Pasture weeds require vigilance

From wide acres of rangeland to intensively managed forage stands, there is a huge array of pasture types on the Prairies, each one subject to pressure from a unique set of weeds.

However, one weed that seems to be pretty much ubiquitous is Canada thistle, which moves quickly into a pasture and then spreads through its seed and root.

“We still see quite a bit of Canada thistle out on the landscape, whether that be in an irrigated, tame pasture setting or even some of our areas that are more heavily grazed,” said Todd Green, agricultural fieldman in the County of Newell surrounding Brooks, Alta.

There are herbicide, mowing and grazing options for Canada thistle, including intensive animal grazing in situations where animals can’t select around the weed. It works best when the weed is young and relatively tender.

Another common issue in Newell is weed seeds spread by winter feed.

Often, more competitive perennial plants will squeeze these weeds out when given help, possibly with timely mowing.

Green is a director at the Alberta Invasive Species Council and he said early detection of weeds is key to maintaining productive pastures.

“From the regulated standpoint, we do see some downy brome, we see some knapweed, we have a couple of our areas that have some baby’s breath,” Green said.

“One of our bigger concerns, I guess, is we do have some creek systems that go through and some of them are fairly full of common burdock, which is relatively new to our area.”

He said producers have to use every trick in the book to keep some pastures productive.

“If you’re seeing numbers slip where at one time you could graze 100 animals on a chunk of land and now you’re down to 50, that’s a huge detriment to your rotation,” Green said.

“I would say, as a cattleman, put some money aside for some chemical.”

Trevor Lennox, a range management extension specialist at Swift Current, Sask., provided a list of the most economically important weeds in Saskatchewan.

Number 1 on his list is leafy spurge.

“It’s very aggressive, very invasive and once it’s established, it’s very difficult to eradicate. You can spray it and suppress it for maybe two or three or four years and think you’ve got it, but it will come back from the root again,” Lennox said.

Canada thistle is also near the top of his list because of its abundance, as is foxtail barley.

“Foxtail barley is a native weed, but it still is able to compete quite aggressively in areas where it doesn’t have much competition, such as saline areas, around saline sloughs, or areas that flood frequently,” he said.

“Provide a good strong competition or graze it early in the season and it may not be much of a problem for you.”

Common tansy has a poisonous compound and the plant is causing issues because even if animals are forced to eat it, they can’t eat a lot of it.

Producers usually have to resort to herbicide to control common tansy, as they do with absinthe wormwood.

“Absinthe wormwood is probably one where chemicals are probably the best option because it has a very aggressive root system,” Lennox said.

Knotted thistle, which grows up to two metres tall, is a short-lived biannual present across Saskatchewan and it can be controlled with targeted mowing or by rotating a pasture into hay for a few years.

Snowberry is a shrub commonly referred to as buckbrush that grows to knee-high or mid-thigh height and is increasingly creeping into pastures.

Chemicals can keep this plant in check, as does mowing and intensive grazing.

“In the north, aspen poplar can be considered a bit of a weed. Aspen wants to come back on fields that were recently broken up,” Lennox said.

Mowing, haying, and chemicals can target young aspen, and they will eventually get the hint and become less of a problem on new pastures.

He said if a producer is thinking of putting a field into pasture that is currently being used for annual cropping, they should try to clean it up while there are still more herbicide options.

“Once you go back to perennials it’s harder to control weeds, not saying it’s impossible, just tougher. Having said that a lot of annual type weeds that are more of a problem in an annual cropping situation aren’t as much as a problem in perennial forages,” Lennox said.

There are also more herbicide options for grass hay but they have a downside.

“I hesitate to say seed straight grass because in a sense that helps the weeds compete more too. Alfalfa is an awesome competitor against broadleaf weeds because it has deep roots and can compete head-to-head against these broadleaf weeds,” Lennox said.

Jane Thornton, a forage and pasture specialist for Manitoba Agriculture, said leafy spurge is the biggest problem in her area.

“When I arrived here in the Brandon, Carberry, Shilo area, in that 20 years it spread north of Riding Mountain National Park and it’s everywhere now,” Thornton said.

Producers must identify weeds early and deal with them immediately.

She said pasture rotations and proper stocking rates without overgrazing allows the grass to compete.

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