Glacier FarmMedia – In terms of nightmare plants for pasture management, leafy spurge is a scary one.
It’s an official noxious weed, invasive, deeply rooted, able to spread through both seed and creeping roots and almost impossible to eliminate.
Its thick patches can wreak havoc on pasture productivity for most outdoor livestock species, goats and sheep being the exception.
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Even spraying is often only a costly Band-Aid, said Agriculture and Agri-Food Canada (AAFC) rangeland biologist Mae Elsinger.
“Spurge is one of the most powerful plants I’ve ever seen, with roots that go down 15 feet. They go out 20, 30 feet.”
If it is harmed in one place, she said, it will crop up again in another.
Why it matters: A 2010 report from Brandon University’s Rural Development Institute suggested leafy spurge was costing producers $10.2 million in lost pasture capacity at that time.
Despite often-repeated claims, leafy spurge is not dangerously toxic to cattle, Elsinger noted. Those claims are poorly substantiated. But it does make cattle uncomfortable if enough is eaten.
So, while not a poison threat, thick infestations of spurge can render a pasture unproductive. Research from the U.S. suggests cattle will not stick their noses through the canopy to graze once infestation density reaches 120 leafy spurge stems per square metre, according to Elsinger.
Living with spurge
Management rather than elimination is the likely approach once the weed is established, the AAFC biologist said. Producers have few short-term options except chemical control, and even that must be managed.
Fall preparation for spraying spurge is critical, attendees heard during a field day at the Manitoba Beef and Forage Initiatives (MBFI) site north of Brandon.
The First Street pasture run by MBFI is thickly infested with the weed and has become a prime site for leafy spurge research projects.
A planned blitz will include heavily grazing the area and encouraging hoof trample before the first pass of spray.
“It is important to note that no single chemical treatment will kill this weed,” say documents published by Manitoba Agriculture.
The 2022 crop protection guide counts eight herbicides for leafy spurge in hayfields and pastures, four of which are used only for top-growth control.
“Unfortunately, control with herbicides is often not very cost effective or the weed grows in areas which cannot be reached with spray equipment,” Manitoba Agriculture documents note. “Also, required rates of the herbicides recommended for control of leafy spurge are higher than the suggested rates for use on field crops. However, chemicals are useful as a method to contain existing patches when used in combination with other control means.”
The same documents say certain forage species like brome, wild rye and wheatgrass have better competition with the weed.
Non-chemical options
Other projects at MBFI have nothing to do with the spray tank. The site has spent several years putting flea beetles to work.
Although the words “flea beetle” are guaranteed to make canola growers flinch, the black dot and brown dot species used in spurge control are not considered pests for the oilseed. Their menu of choice is spurge.
Elsinger’s first project, alongside now-retired Manitoba Agriculture researcher Jane Thornton in 2015, looked at changes in beetle and spurge populations in grazed versus ungrazed areas. That project ended without usable results when researchers realized there were already spurge-eating flea beetles on site that complicated their data.
Nor were flea beetles the only spurge predator. Researchers found leaf tier moth and leafy spurge hawk moth caterpillars and gall midges. The findings led to a new survey on leafy spurge and biocontrol populations on the site.
Although it looks like a thick infestation, Elsinger found that most areas of First Street pasture were still below the threshold of 120 leafy spurge stems per square metre.
This year, she started managing the beetles more purposefully. New efforts include “herding” the flea beetles in efforts to concentrate populations in specific problem areas to see if there is better control.
“Historically, it’s been one release of 2,500 beetles or whatever … and that’s to cover what? A quarter-section or 80 acres or something like that? The tendency has been just kind of to walk away from that and not manage it in any particular fashion or even to monitor it.”
A now completed project looked at increasing cattle’s tolerance for leafy spurge. Led by Elsinger and Thornton, the project drew on similar projects by Kathy Voth, a U.S. grazing consultant and author.
Voth’s training regimen gradually increased a cow’s tolerance and consumption of leafy spurge, with a possible increase up to five per cent of the animal’s diet.
“It entices the cattle to not be afraid of new feeds and it gets them curious about trying new feeds,” Elsinger said.
Cattle were put through several rounds of training. The project included another spurge population survey to determine if cattle were grazing or trampling the weed.
Researchers found that cattle consumed anywhere from none to 14 per cent of stems, and that even with training, they grazed very little spurge when compared to sheep or goats.
A 2019 snapshot of the various projects noted that grazing and trampling, in conjunction with other biocontrols, likely affected spurge vigour, although there was no meaningful reduction in the weed’s population since beetles were first released in 2015.
Now years after the project ended, it is hard to tell if the training had any long-term impact on herd behaviour, Elsinger said.
A quick look at grazed sections of the First Street pasture showed lots of spurge left behind. It is obviously not the food of choice. At the same time, the closely grazed areas around each spurge stem suggest that cattle were willing to stick their noses around the noxious weed, said Elsinger.
The 2019 snapshot indicated that leafy spurge remained but hadn’t rendered the pasture unproductive. Researchers suggested beetle populations were keeping things in check.
“What I encourage people is to think about this: You’re not going to get rid of spurge. So, can you manage it so you can tolerate it? That’s why I always talk about these thresholds with cattle grazing… Can you get the spurge under 120 stems per metre squared?”
– This article was originally published at the Manitoba Co-operator.
