Palmer amaranth: weed seed doppelganger

Similarity between noxious American weed and redroot pigweed a concern for Canadian forage growers

Concerns over the presence of Palmer amaranth in the United States are beginning to reverberate through the Canadian forage seed industry.


Palmer amaranth is an invasive weed common throughout much of the U.S. Its presence has not been confirmed on the Canadian Prairies but south of the border, its destructive potential is widely known.


The weed has already been identified in South Dakota and growers in North Dakota are on high alert.


Palmer amaranth has been designated a prohibited noxious weed in Minnesota and Ohio, meaning the presence of a single seed in a shipment of commercial forage seed could disrupt trade and cause significant economic damage to the Canadian forage seed industry.


To make matters worse, Palmer amaranth seeds are visually indistinguishable from the seeds of redroot pigweed, a well-established weed in many parts of Western Canada.


Paul Gregory, a seed marketer and retired forage seed producer from Fisher Branch, Man., said forage seed growers in Canada must be aware of Palmer amaranth and its similarities to redroot pigweed.


“In Canada, there are a lot of forage seed producers that aren’t even aware of this (situation),” said Gregory, who runs Interlake Forage Seeds.


“It’s kind of scary to think that a couple of redroot pigweed plants could potentially take out a quarter-million dollars of your farm income, or at least a significant part of it.”


“This has the potential to impact forage seed growers right across Canada … so it’s probably good for all producers to know about this.”


Palmer amaranth is a type of pigweed.


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The fast-growing species has the potential to produce up to a million weed seeds per plant. The species can be extremely hard to control if left unchecked.


Glyphosate-resistant populations have been documented in several U.S. states. New herbicide chemistries and weed control platforms are being developed to control the species in corn and soybean crops.


Palmer amaranth has already emerged as a significant problem on land under the U.S. federal Conservation Reserve Program (CRP). 


American weed scientists attribute its spread to a number of factors, including contaminated grass seed supplies that were planted on CRP lands.


Gregory thinks heightened awareness of the Palmer amaranth threat in the U.S. has potential to disrupt Canadian forage seed exports.


Because seeds from Palmer amaranth and redroot pigweed are visually indistinguishable, forage seed supplies that contain a single redroot pigweed seed could cause costly trade disruptions or lost sales, he said.


“There is (a diagnostics company) that will charge $250 per seed to do genetic testing … but I’m finding that most companies just don’t want to deal with that,” Gregory said.


“Even if you can properly identify the seed as pigweed, not Palmer amaranth, it still takes time and it’s expensive.


“It (genetic testing) opens up a can of worms that nobody wants to get into, so it’s much better to ensure that your forage seed is pigweed free.”


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Typically, the vast majority of pigweed seeds are removed from grass or forage seedlots during cleaning.


However, it can be difficult to remove all weed seeds because of their size. Pigweed seeds are small and can become lodged in the awns of certain grasses. 


Earlier this year, the Manitoba Forage Seed Association published an article in its newsletter warning producers about potential confusion between Palmer amaranth seeds and redroot pigweed seeds.


According to the article written by MFSA administrator Heather McBey, Minnesota law prohibits the sale of any agricultural seed containing a single prohibited noxious weed seed. Ohio prohibits the sale of seed that contains small amounts of restricted weed seeds.


“Although Palmer amaranth has not been confirmed as being present in the Canadian Prairies, it is a weed noteworthy of producers’ concern as customers will be wary of any shipment containing pigweed,” McBey wrote.


“This may limit the marketability or cause outright rejection of your lot of seed.”


Kyle Willis, president of the MFSA, said he is not aware of any incidents where a shipment of Canadian forage seed to the United States was rejected due to the presence of pigweed. 


However, he said forage seed growers should take extra precautions to eliminate pigweed from forage stands.


Producers should also ensure that seed lots are thoroughly cleaned and free of pigweed.


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