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Description: Selenium is an essential trace mineral but can be toxic in relatively small amounts. The concentration of selenium in feedstuffs commonly ranges from 0.05 to 0.3 ppm with higher amounts accumulating in plants grown on alkaline soils and in certain "selenium accumulator" plants, especially in the Rocky Mountain region and the Great Plains States of the western USA. The maximal tolerable selenium intake in horses is estimated at 2 mg/kg of total ration (NRC, 1980, 1989) though signs of chronic excess can be seen with lower intakes in the author's experience. Some have hypothesized that copper interferes with Se absorption and high (>15-20 ppm) Cu intakes may be protective in some chronic Se excess situations. Signs of chronic excess include loss of hair in the mane and tail, as well as horizontal cracking of the hooves, which may start at the coronary band but worsens as the hoof grows out. If severe excess exists then the lower half of the hoof wall and sole may slough off, leaving the pedal bone exposed. The hoof in the picture on the left has a horizontal crack forming just below the coronary band pointed out by a small black arrow. This horse had developed an abnormal appetite for gumweed (a selenium accumulator plant) in northern Colorado. The image on the right shows a tail shortened by hairloss. Both are examples of minor chronic selenium toxicity.

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