Littleleaf horsebrush is a strong-scented shrub that often reaches a height of 3 feet. Stems are abundantly branched; dark green leaves are slender and come to a sharp point. Flowers of the littleleaf horsebrush are yellow when they first appear. After they mature and dry, they turn brown.
Gray horsebrush gets its silvery appearance from hairs that cover stems and leaves. Leaves are about 1 inch long-longer and broader than those of littleleaf horsebrush.
Gray horsbrush reaches a height of about 1 to 3 feet. It is often mistaken for nonpoisonous sagebrush. Like littleleaf horsebrush, gray horsebrush is a perennial in the sunflower family.
Two species of horsebrush that grow in the Great Basin region of the West are poisonous to sheep. Littleleaf horsebrush is also called smooth horsebrush, spring rabbitbrush, coal-oil brush, lizard shade, ratbrush, and dogbrush. Gray horsebrush is also called spineless horsebrush.
Sheep that feed on horsebrush and then are exposed to bright sunlight may develope a characteristic swelling of the head. This is called bighead.
Most losses from horsebrush occur when sheep are trailed through heavily infested areas that do not support good forage, or immediately following a storm. Hungry sheep often eat toxic amounts of horsebrush after they are watered.
Littleleaf horsebrush is more than twice as toxic as gray horsebrush. All plant parts of both species are poisonous, but sheep eat only the leaves and fine stems.
Plants are most toxic during active growth-from March to May. Toxicity decreases rapidly after the plants have flowered and are doemant.
Where and When It GrowsLittleleaf is most abundant on benchlands, well drained slopes, and low elevations on winter ranges. It often is found on or near lava formations.
Littleleaf horsebrush is one of the earliest desert range plants to start growth in the spring; it usually is in full flower by the end of June. Leaves dry up and drop off in early July. The plant is dormant until next spring. It is semidormant in dry seasons.
Gray horsebrush is most abundant in sagebrush areas and foothill regions; it is also scattered throughout littleleaf areas.
Gray horsebrush starts growth later than littleleaf horsebrush.The plant flowers in June or July and usually remains green until fall.
How It Affects Livestock
Scientists have shown that horsebrush causes severe liver damage and a condition that sensitizes the skin to sunlight. Swelling occurs only in light-skinned animals; it becomes more severe in direct sunlight. Sheep often recover from bighead caused by gray horsebrush. Sheep affected by littleleaf usually die.
Sometimes animals die within a few hours of eating the plant-so quickly that they do not develope usual signs of poisoning.
Many ewes abort or become sterile after eating a toxic dose of horsebrush. Usually, 1/2 pound of leaves and fine stems of littleleaf or 1 1/4 pounds of leaves and stems of gray horsebrush will cause bighead in a 100 pound sheep.
Signs of Poisoning
3. Swelling of head, neck, ears, eyelids, and nose
4. Heavy, drooping ears that hang straight down
5. Loss of milk in nursing ewes
6. Peeling of skin and wool from head, ears, and back
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