Weed & Pest
Larkspur control Research suggests resistance in cattle
“Larkspurs are poisonous plants that negatively impact the earning potential of beef producers in many western rangelands of North America,” says Ben Green, et. al. in their article, titled, “Mitigation of Larkspur Poisoning on Rangelands through the Selection of Cattle” that appeared in Rangelands magazine. “If ranchers are unfortunate enough to have large stands of toxic larkspur in their pastures, yearly herd mortality can be as high as 10 percent.”
Green, research pharmacologist at the USDA Agricultural Research Service (ARS) Poisonous Plant Research Laboratory, has studied larkspur for 11 years and believes that larkspur toxicity can be mitigated through cattle selection.
Green notes that poisonous plants have been studied by USDA since 1894, and larkspur has been studied at the lab for many years, but he was particularly impacted while visiting a producer who had suffered cattle losses from the plant.
“We went to a producer’s field to try and understand what was going on,” explains Green. “He lost 30 animals and his bulls, and on seeing that, I understood how important this subject is for ranchers.”
He further adds that it was interesting to see that, while a number of animals died as a result of larkspur toxicity, another 30 to 40 cows in the pasture were unaffected.
“The other cows looked normal and healthy,” Green says. “We believed that the surviving animals must be resistant and have worked to identify those animals with a natural resistance to larkspur poisoning.”
In setting up the project, Green says they first halter broke a set of steers.
“We gentled all the steers and trained them to an exercise regimen,” he says. “Then, we administered a low dose of larkspur – strong enough to cause muscle weakness but not so strong to have other impacts.”
After administering the larkspur, Green notes that cattle were exercised around a track until they showed clinical signs of muscle weakness – a classic signal of larkspur poisoning.
“When they showed signs of weakness, we immediately let them recover,” Green says. “We measured the time from when they started walking until they showed weakness.”
It was easy to see which cattle were poisoned and which were more resistant, Green says.
“Very susceptible animals were so weak that they tired quickly,” he comments. “Resistant animals would walk and walk as if they weren’t poisoned at all.”
The study looked at five breeds of cattle – including Line One Herefords from USDA ARS Fort Keogh in Miles City, Mont., Angus, Holstein, Jersey and Brahman – to determine if breed affected susceptibility to larkspur poisoning.
“Some breeds have more susceptibility to larkspur, especially the Line One Herefords,” he comments. “Those animals are a group that have been line bred for more than 50 years.”
Additionally, Brahmans were very susceptible, and the beef breeds in general were more susceptible than dairy cattle breeds.
“Every breed has resistance to larkspur poisoning,” Green comments. “The important thing is finding those resistant animals.”
Green also adds that the results of the study were very repeatable and resulted in very good data.
After identifying that some animals within each breed do have resistance to larkspur, Green notes they have isolated DNA from both resistant and susceptible animals and sent it to GeneSeek, a genetics company, for genotyping.
“Right now we are working on the genetics with John Keele and other researchers at the USDA ARS U.S. Meat Animal Research Center,” Green comments. “We are hoping to find gene markers for both susceptibility and resistance, but if we find a genetic marker for one or the other, that would also be great.”
By identifying markers, he notes that producers would be able to identify susceptible or resistant cattle using genetic tests.
They will test their genetic markers by attempting to identify cattle in the lab that will be resistant and susceptible to larkspur, treating them with larkspur and testing them for muscle weakness, as was done in the initial study.
“Hopefully we have useable results for cattle producers within five to 10 years,” Green says.
USDA Agricultural Research Service Research Pharmacologist Ben Green notes that larkspurs vary in toxicity from location to location.
“At our lab, we do analyze larkspurs for producers who send samples in to determine their level of toxicity,” he says. “If producers have larkspur, we recommend they have it analyzed.”
Green notes that producers can obtain more information by calling the lab at 435-752-2941.
Lupine, another toxic plant, is also a subject of research at the USDA Agricultural Research Service Poisonous Plant Research Laboratory.
“Our lupine study is similar to the larkspur study,” says Research Pharmacologist Ben Green. “Pharmacologically, larkspur alkaloids block nerve transmission and lupine alkaloids activate nerve transmission, so we had to come up with a slightly different study.”
Lupine causes crooked calf disease and is also economically important for cattle producers.