"Last year's drought meant that Missouri's hay crop, which is usually balanced very well for a horse's nutrition, was much poorer than usual," said Philip Johnson, professor of veterinary medicine and surgery. "Because of the poor Missouri hay crop, horse owners imported hay from other states nearby and possibly fed their horses hay that was too high in selenium. This can have very grave consequences for horses. Owners also may have fed their horses poor quality hay from Missouri or other places, which led to deficiencies in vitamin E, another very dangerous problem for horses."

Selenium poisoning
Selenium is a naturally occurring element and is an essential part of horse diets. However, too much or too little can create problems for a horse. When chronic selenosis, or selenium poisoning, occurs from eating too much of the element, horses can lose the hair in the mane and tail and develop a form of laminitis, a painful condition that affects the hoof. If left untreated for too long, a horse with chronic selenosis may require euthanasia as a result of severe laminitis.

Johnson said that the amount of selenium in hay can vary by county throughout the nation, but that Missouri hay typically has just the right amount of the essential element. For a small fee, horse owners can have their hay tested to determine if it has the right amount of selenium in it.

Vitamin E deficiency
In addition, hay that is not fresh can lack vitamin E, an antioxidant which is important for nerve health in a horse. Some horse owners unknowingly compensate for this deficiency by feeding their animals with nutritional supplements. Those horses that suffer from a vitamin E deficiency typically show symptoms that include weakness, loss of weight, trembling and changes in the retina at the back of the eyeball. A quick blood test can determine if the animal is suffering from a vitamin E deficiency. Johnson recommends that horse owners who imported hay from unknown sources last year either have the hay tested or keep a close watch on their horses. Horses that do not have access to green grass and that are being fed old yellow hay are at risk.

"Usually, by the time the horse is showing symptoms of either problem, it may be too late to reverse the disease completely," Johnson said. "However, if a horse owner has other horses that are feeding from the same food source, it's important to have those animals treated before the damage is permanent."