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Weeds develop defences

Kochia gets hairy and lamb’s quarters get waxy under harsh conditions

Kochia was emerging in Manitoba fields last week and most of it is presumed resistant to Group 2 herbicides, says Manitoba Agriculture weed specialist Tammy Jones.

Many could also be glyphosate tolerant, but it’s hard to know to what extent, she said in an interview April 24.

“I know that it is distributed fairly thoroughly across the province, but there aren’t a lot of people who test for it and so I think it’s under-reported quite significantly,” Jones said. “And some people don’t bother testing because they just assume that it is.”

But when a herbicide fails to kill a weed it’s not always because the weed is herbicide tolerant, she said. It could be a combination of factors, including the environment and the weed’s own natural defences.

“Kochia gets super hairy and lamb’s quarters develop lots of wax on the leaves,” Jones said, adding those features make it hard for a herbicide to stick to them. “Sometimes the environment and the dryness impact more on that weed so that it’s harder to kill…

“And if the plant is not as actively growing because it doesn’t have abundant moisture it’s not maybe picking up that herbicide as quickly either and that can affect how well our herbicides work.”

Lamb’s quarters’ waxy leaf.
photo: Tammy Jones, Manitoba Agriculture

The environment can also affect pre-plant or pre-emergent herbicide efficacy, she said. Most require a certain amount of moisture to be activated.

“As much as you try, nothing is perfect,” Jones said. “You need a backup plan or another option when the first one doesn’t work. It’s tough with all the weed pressure and all of the resistance (to herbicides) to have that strategy mapped out all the time.”

Kochia, in addition to being in some cases resistant to Group 2 herbicides and glyphosate, is always germinating under the right conditions, she said.

“So a one-pass system — cultivation or herbicide — you’re never going to control the entire population because it just keeps germinating,” Jones said.

The reduced threat of overland flooding along the Red River this spring also reduces the risk of common waterhemp and Palmer amaranth spreading here from the United States. Both are ‘Tier 1’ weeds under Manitoba’s Noxious Weeds Act.

Farmers with Tier 1 weeds are legally required to destroy the plants and seed, “because they are a significant threat to our economy,” Jones said.

Jones isn’t aware of any Palmer amaranth in Manitoba yet.

“Hopefully it’s not something we’ll see any time soon,” she said.

Some common waterhemp has been found.

“There has been some pretty significant efforts to make sure those fields have been monitored and those weeds have been destroyed.”

Both weeds are in the same family as redroot pigweed, which is a prolific seed producer and competitive with many crops.

Palmer amaranth in the U.S. is resistant to several herbicide groups, including glyphosate.

“Common waterhemp grows 50 to 70 per cent faster than redroot pigweed,” Jones said. “Palmer amaranth is even more aggressive than common waterhemp as far as growth rate and seed production.”

About the author

Reporter

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.

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