Editor’s Note: This is an excerpt from Understanding Equine Preventive Medicine by Bradford G. Bentz, VMD.

Feeding of young, growing horses requires a higher amount of digestible energy and specific attention to certain nutrients.  Young horses frequently need more protein, calcium, phosphorus, zinc, and copper than that available in most grains and forages. Inadequate dietary protein intake in young horses can reduce growth rates. Slower growth rates reduce the need for some of these nutrients, but the mature horse ultimately may be smaller if growth rate is significantly slowed.  As normal or faster-than-normal growth rates are reached, a deficiency of one of the aforementioned minerals becomes more likely (calcium, phosphorus, zinc, and copper). If these deficiencies are not prevented, rapid growth and mineral deficiencies may lead to developmental orthopedic diseases such as osteochondritis dissecans (OCD), wobbler’s syndrome, or other bone-related problems. Fast growth rates do not lead to increases in mature size.

Horse owners commonly make the mistake of oversupplementing minerals, which can cause problems as well. Unlimited access to trace-mineralized salt high in copper and zinc usually meets the demand for these minerals. In areas of selenium deficiency, selenium in the mineral block may be worthwhile, but over-supplementation of selenium should be carefully avoided. Trace-mineralized salt does not contain calcium and phosphorus. Generally speaking, only if and when the proper amounts of calcium and phosphorus are in the grain mix or in the forage is a growing horse likely to receive the proper amounts of these minerals.

Grain feeding is often necessary to complete the energy demands of the growing horse and to reach the maximum mature size. However, dietary caloric intake must be avoided for the previously outlined reasons.  A summary of suggested energy intake and feed content for growing horses is presented in tables 1.1, 1.2, and 1.3 on pages 56 and 57 of Understanding Equine Preventive Medicine.  In addition to feeding adequate levels of protein and minerals to make up for the amounts not supplied by the forage alone, use grain mixes high in the amino acid lysine.  Inadequate levels of this amino acid may also reduce growth rates.  Soybean meal and canola meal are protein sources that are high in lysine and may be added to mixes insufficient in this amino acid