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Resistant waterhemp devastates soybean yields

Yield reduction up to 75% reported as weed continues to spread through Ontario and Quebec

Agronomists in Quebec are developing protocols to deal with infestations of waterhemp that have caused significant yield loss in soybean fields.

There are two things that makes this new weed in Canada so difficult to control. First, it’s resistant to many commonly used chemistries in soybeans. Second, it’s prolific and emerges through the entire growing season.

“Herbicide-resistant waterhemp is the number one problem in the U.S. and even in Ontario,” says agronomist Jean-François Lemoine co-owner of the Farnham Agrocentre located southeast of Montreal.

Lemoine’s team was one of the first to detect the presence of resistant waterhemp in the province, a weed that is gaining ground quickly in Quebec. That was in 2018 in a soybean field.

“We lost a good part of the crop,” recalls Lemoine.

Since then, the number of resistance cases have been multiplying. In some places, the fire is raging. “You can lose up to 75% of your yield,” he says. “Not to mention that waterhemp can complicate harvesting and stain the beans.”

To achieve control of this tough weed, the agronomist advocates an integrated management strategy. “In terms of herbicides, the principle is to make several applications with multiple sites of action while avoiding Groups 2, 5, 9 and 14, for which cases of resistance have already been diagnosed.”

In soybeans, Authority® Supreme herbicide is a mainstay of his approach. He says that’s partly because it combines Groups 14 and 15. Waterhemp is not resistant to Group 15, and it's not resistant to Group 14 at the pre-emergence stage either. “The idea is to intervene as early as possible,” he says.

Jean-Francois Lemoine points out that Authority Supreme herbicide offers several advantages. “Compared to other herbicides in its class, it requires less water to be activated,” he says. “In addition, it is not phytotoxic to soybeans. Most importantly, it has long-lasting efficacy, which is crucial, as waterhemp emerge throughout the entire season.”

Along with herbicide treatments, the agronomist advocates crop rotation, use of cover crops, hand pulling and mechanical weeding.

“Each waterhemp plant can produce up to four million seeds,” he says. “Plus, because it's a dioecious species, its genome evolves quickly. You have to be proactive about this threat.