Controlling the weed is more complicated that simply spraying it, experts say.
The toxic weed tansy ragwort has spread this year around Western Oregon with particularly large populations in Marion and Clackamas counties.
The outbreak is the worst Sam Leininger, WeedWise program manager, has seen in the program’s eight years of operation. WeedWise began in 2008 to “support more effective management of invasive weeds in Clackamas County,” according to its website.
Tansy is dangerous to humans and livestock because of a poisonous alkaloid in the plant’s tissue that causes liver damage when eaten. Signs of poisoning are consistent with liver failure, said Dr. Charles Estill, Oregon State University Extension veterinary agent.
“Horses get jaundiced, lethargic, slow, depressed, stop chewing in the middle of eating and wandering — some die from the wandering, from either walking off a cliff or walking into a pond and drowning,” said Estill.
While grazing animals generally avoid eating tansy, heavily infested pastures or hay contaminated with the weed can make it nearly impossible for the animals to avoid consumption. If tansy is more than 5 percent of the plants in a pasture, it creates an opportunity for toxicity, Estill said.
Young animals are especially susceptible because “they have different physiology or not as much life experience,” he said, as well as being less discriminating in what they eat.
Tansy can take anywhere from two weeks to six months to kill an animal. No treatment is available.
“(An animal) can ingest it now and die at Christmastime,” Estill said. Most cases are suspected and not confirmed.
There haven’t been any reported cases so far this year of animals killed by tansy consumption, but it is too early in the year to know, said Dr. Morrie Craig, professor of toxicology at Oregon State’s veterinarian research laboratory.
Unlike horses and cattle, sheep have ruminal microbes in their stomach that can eat the alkaloids that the tansy produces. Craig said research is underway to take advantage of these ruminal microbes.
“We’re thinking we’re going to try a way to capsulate ruminal microbes, and make a probiotic out of it that you can give to a cow,” Craig said, “and then it’ll have the microbes in its stomach to break it down.”
Tansy ragwort is identifiable by its flat yellow flowers at the top of the plant. The stems are green — occasionally with a reddish tinge — and the leaves are ruffled and dark green, according to WeedWise. At maturity, the plant can grow up to 6 feet tall and produce up to 200,000 seeds that remain in the soil for more than 10 years.
The infestation is most likely caused by the cool, wet and mild springs that Oregon has had for the past two years, Leininger said. The conditions have undermined biological controls such as the Cinnabar moth and flea beetle, which during the larval stage eat the roots of the plant. They were introduced from 1960 to 1971 by the Oregon Department of Agriculture. By the mid-1980s cattle deaths by tansy had been reduced by more than 90 percent, a report from the ODA said.
Tim Butler. manager of ODA’s Noxious Weed Control Program, explained that biological controls work on a cycle with the plant.
“With a crash in the tansy population we have the mirror image of the bio-controls dropping off. Then when the (tansy) comes back there will be a lag time for the fleas to come back,” he said. “People who want to get more bio-controls, we can’t supply anything that’s going to speed up the natural process. It won’t make a significant difference.”
At this point, it’s too late to spray the weeds with herbicide, he said.
“Spraying won’t do anything. The time to spray is early spring when it’s a rosette, or in fall after the rain,” Butler said.
Mowing the tansy when it’s in full bloom isn’t a solution, either, because it can cause plants to become short-lived perennials, and grow back next year.
However, Butler understands the motivation behind mowing, whether it’s to appease neighbors or make the pastures look better.
WeedWise encourages removing the seed heads and putting them into bags to keep seeds from spreading. If there is a concern about exposure to cattle and horses, Leininger said to remove the tansy from the field.
“Some (people) want to pull it and leave it, but the plant’s potability can increase in its wilt phase,” Leininger said. “You have to understand it’s a big task, but preventing seeding is the ideal scenario.”
Craig said that grazing sheep on tansy-covered pastures could be a way to clean up the weed.
Tansy populations may have increased, but it doesn’t compare to the infestation in the 1970s and 1980s. Butler said that historically the Tillamook Valley at this time would be yellow with tansy, but this year there are few plants.
“We have to look at where there isn’t tansy; that’s the real success story,” Butler said. “It shows areas that are doing quite well and free from tansy. Every year there’s going to be pockets of tansy. It’s frustrating and I understand that, but it’s not the problem it once was.”
To prevent future infestations Leininger recommends spraying in the fall. He said that large-scale operations should apply a broadcast herbicide, but smaller operations can control it with a backpack sprayer.
“We are definitely encouraging people to control it as much as they can,” Leininger said. “It’s not something that any singular entity can take after, it takes a community.”
There are no silver bullets when it comes to tansy, Butler said.