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, 6th 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 signs. The severity of the deficiency signss may depend on the degree of the deficiency and the time period over which the deficiency exists:

• A sub-clinical deficiency 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 six months, the horse would lose 45 pounds. Over the course of a year, the horse becomes almost 100 pounds 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 okay, but the hair coat might get dull, hoof quality might deteriorate, and 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. Mineral deficiency signs might show as either bone problems or reproductive problems