California Riding Magazine • June, 2011

The Gallop
Recent Cases of Selenium Deficiency
found in Southern California Horses.

by Ken Feigner, DVM

The veterinarians at San Luis Rey Equine Hospital in Bonsall have recently diagnosed over a dozen horses located in the Southern California area with severe selenium deficiency. Historically this has not been considered a major problem in the southern parts of the state but these cases suggest that the problem may have been under-diagnosed for this reason.

Initially the clinical signs in these horses were primarily related to colic and cardiac issues. The affected horses had elevations in heart and muscle enzymes associated with cardiac damage and rhythm abnormalities that in one case resulted in death. The selenium levels present in the bodies of these horses was much lower than required, and in a few cases was undetectable.

Horses get their selenium primarily from the hay that they eat. The hay grown in California gets its selenium content from the soil in which it is grown. If the soil is deficient in selenium then the hay grown here will also be deficient in the nutrient. A map available online (at shows the selenium content of the soil in each of the counties of California. As seen, the soil content of selenium in the majority of Northern California and several of the southern counties is very low. San Bernardino county and a few other areas in Southern California where large amounts of hay are grown are in this deficient category.

It is easy enough to supplement selenium in the diet with any number of available vitamin-mineral supplements. The recommended daily intake for an average sized horse is about 1 mg per day. Since the hay grown in other areas can actually have an excessive amount of selenium, however, it is not always wise to simply supplement all horses. Too much selenium in the diet can be harmful as well.

Blood tests can determine the selenium status of an individual horse which can be helpful in the short term if selenium deficiency is suspected. For longer term management, however, it is more helpful to know the selenium content in the hay you are feeding. Hay analysis can be run fairly inexpensively, but simply knowing the area that the hay you are feeding was grown in can be a good start to knowing what the selenium content is likely to be.

There are countless websites and online articles making recommendations for horse nutrition that are not always accurate or that are simply trying to sell a product that may not be in the best interest of your horses. Working with your veterinarian to develop a good nutritional plan for your horses is the best way to help keep them healthy and happy for a lifetime.

Author Ken Feigner DVM, is a veterinarian with San Luis Rey Equine Hospital in San Diego County's Bonsall. He is an ambulatory veterinarian with a sports medicine specialty.