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Absinth. An obnoxious weed for pastures

As the rains of 2010 brought signs of an end to a battering 10-year drought, Marj Veno could see that absinth wormwood was getting the upper hand in pastures around the ranch’s home place northeast of Hanna, Alta.

Absinth first reared its ugly yellow-flowered head in the pasture west of the yard on the old road and in the ditches after a new stretch of grid road was built in 1987. It fanned out in all directions, eventually to a large slough on that quarter, across the road to the shelterbelt trees along a small pasture near the house and on out into the native range on the home quarter.

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Veno says the most likely source of the weed was the grass seed mix that the Special Area crew used for reclamation work because she had never seen absinth before it invaded her three-generation ranch in the Special Area.

Special Area is a municipal designation, similar to a county, but administered as one large region running from Hanna to the Saskatchewan border, Veteran on the north, and Empress to the south, about an hour north of Medicine Hat.

Absinth is a long-lived shrub-like perennial notorious for yielding profuse amounts of seed that remains viable for three or four years and takes hold in any spot of disturbed ground. Gopher mounds, coyote dens, cow trails, even buckbrush patches in pastures are jumping off places for new plants to take root and spread their seed.

It seemed like the weed had just been mocking her attempts over the past 25 years to keep it under control by mowing the reclaimed roadway to try to prevent seed set, and tilling, mowing and spot spraying the shelterbelt area.

“When the rains finally came the prairie was so beat up from grasshoppers and had gone dormant, but this stuff was ready to jump all over that moisture and absolutely erupted. I think that’s why it got away on me, because it got the jump on the prairie grasses,” Veno says.

“To say it only attacks overgrazed pastures just isn’t true, though,” she says driving past places where the silvery leaves of absinth plants poke out from the healthy prairie wool stand with a thick thatch cover in the bottom.

Veno and her husband, Murray McArthur, run 300 commercial and 300 purebred cows on 12,500 acres of predominantly native range across three ranch sites in the area. They keep detailed grazing records to manage for year-round grazing as far as possible and the range east of the yard is an important part of their winter grazing program.

“With a bit of rain in September, that prairie wool greens up and the cattle get fat fast on it. It’s powerful grass because they eat less and get more good out of it. Lots of winters they see very little extra feed. It’s just what they can rustle up themselves out there and they’re always fat,” she says.

She is very grateful to the folks at the Chinook Applied Research Association and manager Dianne Westerlund for getting her pointed in the right direction with the use of the herbicide Restore II. Westerlund organized a demonstration project at the ranch as part of a Commission for Environmental Co-operation project intended to demonstrate various beneficial management practices for sustainable use of grasslands at project sites in Mexico, the U.S. and the prairie provinces.

Side-by-side applications of three herbicides (2,4-D, Reclaim, and Restore II) were carried out in spring 2014 with additional treatments made in spring 2015 in the pasture near where the absinth problem originated. Headland areas and fence lines were also sprayed to try to prevent further spread. Large areas of absinth were mowed with a three-point-hitch mower, as were the ditches along the road to prevent seed set.

Veno purchased additional Restore II to spot spray individual absinth plants in that pasture and the native grass pasture across the road.

Restore II is a broad-spectrum broadleaf herbicide designed specifically for use on rangeland and permanent pasture with residual activity in the soil to help control germinating seeds and emerging seedlings. There are no grazing restrictions following application except for lactating dairy cows, but there is a 30-day wait time before cutting for hay or silage.

Mounds on one side of this pond (left) were sprayed in 2014 with Restore II, the other side went unsprayed.

Mounds on one side of this pond (left) were sprayed in 2014 with Restore II, the other side went unsprayed.
photo: Marj Veno

Veno was impressed with the results. After two summers of vigilant spot spraying by wand from a sprayer mounted on the back of the side-by-side, she had the shelterbelt area cleaned up and a good start on the native range. At least there is hope now that eventually she will be able to say the same about the native range.

Her concerns are twofold. Not only is the absinth-infested area important winter grazing range for their operation, but the Berry Creek that runs through the pasture is the major tributary running out of the region to the Red Deer River, approximately 75 miles to the south.

The wintering range is in a flat flood plane where three creeks converge into the Berry Creek. Spring runoff spills over the banks to flood irrigate about 600 acres of native grass. When the water is coming in full force, she opens a control structure on the creek built by the PRFA in the 1940s to let water flow downstream to flood irrigate a neighbour’s 300-acre hay meadow. Shutting the gate diverts it into a channel that runs to Ducks Unlimited’s big wetland restoration project, Contra Costa, approximately three miles southeast of the pasture. There, it floods about three sections before the water runs over the spillway, back into Berry Creek and on to the Berry Creek (Carolside) Reservoir east of Drumheller near Sunnynook. Not only is the reservoir a major recreational fishing site, but it supports a lot of farms and ranches on an irrigation project south of there. From there, the water continues on its path to the Red Deer River.

“It just scares me because the seeds fall down here, the water comes in, and away the seeds go. There’s potential to make a massive mess,” she says, adding that the land around Contra Costa is already infested.

To make matters worse, the weed popped up at a gas-well reclamation site on their east ranch about two weeks after the company sowed the grass. Veno attacked it and brought the problem to the attention of the gas company, which followed up with another treatment.

“I see a big problem because thousands of old well sites will be reclaimed in central Alberta in the coming decade. People need to be aware of this weed. I don’t want to see this happen everywhere because this is such a unique part of the province,” Veno says.

Absinth is starting to get a bit of attention, but so far not many resources are available to help with control, Westerlund says. The provincial government designates prohibited weeds and it’s up to the counties to declare noxious and problem weeds in their jurisdictions. The Special Area board has been aware of the absinth problem since the early 1990s and manages it as a problem weed, but so far it hasn’t made it onto the list of noxious weeds, although at least one county to the west of the Special Area has recently added absinth to this category.

Saskatchewan Control Projects

Approved herbicide costs for control of the specified noxious weeds are rebated at 100 per cent for public land and 50 per cent for private land. The program also rebates 100 per cent of approved herbicide costs to control prohibited weeds on public and private land.

A project evaluating the effectiveness of five herbicide options for absinth control was carried out by Bart Lardner and Daalkhaijav Damiran with the Western Beef Development Centre and Nadia Mori, forage specialist with Saskatchewan Agriculture.

Once an absinth plant is established it spreads rapidly in native pastures.

Once an absinth plant is established it spreads rapidly in native pastures.
photo: Marj Veno

Based on effectiveness reducing absinth up to 12 months after application at four sites across the province, the ranking was 2,4-D < Banvel II < Rejuvra XL < Grazon < Restore II < Reclaim.

Based on cost per acre, the ranking was 2,4-D < Restore II < Reclaim < Grazon < Banvel II. Rejuvra XL wasn’t commercially available in Canada until after the project wrapped up.

All products increased the percentage of grass in the canopy, decreased the percentage of legumes and other weeds, and increased the percentage of bare ground in the 12 months following treatment.

All except 2,4-D reduced the percentage of absinth. It did provide some top-growth control, but absinth actually increased from an average of 8.3 per cent before treatment to 10.6 per cent of the canopy a year later, suggesting that multiple applications would be necessary.

The economics didn’t work out for Banvel II considering that it was the most expensive per acre and absinth control averaged only 55 per cent across the sites.

This compares to 99 per cent for Restore II and Reclaim at lower price points. The one site where a few absinth plants did reappear was treated when the temperature was above the recommended maximum of 28 C. Grazon was very effective as well, but has limitations because it is highly mobile in soil.

Restore II, Reclaim and Grazon are Dow AgroSciences products designed specifically for use on rangeland and permanent pasture. Secondary herbicide activity may occur through soil uptake to control seeds that do try to germinate. The authors suggest that if these products are able to provide multiple years of control, they may be more economical than lower-cost options that require multiple applications.

Rejuvra XL is a DuPont product, also designed specifically for control of broadleaf weeds and some brush species on permanent pasture, with additional control of young, actively germinating weeds through uptake by roots and shoots. Although absinth isn’t listed among the broadleaf weeds the product controls, it did reduce absinth from an average of 4.3 per cent to 1.1 per cent and totally wiped out all other broadleaf weeds.

The authors note that the greater the weed and legume density in a forage stand, the more bare ground there will be following an effective treatment and the greater the opportunity for more weeds to take hold unless a contingency plan is in place for filling them with desirable species. When legumes make up more than 18 per cent of the initial stand, legume loss after treatment can negatively affect total forage biomass and quality of the stand due to loss of nitrogen fixation and cross-fertilization to grasses. The fact sheet with full results is available at www.wbdc.sk.ca.

Weed wiper shows promise

The rope-wick wiper is mounted on the front of a quad. A PVC tube holds the herbicide solution and a rope threaded in and out of the tube spreads the herbicide on contact with vegetation. However, there is no way to turn off the flow once the tube is filled. The kill rate on the target weeds was only 15 to 30 per cent across the three sites in a recent trial.

The rotating drum wiper pulled behind a quad gave an overall 80 per cent kill on the target weeds. The drum is covered with a thin carpet-like material wetted by sprayer nozzles along the drum under a covered boom. It was easy to stop flow of the herbicide in progress and to clean out.

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