Milkweed roots contain the lowest amount of toxins. The toxins are highest in the bitter, milky sap, found throughout milkweed stems and leaves, hence the vulnerability of grazing animals. Sheep, cattle and sometimes horses may be poisoned by milkweed when they eat it in bulk. These poisonings usually happen when animals are penned in corrals with nothing to eat but milkweed, which grows almost anywhere, or when they eat hay containing large amounts of milkweed. Milkweed is also toxic to poultry.