Milkweed (Asclepias spp.).
Greatest losses have occurred in sheep on western rangelands, but all animals are susceptible to poisoning, especially when other forages are scarce or milkweed is incorporated in their hay.
Milkweed is an erect perennial herb that has either 2.5- to 5-inch (6- to 12-cm) broad-veined leaves or narrow linear leaves seldom more than 1 to 1.5 inches (2 to 4 cm) wide arranged either alternately or in whorls. Most species of milkweed (except butterfly weed, or A. tuberosa) contain a milky sap from which the plant derives its name. The flowers are produced in terminal or axillary umbels consisting of two 5-parted whorls of petals, the inner one being modified into a characteristic horn-like projection. The color of the flowers varies from greenish white to red. The characteristic follicle or pod contains many seeds, each with a tuft of silky white hairs. The narrow-leafed species generally tend to be the most toxic. They grow in open areas along roadsides, waterways, and disturbed areas, preferring the sandy soils of the plains and foothills. Overgrazing will enhance the encroachment of milkweed.
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