Without milkweed, the monarch butterfly would become extinct as this is the only plant that the monarch caterpillars eat. But, what about other animals, birds or insects? Is this a plant we want in our pastures or hay fields?
Milkweed is toxic to poultry and some livestock including sheep, goats, cattle, and horses with cattle, sheep and horses being the most susceptible. Milkweed contains two toxins: cardiac glycosides and an unidentified neurotoxin. Both of these can cause death. While the neurotoxin is the most lethal, the cardiac glycosides can produce digitalis-like signs that can contribute to death.
The amount of milkweed that your horse would have to eat to become ill or die is uncertain. A lot is dependent on the horse but, it could be as little as 0.0005% of your horse’s weight to as much as 2.0%. For a 1,000 pound horse, the amount could vary from ½ pound to 20 pounds.
Dry milkweed that has been baled in the hay DOES NOT lose its toxicity! While the green plant is more toxic, the dry plant can still make a horse sick or even die!
The symptoms of cardiac glycosides include depression and reluctance to stand, irregular heartbeat, colic, dilated pupils, muscular weakness or tremors and uncoordinated gait, and labored breathing. Death typically occurs within 24 hours.
Neurotoxins affect the nervous system. The symptoms include severe colic, dilated pupils, muscle tremors and falling down, incoordination, violent convulsions, and respiratory failure. Death occurs within 24 hours of ingesting the toxin.
For the most part, milkweed is not a vegetation that a horse would eat unless there is no other grasses around. If your horse is on pasture, be sure to monitor how much grass is available to your horse. Check to see if there is any milkweed growing in the pasture and eradicate it immediately.
The greater threat is in baled hay. The whorled-leaf milkweed has a finer stem and leaves that make it harder for horses to separate the weeds from the hay. Always inspect baled hay for the presence of weeks, especially the toxic weeds.
Even though milkweed may not be a “favorite” feed for horses, the potential for death is high. Always take measures to make sure that milkweed is not growing in your pastures or baled in your horse’s hay.