Aggressive weed Palmer amaranth threatens Wisconsin crop fields

Early last summer many Wisconsin farmers – particularly in the northeast and southwest areas of the state – discovered an unfamiliar weed growing in their corn and soybean fields.

By harvest time, large portions of many of the fields had been taken over by the aggressive weed – Palmer amaranth – that significantly reduced the corn and soybean crop yields.

Daniel Smith, University of Wisconsin-Madison & Extension regional outreach nutrient and pest management specialist, spoke about the threat this weed poses to crops during a recent online presentation that was part of the 2021 virtual  Cow College program.

Smith said Palmer amaranth (Amaranthus palmeri S. Watson) is one of a number of pigweeds that are problem weeds in crops throughout the United States. 

“Pigweeds are warm-season annuals, grow quickly and aggressively, compete well with crops, reproduce by seed, are frost sensitive and have a high percentage of hard seed. Palmer amaranth has caused severe yield losses up to 91 percent in corn and 79 percent in soybeans,” he explained.

Plant characteristics

Palmer amaranth is sometimes mistaken for waterhemp because both have similar characteristics. “Both seeds are almost identical. They’re very small and very hard to distinguish,” said Smith. “Palmer amaranth has 26,000 seeds per plant, and 99% of those seeds are retained until that plant is harvested. Water hemp is very similar, with 25,000 seeds per plant and 99% retained.”

Palmer amaranth can grow up to 8 feet tall and has one reddish central stem that is smooth, relatively hairless, with many lateral branches. In comparison, redroot pigweed seedlings have pubescent or hairy stems and leaves. 

Palmer amaranth leaves are alternate and grow symmetrically around the stem, giving it a poinsettia appearance when viewed from above. Leaves are hairless, lance to diamond-shaped, 2 to 8 inches long and one half to 2.5 inches wide, with a prominent whitish vein on the leaf underside. The leaf petiole (stalk) is longer than the leaf itself.

“Seed heads on female Palmer amaranth plants can reach 3 feet long and have stiff, sharp bracts, giving them a prickly feel. It is a prolific seed producer, producing small brown-black seeds that remain viable for up to five years,” Smith emphasized.

How it gets to fields

Palmer amaranth is a desert plant native to the south, and is very common in cotton fields in southern US. 

“Over the past few years we’ve seen anecdotal reports of Palmer amaranth plants escaping on dairy farms, and the hypothesis was that it was being brought in through cottonseed, which many dairy farmers feed; in one case we found a cotton plant growing in the same field as the Palmer amaranth,” Smith reported. 

Palmer amaranth seeds can also make their way to the fields when manure is spread. “Research tells us that many of our common weed seeds will survive most manure handling processes,” explained Smith. “One of the few ways that weed seeds will become non viable is by composing. But it has to be done right. We have to hit those target temperatures to be able to kill the seed germination.”

Many of the soybean fields in east-central Wisconsin where Palmer amaranth was found in significant amounts haven’t had any manure applied for several years, so its highly likely that some of seed got mixed in with the soybean seed.

Smith isn’t trying to discourage farmers from feeding cottonseed. “But if you’re going to have some of these products on your farm, or importing straw from another state, look to buy certified products, ask questions about where these products are coming from,” he advised. “We also see a lot of that seed shattered when it’s run through harvesting equipment, which means that the weed seed is spread for future years and transplanted to other fields.

“We see these species holding on to their weed seeds until we we potentially harvest the crop like soybeans and silage corn. And we can bring all that seed back with us in that grain and silage,” Smith said.

Controlling Palmer amaranth

Palmer amaranth has the potential to become a major agronomic problem in many states. 

Harvesting soybean fields that are infested with Palmer amaranth can be especially challenging. The thick stalks can quickly plug the combine even when trying to harvest a swath about half the width of the combine’s grain head.

The process often can be very time consuming as well, taking more than twice as long to harvest less than half the normal yield.

In the Midwest, Palmer amaranth emerges from May through September, forcing producers to manage it throughout the year. It can hybridize with other pigweeds, and its reproductive habits allow it to readily adapt to new environments and develop resistance to herbicides. It is already resistant to glyphosate and other commonly used crop herbicides.

Smith says in silage corn it can be controlled with pre-emergent herbicides. “If we let this escape to other fields, we’re looking at yield loss, it’s going to compete heavily for light, water and soil fertility.”

He advises farmers to ask questions about where their seed is coming from, and make sure they scout for weed seed escapes. “Stay in contact with your crop adviser for the latest herbicide information, and scout for weird looking weeds on the farm. Preventing them from going to seed and being spread can be really important,” Smith stressed.