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Managing herbicide-resistant waterhemp in Manitoba

04/06/2018

The weed’s movement into Manitoba calls for more vigilant field scouting

Waterhemp was found for the first time in Manitoba in 2016, southeast of Winnipeg in the area around the RM of Taché. The discovery led to the establishment of a waterhemp surveillance program led by Manitoba Agriculture in 2017. More waterhemp was found at a second site closer to the U.S. border. In both circumstances, the weed was very likely glyphosate resistant, being found in glyphosate tolerant crops.

When waterhemp first appeared it surprised agronomists, said Manitoba Agriculture extension specialist Ingrid Kristjanson. “We assumed that because the likely movement of waterhemp is from south to north, we expected to see it closer to the border first,” said Kristjanson. “So that was a bit of a surprise for us.”

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Waterhemp isn’t native to Manitoba. It was spotted in a soybean crop where it stood four to five feet above the crop canopy. “The reason it was spotted was because it was in a glyphosate-resistant variety soybean field,” said Kristjanson. “The suspicion is that it came in with chicken feed and due to the pattern in the field, the assumption is that it was spread with manure and survived.”

“In the 2017 field, it was in just a small area of the field, and there the suspicion was that maybe it came in with geese or with ducks,” she continued. “That’s another confirmed means of spread of the weed.”

Other ways that the weed can spread is by water and on equipment. “In the future, we’ll likely see further spread with water movement,” she said.

Identifying waterhemp

Waterhemp is in the amaranth or pigweed family. Initially, it could be misidentified as redroot pigweed. The cotyledons are smaller than redroot pigweed, and the leaves are longer, pointier and more lance shaped than redroot pigweed.

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“People talk about there being a kind of glossy look to those leaves,” said Kristjanson. “The struggle we always have is redroot pigweed, of course, has bright red on the undersides of the cotyledons and along the stem. You assume that that would be a good identifying characteristic, but it’s not always there.”

The differences in leaf shape, said Kristjanson, provide a better aid for identification.

Because it’s a new weed, probably the first thing growers will notice is stray weeds sitting above the crop canopy. “If you haven’t spotted it above the canopy prior to harvest, anybody who’s on the combine should be keeping close track of any oddball patches that they see,” said Kristjanson.

Managing waterhemp

Waterhemp is a prolific seed producer. A single plant that hasn’t faced a lot of competition could produce anywhere between one and five million seeds. Waterhemp seeds are similar in size to those of redroot pigweed. They germinate from a shallow depth.

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Further complicating the problem is the fact that waterhemp may not germinate until after weed control measures have already been taken, and it can easily germinate all the way into August. In open-canopy crops, like soybeans, where rain and sunlight can reach the ground, waterhemp can be particularly problematic. Waterhemp is less competitive in wheat, which is more aggressive and competitive early on.

“The seedlings of waterhemp aren’t overly competitive,” said Kristjanson. “As it gets bigger, that’s where it’s gaining speed and can outcompete other things if there’s good opportunity for moisture, nutrients and light.”

Rotation is key to management. Kristjanson recommends juggling crop types and encouraging more competitive crops with good seeding rates and early canopy closure. Different crops have different seeding dates, allowing for more time to manage the weed. She also suggests looking at different herbicide management strategies.

In Manitoba, the waterhemp found is highly likely to be resistant to glyphosate. In the U.S., waterhemp has shown resistance to both glyphosate and Group 2 herbicides. U.S. agronomists have found waterhemp that’s resistant to other groups, as well. “Straight glyphosate is not the answer,” said Kristjanson. “If you do have that weed, you should be looking at doing some sort of pre-emergent application.”

For growers who insist on using glyphosate, Kristjanson emphasized the importance of also using a product from another group. She suggested a tank mix in crop or sequential treatments of glyphosate followed by something else. Talk to your local agronomist for more specific solutions for your farm.

“We’ve got a terrific product in glyphosate and it’s an important tool for farmers,” Kristjanson concluded. “Part of the reason of having these discussions is making sure that farmers are considering some of these issues because we want to keep this tool in the toolbox for as long as possible. We don’t want to lose use of that product.”


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