COMMON NUTRIENT DEFICIENCY IN HORSE

Vitamin E

This potent antioxidant enhances immune function, facilitates cellular respiration, aids DNA synthesis, and improves the absorption and storage of vitamin A, among other roles.

 Fresh grass contains high levels, but these drop once the grass is cut for hay and stored. There are two diseases associated with a vitamin E deficiency: equine motor neuron disease, a rare oxidative disorder that impacts motor neurons in the spinal cord, and equine degenerative myoencephalopathy, a disease characterized by brain stem and spinal cord deterioration. 

Testing is the only way to know for sure if a forage is deficient in E; horses on hay-only diets might need vitamin E supplementation.

Vitamin A is important for vision, bone and muscle growth of young horses, reproduction, healthy skin, and immune response to infection. 

"When green, grass has plenty of carotene, which is the source of vitamin A," 
"But when you start feeding only hay, sometimes horses don't get enough A. Usually A isn't a vitamin of concern because it is added into almost all commercial horse feeds, and if a horse gets enough green grass it's not an issue at all."

Vitamin C is an antioxidant that helps boost the immune system, notes Crandell. Horses' bodies produce vitamin C naturally, but when stressed they might not make enough. 

"We're talking long-term stress," she adds. "Weaning a foal would be an example. In a study looking at blood levels of vitamin C in foals being weaned, the levels dropped during that stressful time."

Horses off their feed due to illness and/or stress can develop B vitamin deficiencies. The equine hindgut (the large intestine, which consists of the cecum and the colon) contains billions of microbes that digest forage and produce B vitamins, which have a variety of functions such as maintaining healthy skin and muscle, promoting cell growth and division, and enhancing the immune and nervous systems.

"If a horse isn't eating enough or if their hindgut is too acidic (due to too much grain and not enough forage) or if the horse has a fever or colic, the hindgut can become out of balance, which compromises production of B vitamins," Crandell explains. "But again, good-quality feeds have B vitamins included in the mix."

"People are feeding magnesium in therapeutic doses to calm a horse, and some say it reduces the thick, cresty neck and the risk of foundering in insulin resistant horses," 

A diet can be salt-deficient, causing metabolic issues, if a horse does not have access to salt. "Forages are low in sodium and chloride (together they make salt), and commercial feeds usually only add less than 0.5% salt to the feed," 

"Calcium and phosphorus need to be balanced in a 1-1 ratio for adult horses; growing horses need 2-1 (ratio, calcium to phosphorus),". "If that ratio is inverted you'll have definite bone problems, such as osteochondritis dissecans--a cartilage disorder characterized by the presence of large flaps of cartilage or loose cartilaginous bodies within a joint--and epiphysitis (inflammation of the ends of the long bones), in your young horse."

References

http://www.thehorse.com/articles/31995/common-nutrient-deficiencies-in-horses