Q: My horse occasionally grazes as I ride him on the trails---he knows when he is allowed to eat and when he is not. Normally he’ll snack on grasses, dandelions and clovers, but he will also take an occasional bite of milkweed. I have been told that milkweed is dangerous and contains a neurotoxin for horses. Is this true? How much must he eat before damage occurs, and what is it about milkweed that attracts a horse to eat it if it’s not good for him?
A: Whether or not a horse is poisoned by milkweed very much depends on the species he has eaten and how much of the plant he consumes. More than 100 species of milkweed grow in North America, and all contain variable amounts of different toxins.
In general, species of milkweed with narrow leaves (less than half an inch wide) attached to the stems in whorls (verticillate) contain toxins that affect the nervous system. These narrow-leaved milkweeds are quite palatable when green and especially when dried into hay. Most livestock losses are associated with the narrow-leaved species such as whorled milkweed (Asclepias subverticillata and Asclepias fascicularis).
The other group of milkweeds that can cause poisoning in animals are the broad-leaved species containing cardenolides, which affect the heart and can cause sudden death. Horses and other animals rarely eat these milkweeds because they are far less palatable. If your horse is routinely nibbling a particular type of milkweed, it would be wise to take a sample to your local extension office for identification.
The quantity of narrow-leaved milkweed necessary to cause severe poisoning and death of a 1,000-pound horse is approximately two to three pounds of the green plant. Occasional browsing on milkweed is not likely to cause any significant problem.
As for why a horse would eat a toxic plant---animals do not inherently know what is poisonous. When grazing freely, horses get by because they tend to eat only small amounts of a wide variety of plants, thereby never eating enough of any one toxin to become ill. However, a hungry horse confined in an area with little else to eat may consume large quantities of milkweed and can be poisoned.
Anthony P. Knight, BVSc, MS, DACVIM Professor EmeritusColorado State University
This article first appeared in EQUUS issue #455, August 2015.