Nutrient Deficiencies in Equine Diets

Horses, like all animals, have a range of nutrient requirements to meet their daily needs. 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-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.


Clinical Deficiencies

The easiest example is the deficiency of energy (calories) in the diet, the more severe the deficiency, the faster the horse will lose weight. If a horse is losing 100g per day, the loss will take some time to be visible. Over the course of 6 months, the horse would lose 18 kilograms or about a full body condition score. Over the course of a year, the horse would be almost 40 kg underweight or goes from a body condition score of 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, which has adequate energy content, but is short in amino acids or other nutrients.

Sub-Clinical Deficiencies

Mineral and vitamin imbalances are the most common as they are not easily visualized.
  • Zinc excess - might produce symptoms consistent with copper deficiency such as improper cartilage development in foals.
  • Phosphorus excess - might create an inverted calcium to phosphorus ratio (less than 1:1) and can produce nutritional secondary hyperparathyroidism or “Big Head” disease as well as other bone issues.
  • Phosphorus deficiency - is most common in horses being fed poor quality pasture and hay, which results in weakened bones.
  • Selenium deficiency - might cause loss of mane and tail hairs and sloughing of the distal portion of the hoof. (hoof wall separation)
  • Vitamin A deficiency - may develop if dried; poor-quality roughage is fed for a prolonged period. If body stores of vitamin A are high, signs may not appear for several months and can result in impaired reproduction as well as brittle hooves.
  • Vitamin D deficiency - If sun-cured hay is consumed or the horse is exposed to sunlight, it is doubtful that a vitamin D deficiency will develop. Prolonged confinement of young horses offered only limited amounts of sun-cured hay may result in reduced bone calcification, stiff and swollen joints, stiffness of gait, irritability and reduced serum calcium and phosphorus.
In order to avoid these deficiencies we must first understand where these nutrients come from in the equine diet.


Horses by nature are grazers, they have a large hindgut specifically for digesting fibre; they are designed to eat for about 18 hours a day. In the wild, horses grazed on whatever shrubs, grass and weeds were available as they moved with their herds, these roughages had varying nutritional values. Today we have access to improved grass species and better pasture management practices that provide high quality pastures with increased nutrient availability and longevity. Hay is a direct reflection of the pasture nutrient availability and conditions at time of cutting. Pasture composition can still be highly variable. It is affected by plant species, soil fertility, water availability, climate, stage of growth and management practices. Applying fertilizer to the soil increases, yield, protein and nutrient content of forages subsequently grown. High temperatures generally increase the concentration of fibre in forages, which has led to a subsequent decline in digestibility. When there is a decrease in leaf proportion there is typically also a decrease in crude protein (CP) concentration and an increase in the concentration of non-digestible fibre. This can also lead to decreased palatability of forages. Temperate grasses tend to have higher digestibility than tropical grasses. Digestibility declines as forage matures, the most effective way of avoiding low digestibility is by maintaining forage in a young vegetative stage of growth by regular cutting or grazing. Nutrient Deficiencies in Equine Diets - Figure 1

Hay and pasture should always make up the majority of the horses diet. Access to adequate amounts of good quality pasture and hay can provide horses with minimal nutritional requirements such as barren mares or non-exercising mature horses with most of their nutritional needs especially energy and protein. Pasture and hay however, are typically always deficient in several trace minerals such as copper, zinc and selenium so it is important to supplement these deficiencies. The diet illustrated in figure 1 may produce some sub-clinical issues over time. The diet in figure 2 shows one option of correcting these issues by adding a low intake vitamin and mineral concentrate such as HYGAIN® BALANCED® or HYGAIN® SPORTHORSE®. Feeding commercial feeds higher in energy and protein therefore only becomes necessary when horses do not have adequate access to good quality pasture and hay or there are increased demands on the horse, such as pregnancy, lactation, growth or performance.

Nutrient Deficiencies in Equine Diets - Figure 2

Commercial Feeds

Even if you are feeding a commercial feed as well as hay and pasture to your horse it does not necessarily mean you will avoid deficiencies. Following the feeding directions outlined on the product is critical in order to supply adequate nutrients from the feed. If the feed states a 3 kg feeding rate for a 500 kg horse and you only feed 1.5kg then you are halving all of the nutrients supplied by the product. This may be fine for the energy portion of the feed but it will also cut in half the amount of vitamins and mineral provided by the product. Just because a horse receives enough energy from a product and is in good body condition does not mean that his diet is adequately fortified.

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