amount of pigweed in a given field. You have to stay on top of it. If you run one contaminated combine or you let a field of pigweed go to seed one year, you’re right back at ground zero.”

palmer_amaranth_pigweed_regrowth_following_mid-season_mowing_iowa_state_university1University of Tennessee weed scientist Larry Steckel said West Tennessee farmers have been using a post-harvest herbicide program for seven years or more.

“In West Tennessee, we’re cutting corn in August and September, and we don’t get a first frost until October. So in that 8 to 10 weeks, pigweed can produce a lot of seed. Corn is drying down, the sun is hitting the ground and pigweeds really start to take off,” Steckel said.

Fall Pigweed Control From A Farmer’s Perspective

One farmer making post-harvest herbicide applications is Steve North, who farms cotton, corn, soybeans and wheat in Pemiscot County in Missouri and Dyer and Lake Counties in West Tennessee. North will make a 2-ounce application of Valor after harvest of soybeans and cotton to control pigweed. Earlier, at corn harvest, he adds paraquat to the tank, when small pigweeds have emerged and are growing.

For corn, North makes the application in early October, and for cotton in November. While North does have PPO-resistant pigweeds in his fields, Valor, a PPO, is still fairly effective on them when used pre-emergence, he says.

In early spring, North will burndown with Valor and Roundup to control winter annuals such as bluegrass, henbit and chickweed. “The Valor holds until around the first of April. After that, pigweeds start coming on through again,” North says.

North says he definitely sees fewer of the persistent pests in the spring thanks to the post-harvest herbicide program and the early burndown.

On-Farm Tests Confirm Effectiveness Behind Corn

In 2012, Steckel put in a large test plot on a cooperating farmer’s field to study the efficacy of the program in corn, and added a second location in 2013. The study also looked at the impact of the post-harvest herbicide program on a subsequent wheat crop.

In the two-year study, three applications of paraquat alone or tank-mixed with Dual on corn stubble controlled 91% of existing pigweed, but did not control regrowth. Paraquat mixed with a residual provided an additional 4% to 7% control. Wheat injury was evident in a second location in 2012, but not in 2013. Wheat grain yield was not adversely affected by any herbicide application.

“If you’re going to come back with wheat, there are only two herbicides, Sharpen or Valor, that you can use,” Steckel said. “With Valor, plant back is 30 days. Sharpen can be put out behind the planter pre-emergence. A lot of growers like the flexibility of Sharpen because they can plant wheat or the cover crop at any time.”

Best of all, the fall herbicide program reduced pigweed seed production versus the check by roughly 12 million seeds per acre, a clear success, said Steckel. Today, the program is recommended for corn producers in West Tennessee, and has been implemented by numerous west Tennessee farmers.

Herbicide Resistance Info

One downside: these post-harvest herbicide applications come at crunch time for many farmers. “They’re rushing, trying to get the crop out and get the wheat sowed. It’s hard to shake somebody loose to go spray. But it does pay off,” Steckel notes.

That payoff is greater under a lighter pigweed infestation. “With a heavy infestation, 12 million seeds are like a few leaves in a forest,” Steckel observes.

Scott adds that post-harvest herbicide applications should be an extension of a farmer’s in-season weed control program. Otherwise, a farmer can quickly wipe out gains.

“You can keep a field clean all year, and then blow it if you cut your corn in late August and a lot of pigweed come through because you didn’t do anything to control them,” he says.

Scott says post-harvest herbicide applications for pigweed fit well with two University of Arkansas Extension projects, the harvest weed seed control and zero tolerance programs.

Midwest Perspective: Parallel Horseweed Programs

In Illinois, fall herbicide applications are typically directed at horseweed, according to Aaron Hager, associate professor of weed science, University of Illinois at Urbana-Champaign. “That’s where we are going to have the biggest challenge the following spring, primarily getting horseweed controlled before planting soybeans.”

Horseweed emerging in residue.
Horseweed emerging in residue.

Hager says post-harvest weed control programs are focused on preventing horseweed from producing seeds, but the more important objective is to take horseweed out before they grow to an unmanageable size by the following spring.

“If we target horseweed in the fall at a small stage of growth using a herbicide rate that is effective, we don’t have as much of a headache the following spring,” Hager said.

“On the other hand, if we take out all the winter annual vegetation, there can be times when you see some of your summer annual species emerge a little sooner,” Hager adds.

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