We’re having a discussion about milkweed in our gardening group in South Deschutes. A member is concerned about us growing Milkweed for Monarchs as she fears it is fatal to cattle and sheep and will spread. True? And does this apply to all milkweed?
Yes all milkweeds contain toxins that can be poisonous to livestock, so caution should be used in where it is planted. The two types of milkweed that are native to Central Oregon is Showy Milkweed and Narrow-leaf Milkweed. It is important to keep pastures and hay free from milkweed. I am not aware of any sites in Central Oregon where it has spread vigorously. It may be worth calling the local Weed Vegetation Specialist to see if he has additional information on its spread. Here is the milkweed entry in Colorado State University's Guide to Poisonous Plants that explains it further.
Milkweed is unpalatable to horses, see Milkweed Poisoning of Horses.
There is some additional information in Livestock-Poisoning Plants of California.