Horses, like all animals, have a range of nutrient requirements to meet their daily needs.  These are spelled out as minimum nutrient requirements in the Nutrient Requirements of Horses, 6^th Edition, published by the National Research Council of the National Academy of Science.

If these minimum requirements are not met, the horse may experience deficiency symptoms.  The severity of the deficiency symptoms may depend on the degree of the deficiency and the time period over which the deficiency exists. 

• A sub-clinicaldeficiency may be the result of a small deficiency over a period of time. 
• Subclinical deficiencies may also result in decreased immune response, decreased reproductive efficiency and decreased performance.
• A clinical deficiency is present when there are readily observed or measured symptoms.

Perhaps the easiest example is a deficiency of energy (Calories) in the diet.  The more severe the deficiency, the faster the horse will lose weight.  If a horse is losing a quarter of a pound per day, the loss will take some time to be visible. Over the course of 6 months, the horse would lose 45 pounds or about a full body condition score.  Over the course of a year, the horse is almost 100 lbs underweight or goes from a body condition score 5 to a 3.

If the horse is getting sufficient Calories, but is deficient in protein or essential amino acids, the body condition might appear OK, but the hair coat might get dull, the hoof quality might deteriorate and the muscle tone might be lost.  This is common for horses that are on pasture that has adequate energy content, but is short on amino acids or other nutrients.  If the diet is deficient in key fat soluble vitamins such as A, D and E, it might take longer for the deficiency symptoms to show up as the animal will use up stored vitamins first.  If there are mineral deficiencies or imbalances the symptoms might show up in either bone problems or reproductive problems.

Mineral imbalances can create deficiency symptoms as well.  A diet that contains a large excess of zinc might produce symptoms consistent with copper deficiency.  An excess of phosphorus, creating an inverted calcium to phosphorus ratio (less than 1:1), can produce nutritional secondary hyperparathyroidism or “Big Head” disease as well as other bone issues.

The diet below illustrates a diet that might produce some sub-clinical issues over time.  This is a fairly typical grass hay and oat diet and uses basic NRC requirements for an 1100 lb horse at light work.

Offering salt free choice would take care of the sodium (Na) deficiency, but there would still be some trace mineral and vitamin shortages in the diet.  The extra magnesium in the diet would not be expected to be a problem.

Using a ration balancer horse feed product would be one solution. Using a properly fortified commercial feed that contains meets all of the requirements when fed as directed would also be a good way to prevent the development of clinical or subclinical deficiency symptoms.