Palmer amaranth

While not legally considered a noxious weed by the Nebraska Department of Agriculture, Palmer amaranth is definitely a perennial nuisance to farmers and ranchers in Nebraska.

“It’s important to rogue out even just a scattered Palmer amaranth plant or two in fields before they go to seed,” said UNL Extension educator John Wilson. “They are such a prolific seed producer that what is a small and manageable infestation one year can explode into a major problem in just one or two years.”

The three primary sources of new Palmer amaranth infestations so far have been from the presence of Palmer seed in cotton-based feeds, movement of contaminated combines from Palmer-infested areas and the presence of Palmer seed in seed for conservation plantings, said Mark Loux of Ohio State University.

Another method of seeds being distributed in fields is through the spreading of infested materials, such as manure.

Knowing what the sources of Palmer amaranth are, in theory, means that we should be able prevent any additional new infestations, Loux said.

“Wishful thinking, to some degree at least,” he said.

Among other things, Loux states, this would require that every grain or animal producer, feed dealer and equipment dealer be well-informed enough about this problem to take the appropriate steps: Stop importing combines from Palmer-infested areas; stop using cotton-based feed products from these areas; and test all seed for the presence of Palmer amaranth seed prior to planting.

Stopping the superweed

Dubbed a “superweed” by some, due to its wide genetic variation, Palmer amaranth is herbicide-resistant, water-efficient, heat-tolerant and is a very fast-growing pigweed species, said Dale Strickler of Green Cover Seed of Bladen, Neb.

“It is a highly competitive weed,” he said. “Probably the most competitive of the pigweeds.”

Populations have developed resistance to multiple classes of herbicides with different modes of action, including glyphosate, making it very difficult and expensive to control, particularly on productive farmland.

Palmer amaranth has a fast growth rate of approximately two or three inches per day and commonly reaches heights of six to eight feet, greatly inhibiting productive crop growth.

“It also produces large quantities of seed,” Strickler said. “Up to a half-million seeds per plant.”

Therein lies its weakness. The seeds are numerous, but they are miniscule.

“The seed is tiny, so it contains a limited amount of energy,” Strickler said. “The seed can support about a half-inch of growth before it will need sunlight to prosper.”

A good cover crop mulch would prevent that, he said. Researchers have found that cereal rye in particular consistently reduced the density of pigweeds, even in the absence of herbicides.

“Producers need to think about what’s going on in the soil in addition to what’s going on above the soil,” said Joe Roberts, owner of Roberts Seed, Inc., in Axtell, Neb. “The bacteria and nematodes boost soil and plant health.”

If weeds are growing in a field, it’s because that field is lacking something. Killing everything in the soil to help grow a crop will not be beneficial in the long run, he said.

“There’s a lot more management that goes into the use of cover crops,” he said. “Overall, using cover crops is going to be more beneficial to producers than not.”

Roberts also agrees with Strickler. Nature wants the ground to be covered, he said.

“I have an agronomist friend who says that every day you’re not growing something in your field — you’re going backwards,” Roberts said.

For example, Palmer amaranth requires nitrates to thrive. Robbing the soil of nitrogen will offer another obstacle to the pigweed.

“If you have a wheat crop following a soybean or well-fertilized corn crop, you could have 50 pounds of nitrogen in the soil,” Strickler said. “That will feed the weeds.”

Wheat only needs a small, but important amount of nitrogen in the fall. Palmer amaranth will use that nitrogen to the detriment of the crop. It can also be toxic to livestock animals due to the presence of nitrates in the leaves.

Allelopathy is the answer, Strickler said. Allelopathy refers to the beneficial or harmful effects of one plant on another plant; in this case, the removal of nitrates by a cover crop.

“They don’t have the groceries — so they starve,” he said. “Cereal rye or triticale; either one will suck up nitrogen and hold it away from the weeds.”

Upon termination, the mulch of the cereal rye will take from two to three weeks to rot down and release the nitrogen from its biomass. This also gives the producer time to spread a starter fertilizer application where the crop can access it early in the process, he said. The water-soluble rye also provides compounds that suppress pigweed.

“Cereal rye has the most potential to contribute to Amaranthus spp. control by reducing weed population density,” Loux said. “As a result, there is a better opportunity to reduce selection for weeds resistant to herbicides used in post-emergence treatments.”

Some producers have been reluctant to use cover crops for various reasons, Roberts said. One reported reason was the fear of Palmer amaranth contamination in the cover crop seeds themselves. This is due to a report of CRP seeds being blamed for Palmer amaranth outbreak in Iowa.

However, the Nebraska Department of Agriculture has no reports of any cover crops seed being contaminated with Palmer amaranth.

“We are not aware of any situations of cover crop seed used in Midwest states being a source of Palmer amaranth,” said Dr. Bob Hartzler, professor of agronomy and an extension weed specialist for Iowa State University.

He was also sure that the companies selling cover crop seeds would make an effort to not have any pigweed seed in their mixes if at all possible, just for the sake of keeping their customers happy.

For a more in-depth look at pigweed control, Strickler offers a PowerPoint presentation available on YouTube called “Innovations in Pigweed Control.”

Jon Burleson can be reached at