Stop the spread of harmful pigweeds

Across the Corn Belt, farmers grapple to gain the upper hand against pigweeds. With the potential to produce more than 500,000 seeds and with increasing resistance to herbicides, one missed plant can spell disaster for years to come.

The most notorious pigweeds for farmers are Palmer amaranth and waterhemp. Left uncontrolled, both can have disastrous effects on yield.

“With waterhemp, our worst-case scenario is somewhere around 42 to 43% yield loss if left uncontrolled the entire season,” says Aaron Hager, a weed specialist at the University of Illinois. Palmer amaranth can have 100% yield loss if it’s left uncontrolled, Hager adds, either through its effect on yield potential or growth of a dense stand that would make harvesting next to impossible.

Waterhemp is the major pigweed across the central Corn Belt, while Palmer amaranth is most often found farther south. Though less of a concern, redroot and smooth pigweed species are also common across the Midwest.

The growth of resistance

Farmers’ heavy reliance on glyphosate, paired with pigweeds’ innate ability to adapt and survive, has caused significant problems when it comes to managing both Palmer amaranth and waterhemp.

“Waterhemp and Palmer amaranth are dioecious, which means you have separate male and female plants, so pollen from a male plant has to land on a female plant in order for the species to survive,” says Bill Johnson, a weed science professor at Purdue University. “The plants that are more adept at surviving are going to be able to spread their genes and repopulate the area. That’s going to result in a fitness advantage and the ability to spread herbicide resistance.”

These weeds’ prolific ability to produce seed that stays viable in the soil for extended periods is also concerning.

“Sometimes the most effective strategies are a relatively narrow range of herbicides,” Johnson says. “What ends up happening is the same herbicide gets used over and over and over because it’s successful. When you do that repeatedly, you end up with resistant plants that cross with one another and become the dominant species in the landscape.

“It only takes one year for these pigweeds to produce so much seed that it populates a field that might need five to 30 years of management before we can really knock that population back,” he adds.

This increased resistance means pigweed management has had to change significantly over the past decade. Farmers who may have once relied on glyphosate as a silver bullet should be using a multi-tactic approach today.

“We have to go after these species with both barrels, so to speak, because they are so problematic and they have the ability to escape many of our different management tactics,” Johnson says.

Managing for pigweeds

Pigweed management should focus on inhibiting plants from developing seeds. Not only are seedlings more vulnerable to herbicides, but preventing seed spread can avert future populations from getting established.

“The big thing we can do is use our residual herbicides more effectively…on every acre that has pigweeds,” Johnson says. “I can’t stress that enough.”

Farmers should identify which active ingredients in premixes work best against pigweeds and increase the rates of those ingredients.

They should also consider control methods beyond herbicide use. Tillage and cover crop management can be effective at knocking back populations and can help limit the reliance on herbicides.

“The only thing that we know with certainty is that if there’s no seed produced at the end of the year, there’s no change in the frequency of any resistance mechanism,” Hager says.

If farmers employ a sound management plan for four consecutive years and no waterhemp seed is produced, populations will literally plummet because the seed does not remain viable in the soil that long, he explains.

“That’s how we win this battle,” Hager says.