Dallas Peterson’s weed-research plots rival the rankest weed-ravaged fields anywhere. And head-high pigweed offers examples of what can happen to corn and soybeans fields if the yield-punishing plants aren’t controlled early.

Whether resistant to glyphosate or not, Palmer amaranth, waterhemp or any of the other pigweed species can reduce production and even strangle harvesting efforts, says Peterson, Kansas State University Extension agronomist/weed scientist.

“A program approach and timing are critical for controlling pigweed in soybeans or corn,” Peterson stresses, as he looks over weed species just outside the K-State Manhattan campus.

“On Roundup Ready soybeans, many growers historically have waited until they think most of the pigweeds have emerged before they spray glyphosate. That’s probably not a good idea.  

“Relying solely on glyphosate and delaying treatment can result in yield loss, poor control and increased potential for developing glyphosate-resistant weeds,” Peterson says. “And, it’s not going to work with a lot of other herbicide chemistries either.”

Glyphosate-resistant waterhemp has become fairly common in eastern Kansas and other areas. Glyphosate-resistant Palmer amaranth, first noticed in the South several years ago, also continues to expand. “It’s not been documented here yet but we know it’s coming,” Peterson says.

“Growers need to consider two-pass weed control programs that include residual herbicides at planting for good weed control and to protect yields,” he says.

For glyphosate-resistant corn, Peterson suggests using a preplant/pre-emergence herbicide such as Lexar (Syngenta), Balance Flexx (Bayer CropScience) or Corvus (Bayer CropScience).

“That will help tremendously,” he says. “At postemergence, consider tankmixing glyphosate with Calisto (Syngenta) Impact (AMVAC), Laudis (Bayer CropScience) or Status (BASF). Adding these herbicides to the weed-control program adds another mode of action and will help with herbicide-resistant management.

In addition to the pigweeds, these herbicides can also help control glyphosate-resistant kochia, a major problem in the High Plains.

Pigweed control in soybeans also should begin with a preplant or pre-emergence residual herbicide, Peterson says. He recommends several herbicides, including Prefix (Syngenta), or the Authority (FMC) and Valor-based products from Valent.

“However, be aware that the lower labeled rates of these products may not be adequate if you have glyphosate-resistant weeds,” Peterson says. “The low rates worked fine if you could clean up escapes and late emerging weeds with glyphosate, but higher rates will be needed if you can’t rely on the glyphosate for control.”

He says Fierce, a new Valent herbicide expected on the market in 2012, has performed well in K-State research. “It’s a combination of Valor and a new chemical called pyroxasulfonea. Pyroxasulfone works like Dual, but is better on broadleaf control. The Fierce rate is expected to be 3-4.5 oz./acre.”

In addition, Peterson says Cobra (Valent), Flexstar (Syngenta) and Ultra Blazer (BASF) can be used postemergence for pigweed control in soybeans, “but weeds need to be small and actively growing to expect good results.”

Peterson says one of the reasons Palmer amaranth and waterhemp may be prone to develop herbicide resistance quickly is because they’re dioecious species, which means they have both male and female organs. “That results in a constant mixing of genes,” he explains.

“The resistant trait is passed through the pollen and can spread rapidly. Fortunately, only half the plants produce seeds. Unfortunately, a single Palmer amaranth or waterhemp plant may produce up to a million seeds per plant.”

Peterson advises growers to discuss various pigweed and other weed-control programs with their crop consultant, chemical provider or Extension weed specialist.

“We just have to diversify and change our weed control programs,” he says. “Roundup has been effective, but it’s been too easy, too simple.”