Fields flooded by worrisome water hemp


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Prairie farmers don’t have to mismanage their chemical control of water hemp. Their American neighbours have already done that.

When water hemp seed arrives sequestered in farm equipment imported from the United States, the stowaways are already highly resistant.

That means when water hemp seed shows up in a Canadian field, it has already been bred and selected to have full immunity to every herbicide group a Canadian farmer might throw at it in a solo application.

Tank mixes are the only hope, although it’s not quite that easy according to Adam Pfeffer, Bayer’s agronomic systems manager for Canada. Pfeffer says water hemp is pigweed, but with waxy shiny leaves instead of hairy leaves.

“Water hemp is a unique weed, extremely difficult to manage. Water hemp and glyphosate-resistant kochia are my two main concerns for the eastern prairies,” Pfeffer said.

“I’m focusing on water hemp, because that’s the really bad one. It has a high natural propensity to develop resistance to different modes of action quite quickly. It maybe doesn’t spread quite as fast as kochia, but it’s a lot more difficult to manage long-term. If you have a water hemp plant in your field, you can assume it’s glyphosate resistant. They all are.”

Pfeffer says water hemp in Canadian fields can all be traced to implements imported from south of the border. They are not natural Canadian plants.

Farmers in areas such as Manitoba’s Red River Valley, that see occasional severe flooding into the lowlands, are accustomed to seeing new weeds in their fields. Migratory birds are another source of water hemp seeds, especially along flyways.

Water hemp first became a significant problem in U.S. border states about a decade ago. It has taken that long to make its Canadian debut, but now that it’s here, Pfeffer says it’s going to continue to spread. He predicts it will become a major weed problem in five to 10 years.

“In Manitoba, it’s resistant to glyphosate, Group 2 and Group 5. In other areas it’s also resistant to Group 14, Group 27, Group 15 and Group 4. There’s lots of genetic diversity within the water hemp population, with male and female plants cross-pollinating. Within a square metre you could find five different bio-types. It’s truly an amazing weed. It’s pretty much your worst nightmare.

“The (Red River) Valley might not be too bad because it doesn’t do well in wheat or canola because they canopy so fast. But the herbicide selection on the Prairies isn’t as robust as we have in Eastern Canada. Those short rotations really limit your herbicide options.

“Resting a field for a number of years won’t help. It’s a small-seeded annual broadleaf weed. Once a population is established the seed bank is going to be there a long time.”

In soybeans, Pfeffer says there are chemistries that do an adequate job of controlling water hemp. Group 14 and Group 15 pre-residual products are strong. Probably the two biggest would be Authority Supreme from FMC and Fierce from Nufarm. Group 4 does a good job of knocking it back and providing short-term residuals.

Pfeffer says there are best practices for pre- and post-emergence spraying in soybeans for herbicide selection. But growers need to scout their fields and keep track of their weed spectrums. He says that’s the only way to select the appropriate herbicide mix for each field. If full kill isn’t achieved, chances for herbicide resistance goes up.