An epidemic for soybeans — that’s how Daniel Stephenson has characterized herbicide-resistant Palmer amaranth in the past.
And it remains a crisis. But with good cultural practices and the use of multiple herbicides that provide alternative modes of action, he now says the nightmare of resistant pigweed can be managed.
Stephenson, a weed scientist with the LSU AgCenter Dean Lee Research and Extension Station at Alexandria, La., says farmers are doing a better job of controlling resistant pigweed.
The remedy for controlling the pesky plant is a diversified weed control management program that includes use of residual herbicides applied preplant, at planting, and in-season.
“Crop yield is determined early in the season,” Stephenson says, adding that farmers can’t allow weeds to out-compete the crop for sunlight, water, nutrients and space.
Control must be before planting
“You should start clean,” he says. “If you have emerged pigweed, don’t allow them to grow before you plant. If they are present, apply a non-selective herbicide, like paraquat, for control.
“At planting, treat fields with a soil-applied residual herbicide. The choice of product can depend on which other weeds are in the field.
“In Louisiana, for example, you can have two, three, or four different grass or broadleaf weeds, along with pigweed. So, make sure the herbicide you apply will control those weeds and pigweed.
“Then, follow up with an early-season application of residual herbicide mixed with glyphosate in Roundup Ready soybeans or Liberty in Liberty Link soybeans. That overlays the residuals.”
The hope then is that the crop canopy will provide shading of the soil surface, thus slowing the emergence of weeds. “But at times, it may require a third herbicide application for season-long control,” Stephenson says.
Treat not only the field, but also turn rows, because the weeds that grow there produce seed as well. When pigweed plants are too tall for herbicides to kill, the only remedy is hand pulling, he says.
Zero tolerance of pigweed
“We want zero tolerance with pigweed,” he says. “Growers must watch fields. If they happen to see even one pigweed that survives an application of any herbicide, they should go and physically remove it from the field, then take it to a burn pile. Don’t let it explode on you.”
It’s essential to keep combines clean of pigweed seed, Stephenson says.
“Practice good sanitation on your equipment. If you combine through a field infested with pigweed, avoid the infested area if possible, or try to thoroughly clean the machine afterward. Pigweed seeds are the size of the ball in the head of a ballpoint pen, and they can easily be transported to another field.”
First five weeks of growth
Data indicate that good weed control will boost yields in soybeans, corn, cotton or any other crop, Stephenson says.
“For soybeans, data show that if you maintain beans weed-free the first five weeks of growth, you will maximize yields in Louisiana, and likely most other production areas. The best way to do this is with a program that features a good pre-emerge residual followed by a good post residual application.”
Pigweed control in corn can be tricky in the South, he says.
“Most Louisiana growers are effective at managing pigweed with an application of herbicides containing two to four modes of action at the 8- to10-inch growth stage. But one issue we see in the South, specifically Louisiana, is that by planting early, farmers also harvest early in August and September.
“If they wait until the first frost to kill remaining weeds, that may not occur until November. Weed growth can again get out of control, and these weeds can continue to produce seed, which will cause issues next year. So, we recommend that growers use post-harvest weed management, either tillage, mowing or herbicide applications.”
New herbicide traits are just over the horizon, Stephenson says, that should help farmers get a better jump on resistant pigweed and control of other weeds.
Best management practices
The new Xtend system from Monsanto, Enlist from Dow, as well as other programs expected on the market in the next few years, have proven themselves in numerous trials across the South and other areas.
However, even the new technology will need to be part of a program that utilizes residual herbicides pre- and postemergence, Stephenson says.
Until those new programs are available, farmers must use the best management possible to control weeds.
“That’s zero tolerance for controlling resistant pigweed,” he says. “Start clean, use good pre-emerge and postemerge herbicide programs that match your particular weed problem. Then, physically remove any remaining weeds.”