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Palmer amaranth is a looming concern

This aggressive, herbicide resistance weed has been travelling north, and may be in our fields soon

Female Palmer amaranth plants can grow to over 10 feet tall.

Prairie farmers may soon have a new problem Palmer amaranth, an aggressive, invasive weed species native to the desert regions of the southern United States and Mexico, is spreading into the Northern Plains. A few patches of the weed were spotted last year in Aberdeen, South Dakota; North Dakotans are on high alert.

According to Richard Zollinger, a weed scientist at North Dakota State University, the weed has been chosen as “weed-of-the-year” to increase awareness about its potentially devastating impact. A fact sheet produced by NDSU claims that Palmer amaranth has one of the fastest growth rates known — over two inches per day. It is extremely aggressive and can exploit even small canopy openings. Female plants can grow to over 10 feet tall with a five to six inch stem girth and seed heads over one foot long.

Most species of Palmer amaranth are resistant to herbicides, and weed populations spread extremely quickly, as each plant can produce at least 100,000 and up to one million seeds.

Note this Palmer amaranth seedling’s long leaf stems.
Note this Palmer amaranth seedling’s long leaf stems. photo: North Dakota State University

In the U.S., according to Zollinger, HR (herbicide resistant) Palmer amaranth can cause 78 per cent yield loss in soybean and 91 per cent yield loss in corn.

Neil Harker, a research scientist with Agriculture and Agri-Food Canada, says HR Palmer amaranth has become a major problem in the U.S. due to prolonged overuse of herbicides with the same mode of action. Another contributing factor, he says, is the country-wide distribution of animal feed containing cotton contaminated with HR Palmer amaranth.

“It will not be long, perhaps two or three years, before Canadian growers have herbicide-resistant Palmer amaranth, since it is already found in the neighbouring States of Ohio and Michigan,” says Harker. “South Dakota also has HR Palmer amaranth, so it may be in Manitoba soon after North Dakota gets it.”

Harker says crop scouting is crucial to curtailing the weed’s spread early in the game. “Initially, the weed will likely be present in small numbers. At that time, growers should hand-weed it and burn the plants before seed set and seed shatter, and take any other efforts necessary to prevent it from spreading.”

Many U.S. cotton growers are resorting to hand-removing Palmer amaranth escapes in-field.

Harker says non-herbicidal weed control methods should be employed whenever possible, including higher seeding rates, crop rotation, use of perennial forages and winter cereals, early-cut silage, and chaff removal through baling.

“When herbicides are used, rotate modes of action and employ tank mixtures with at least two effective modes of actions on target weed species,” Harker advises.

A Purdue extension fact sheet titled “Palmer Amaranth Biology, Identification and Management” recommends growers practice deep tillage, which will bury Palmer amaranth seed below its preferred emergence depth. “In a heavily infested field, this practice can reduce the Palmer amaranth population up to 50 per cent,” claim the authors, Travis Legleiter and Bill Johnson.

“Deep till only once, because the buried seed will remain viable up to five years and will redeposit in the top layer of soil if you deep till again within that time,” it says. “Long-term no-till producers must weigh the weed control benefits of tillage against the economic and soil-structure benefits of their no-till system.” The fact sheet also recommends harvesting infested fields last. †

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Julienne Isaacs

Julienne Isaacs is a Winnipeg-based freelance writer and editor. Contact her at [email protected]

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