For ranchers with large stands of toxic larkspurs on their ranges, yearly herd mortality can be as high as 10%. Across the western U.S., this results in annual economic losses of millions of dollars in animal deaths, increased management and treatment costs, and, if animals are deferred from grazing, the underutilization of otherwise highly nutritious pastures and rangelands, according to researchers with the U.S. Department of Agriculture's Agricultural Research Service.
The researchers recently published an article in Rangelands describing research that that sought to identify cattle breeds that are more resistant to specific toxic plants, focusing initially on larkspurs. This large group of Delphinium species is toxic to many animals, including cattle, but how poisonous the plants are varies widely depending on the specific species of larkspur, where it is located and the breed of the cattle.
Steers that eat large quantities of larkspurs can show signs of poisoning in as few as seven hours. They are most likely to eat high-protein larkspur pods, which are also the most poisonous part of the plants.
The current study focused on half-siblings in five breeds of cattle: two types of dairy cattle and three beef breeds. The steers were given the same dose of dried, ground tall larkspur and watched for 24 hours. They were then walked on a halter until their muscles weakened from larkspur poisoning. Symptoms were recorded, and the animals rested until they could walk again without tiring.
The researchers found that all five breeds of cattle weakened on average within about 30 min. but that the Line 1 Herefords from ARS Ft. Keogh Livestock & Range Research Laboratory in eastern Montana averaged fewer than 9 min. The dairy breeds had greater resistance to tall larkspur than the beef breeds, and Jersey cattle, a breed noted for having little genetic diversity, lasted the longest. Brahman cattle were particularly susceptible to larkspur poisoning.
An unexpected result of the study was in the reactions of individual cattle. Some Angus steers walked for 40 minutes and were labeled as resistant, but other Angus steers were too sensitive to the toxin to be exercised. For each breed tested, at least one animal was labeled as resistant to tall larkspur.
The results indicate that ranchers may be able to selectively breed cattle for larkspur resistance. If a gene marker was found, ranchers could submit a blood or hair sample for genetic testing of vulnerability to the toxic plants. The researchers are also working to identify such resistance to other toxic plants, such as lupine.