Scientific Name: Tetradymia glabrata
Distribution: Utah, Idaho, Nevada, Oregon, and California
Type of Poison: Furanoeremophilanes, which cause photosensitization
Signs of Poisoning:
Growth Characteristics: Littleleaf horsebrush is a highly branched shrub, with spreading or ascending branches, growing 1 to 4 feet tall. It is one of the earliest desert range plants to start growing in the spring. It may be green by late March and in full flower by the end of June. Leaves dry and drop off in early July, and the plant is dormant until the next spring. It reproduces from seeds and root sprouting. It is considered one of the "unarmed" horsebrushes because it lacks spines.
Flowers/Inflorescence: Flowers are bright yellow to cream, with pale lemon color being the most common. Long hairs surround the flowers. There are four to six bracts per head; hence the name Tetradymia. The flowers are similar in appearance to rabbitbrush flowers, only larger. The fruit is a densely silky achene.
Leaves: Alternate, blades narrow lance-shaped, one-fourth to one-half inches long, and come to a sharp point. The margins are entire, and the leaf has a prominent mid-vein. The first leaves to appear at the node can be weakly spinescent. Secondary leaves grow from the axils of these first leaves.
Stems: Erect, short, stout, highly branched. Older bark is gray and shreddy. Old leaf bud scars appear swollen, giving the stem a knobby appearance.
Littleleaf horsebrush is found on dry, open valleys, plains, and foothills. These areas are often sheep winter ranges. It is generally common up to 5,900 feet in elevation and has occasionally been found at elevations over 7,500 feet. Littleleaf horsebrush prefers the skeletal, sand, fine-loam, and clay soils found throughout the Great Basin. The soils are often saline or alkaline.
The occurrence of fire or overgrazing greatly enhances the ability of littleleaf horsebrush to dominate a site.
Littleleaf horsebrush causes severe liver damage when large amounts are eaten, and sheep may die within a day or two. The plant is especially dangerous during bud stage, perhaps because it is more palatable. Survivors, if pregnant, often abort and may require weeks to recover. Sheep eating lesser amounts of the plant may become sensitive to sunlight and develop various degrees of photosensitivity. Severe photosensitization results in bighead, wherein the sheep's head is greatly enlarged. Sheep vary considerably in their susceptibility to horsebrush, but only one-half to three-fourths pounds of littleleaf horsebrush will cause bighead.
Whether bighead develops often depends on whether the sheep has recently eaten certain sagebrush plants, notably black sagebrush. The plant enhances the action of horsebrush on sheep.
Most losses occur during stormy periods when sheep change their grazing habits as they are trailed through heavily infested areas. Hungry sheep may also eat toxic amounts of horsebrush after they have watered. To prevent losses, avoid horsebrush ranges while trailing sheep. Do not let animals graze in infested areas immediately after watering or during stormy periods.
Move affected sheep to shade as soon as you notice signs of bighead. Give them water and feed; supplemental hay may be beneficial. If possible, these should should be left undisturbed for a few days.
Howard, J.L. 2002. Tetradymia glabrata. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available:  Accessed 4 August 2009.
James, L.F., R.F. Keeler, A.E. Johnson, M.C. Williams, E.H. Cronin, and J.D. Olsen. 1980. Plants Poisonous to Livestock in the Western States. U.S. Department of Agriculture, Agriculture Information Bulletin 415, 90 pp.