Foal limb soundness begins before a foal is born. It involves considerations of nature (genetic) and nurture (environment). Genetic factors include inherited conformation and behavioral traits from the stallion and mare. Environmental factors include physical space in the mare’s uterus, nutrition, training and disease prevention.

Choosing a mating that has a good chance of producing a structurally correct horse is important since conformation deformities predispose animals to unsoundness. Nutrition of the mare is especially important during the last three months of gestation, as the majority of the foal’s limb growth takes place during this time. Nutrition of the lactating mare and supplemental foal feeding affects the growth of the foal after birth. Imprint training should be done soon after birth as a preparation for hoof care that should begin during the second month of life.

Genetics
Sound legs start with the mating of a sound stallion to a sound mare. They each contribute one half of the whole. Some horses are said to be very prepotent, meaning they are capable of stamping their characteristics on their offspring no matter what the genetics of the other horse. Sometimes, we see exceptional “nicks” or good combinations that consistently produce good horses. In some cases outcrossing is desirable as it produces heterosis or hybrid vigor. The mule is an extreme example since it is usually hardier than either parent. Most horsemen realize that breeding is a “roll of the dice” but they do what they can to control some of the variables.

Unfortunately, many of our modern horse breeding programs are not focused on soundness. Instead, they are focused on beauty, color, disposition, speed, cow sense, early rapid growth, mature size, etc. Structural deformities are accepted where they should be cause for rejection. Structurally incorrect horses are then trained and put into use, eventually becoming unsound. A price must be paid for adopting the short term at the expense of the long term. Structurally incorrect horses may stay sound for a while, but very few unsound horses are structurally correct.

Heritability of skeletal structure in horses is one of the highest for any trait, estimated by some researchers as high as .65. This means that there is a 65 percent chance that crooked or sound legs are due to heredity and a 35 percent chance that they are due to environment. Selection of sound horses for mating is essential.

Prenatal (before birth) Environment
Foals grow to the size of the mother’s womb before birth. Some almost look like spiders when they are born, as their legs are so much more developed than their body size. The limbs complete much of their development in the womb and are largely a product of the nutrition received by the mare.

The cartilage in the leg rapidly proliferates during gestation, and its maturation to bone may be delayed in horses fed diets deficient in some minerals and vitamins. Calcium, Phosphorus, Potassium, Magnesium, Copper and Zinc are especially important minerals, as are Vitamins A and D.

Overfeeding of carbohydrates and protein can also cause problems. Excess protein is treated like excess carbohydrate by the body and may cause hormonal imbalance as well as nutritional diseases. Overgrowing in the confining uterine environment caused by a delayed foaling date causes some limb deformities.

Post Natal (after birth) Environment
Foals are naturally awkward when they are born. Their body size will grow faster than the length of their legs. The small bones of the knee or hock may not be completely formed at birth and are crushed by excessive exercise and body weight. Many times foal legs will straighten as the animal grows older and stronger.

Limb defects can be aggravated by many things. The stall should be safe and large enough that the foal won’t be injured by the mare. Exercise areas should be as safe as possible. Some foals have had a leg broken while in a stall or while exercising in a rough pasture. Other older horses in the same pen may purposely injure foals.

Fences that are sharp or not visible are especially dangerous. The fact that the foal has a short neck in relation to long legs may also aggravate deformities as he spreads his legs to eat off the ground. Standing with one front foot back and the other forward may also cause the development of club foot on the foot that is back if it becomes a habit.

The most important post natal consideration is the lactation of the mare. Too little or too much milk can cause limb development problems and lameness. Mares that are easy keepers and good milk producers have a higher incidence of foals with DOD.

Developmental orthopedic disease (nicknamed DOD) is the name given to various abnormalities of the limbs. These can have genetic or environmental origins. Horses bred to grow fast and mature rapidly are susceptible. Horses fed a high calorie, imbalanced ration are at great risk for epiphysitis – an inflammation of the growth plates of the long bones. Usually, this comes about from excessive grain and protein supplement consumption. However, overeating of high quality alfalfa hay may also produce it.

Types of Neo Natal (new born) Deformities
Foals with the knees or hocks turned in are called valgus. Foals with the knees or hocks turned out are called varus. When both left and right hock (or knee) joints are bent in the same direction, we say the foal is windswept.

A foal with a straight pastern and high-heeled foot is said to have contracted tendons or be club footed. A foal with the fetlock forward due to superficial tendon contracture is said to be knuckled over. Foals with the pastern down and the heels crushed with the toe up are said to have tendon laxity.