Optimal vs maximal growth
Maximal growth of the horse is different than optimal growth of the horse. Feeding a young horse so that it is growing at the maximum rate possible is undesirable because during the growth process bone mineralization, which is responsible for the strength of the bone, lags significantly behind bone lengthening (at 12 months of age the young horse could be expected to have reached approximately 90 - 95% of its mature height but only about 75% of its mature bone mineral content).
Ideally, the young horse should gain weight at a rate that its developing bones can easily support. However, overfeeding (especially energy) can cause a young horse to gain weight so fast that its bones do not have the structural strength to support its weight and/or the rapid weight gain can exacerbate other skeletal anomalies. Under these conditions, the incidence of developmental orthopedic disorders (DOD) and unsoundness increases.
This scenario can also occur during periods of uneven growth. For example if a horse which was underfed and growing slowly is switched to an adequate diet which allows it to grow quickly, the probability of DOD occurring is increased. Foals between the ages of 3 months and 9 months of age are at greatest risk for the incidence of DOD.
Ideally, the young horse should be fed so that it grows at a moderate, steady rate. Recommended rates of average daily gain for horses are found in the National Research Council's Nutrient Requirement for Horses publication. Recommended average daily gain values for horses of different mature body weights range from 0.28 - 0.39% and 0.15 - 0.21% of the horse's body weight for weanlings and yearlings, respectively.
Feeding a young horse so that it grows at a moderate rate does not compromise the eventual size of the horse. Consequently, you do not have to be concerned that feeding the weanling and yearling for a moderate rate of growth will result in a smaller horse at maturity. Monitoring the horse's body weight using a scale or a measuring tape are two ways to track growth over time. With a tape, measure the circumference of the horse's heart girth and the length from point of shoulder to point of buttock. The measurements and the following equation can be used to closely approximate the horse's weight.
lb body wt = [heart girth (in)2 X length (in)] /301 (for yearlings) or 280 (for weanlings).