Regardless of the breed or discipline, a horse must have a good foundation on which to develop strong bones and joints. Nutrition of the growing horse has a great impact on this foundation.
Several nutrients are particularly important in bone growth and development of the horse. Supply and balance of these nutrients is key to optimizing growth and minimizing nutritionally associated orthopedic problems. Whether the foal is a Thoroughbred destined to be the next Triple Crown contender, a Saddlebred aiming to be the next World Grand Champion or a pony that is a dream come true for a child, supplying sufficient nutrients in the proper balance will help foals develop a strong foundation.
Energy and protein
Optimal energy and protein balance are needed to support growth. As the horse matures, the growth rate changes; therefore, energy and protein requirements will also change. Energy is essential to growth and development. The quality of protein in the diet is also important. Crude protein in the diet is a measure of nitrogen and not an indication of protein quality. Protein quality is determined by the amino acid composition of the diet and the digestibility of the amino acids. Thus, the amino acid composition of the diet, rather than the crude protein content, is important to the growing horse. A steady growth rate can be obtained by avoiding excesses or deficiencies in either energy or protein and will help to reduce the incidence or severity of developmental orthopedic diseases (DOD).
Excess dietary energy and protein
If certain nutrients in the diet (vitamins and minerals) cannot support an increase in growth rate due to excess energy and protein, it may result in physitis, weak or brittle bones, increased risk for bucked shins, osteochondritis dissecans (OCD) and flexural deformities. Excess energy will also result in a fat horse. Excess dietary protein alone has not been found to cause bone development problems in horses; however, in practical situations, excess protein usually means excess energy also. Distinguishing between getting fat (body condition) and growing (growth rate) is essential. Both body weight and body condition should be monitored at least monthly and the feeding program adjusted as needed.
Deficient dietary energy and protein
Feeding deficient protein and energy will decrease growth rate and may lead to compensatory growth later when sufficient nutrients are available. If insufficient energy and protein are provided in the winter months (due to lack of feeding, poor quality hay, etc.), the growth rate will slow. In the spring, when pastures are lush and rich in calories and protein, a rapid increase in growth rate occurs. This rapid increase in growth is compensatory growth. It usually occurs post-weaning and may predispose the growing horse to DOD. In addition to decreased growth, protein deficiency may result in decreased feed intake (and therefore possible deficiencies in other nutrients) and decreased protein digestibility. Energy deficiency will also slow growth and may result in DOD.
Minerals: Calcium and phosphorus
Besides energy and protein, several minerals are important for proper bone and cartilage formation and development. Bone is approximately 35 percent calcium (Ca) and 14 percent to 17 percent phosphorus (P). Deficiencies in calcium and/or phosphorus can cause cartilage thickening, decreased growth rate and decreased bone density. While meeting calcium and phosphorus requirements are critical, the ratio in the diet is equally important. The safest ratio range should be maintained between 1:1 and 4:1 (Ca:P). A ratio above 4:1 (excess Ca) may decrease the absorption of other minerals, including iron, magnesium, manganese, phosphorus and zinc, thereby causing DOD. A ratio below 1:1 (excess P) may result in poor bone development, problems with cartilage growth and decreased calcium absorption, possibly leading to chronic calcium deficiency and secondary hyperparathyroidism.
Minerals: Copper and zinc
Copper (Cu) and zinc (Zn) are also essential for proper bone growth. Copper is necessary for synthesis of connective tissue, while bone contains intermediate concentrations of zinc. Deficiencies in copper result in DOD, and deficient dietary zinc decreases growth rate. In general, most forages and grains are slightly below a horse's requirements in copper but have only half a horse's requirement of zinc. Knowledgeable feed manufacturers will fortify their feeds to meet copper and zinc requirements when fed under average feeding practices. In practical terms, a horse's diet should have a Zn:Cu ratio ranging from 3:1 to 5:1. Excess zinc (>500 parts per million dry matter) may interfere with absorption of calcium and phosphorus and proper copper utilization, resulting in physitis, lameness and stiffness.
Vitamins: A & D
Vitamins, particularly A and D, are essential for normal growth and development. Beta-carotene, a precursor of vitamin A, is high in green forages, such as pastures and well-cured hays. Horses must convert beta-carotene to vitamin A. Well-cured hays retain most vitamin A activity, but poor curing conditions (e.g., rain damage, long exposure to drying, etc.) will destroy beta-carotene.
Vitamin A has many functions, including bone remodeling. Among other symptoms, excesses in vitamin A (>7,000 International Units per pound of dry matter) may cause weak bones. Excessively high intake can result when multiple supplements containing substantial amounts of vitamin A are fed. Vitamin A deficiency is not likely for horses consuming adequate green forage. However, horses given old or poor-quality hay and/or with very little grazing time may need vitamin A supplementation. Most commercial feeds and supplements contain more than adequate amounts of this vitamin. Deficiency in vitamin A will result in poor growth, but it has not been directly shown to cause bone problems in horses.
Vitamin D promotes calcium and phosphorus absorption from the intestine, resorption of calcium from bone and reabsorption of calcium by the kidneys. Under sunlight, a substance in the skin is converted to a pre-vitamin form and eventually to an active form by the liver and kidneys, making deficiencies unlikely. Deficiencies may occur if horses are not exposed to direct sunlight and receive poorly cured hay. A deficiency could cause various bone abnormalities to develop. Excesses are also rare, but would be most common if certain members of the nightshade plant family are consumed. Symptoms of excess vitamin D intake would include bone abnormalities and calcification of blood vessels, the heart and other soft tissues.
The ultimate goal for growing horses is to achieve a steady growth rate and avoid orthopedic problems. This can be accomplished by supplying sufficient nutrients in the proper balance.
Seasonal changes in pasture quality and individual body condition make it necessary to adjust the feeding program accordingly.
If feed (grain) is reduced, mineral supplementation may be needed to make up for deficiencies.