We don't have all of the answers when it comes to feeding young horses. A nutrition program that doesn't promote--and possibly helps prevent--developmental orthopedic disease (DOD), including osteochondritis dissecans (OCD), is critical at this stage. Researchers now are steering away from the belief that excess protein is a major culprit in DOD, excess digestible energy now might be a possible factor.

Properly feeding a growing horse is a complex task in which the owner must provide adequate water, minerals, salt, protein, and a host of other nutrients. The nutritional requirements usually can't be met by forage alone. Cereal grains and forages also might not provide the correct calcium:phosphorus ratio needed for proper bone development (more on this later).

Now, let's look at what is known about feeding the young, growing horse, and what you can do with your nutrition program to avoid the problems that can arise.

Start At The Beginning

"My recommendation is to make sure you feed the mare well in late pregnancy in terms of trace mineral supplementation," says Joe Pagan, PhD, president of Kentucky Equine Research (KER), a nutrition consulting company in Versailles, Ky. "If that was done, don't be real concerned about trace mineral supplementation of the foal until he is 90 days old. Start to feed the foal fairly small quantities of a well fortified 16% protein grain mix (about one pound of feed per month of age, i.e. four pounds for a four-month-old foal) about a month before you're going to wean him. I think there's a risk of overfeeding the suckling foal in an effort to try to ensure it has adequate mineral intake."

Jeff Pendleton, general manager for Hallway Feed Company, also emphasizes proper prenatal nutrition for the mare in her final months of pregnancy, especially when it comes to trace minerals, selenium, and vitamin E. Providing these nutrients during pregnancy ensures that the foal has an adequate supply at birth. Research has shown that feeding these nutrients to the young foal cannot compensate for poor nutrition of the mare during pregnancy. "If the mare was not supplemented properly, particularly in the last three months, no matter what you feed the foal, you can't make up for that lost ground," he says.

Growing Horses

Growth rate in the young horse can be manipulated with nutrition. A controversy that surrounds the issue comes from breeders who raise stock for the show or sale ring. In both cases, judges and buyers tend to better appreciate larger, more mature-looking youngsters. Halter horses need to display musculature and appropriate flesh to win classes, and sale horses need to stand out in the crowd to bring top dollar.

Thus, breeders are feeding horses at levels designed to optimize growth at an early stage, not for growth into an athletic adult. This excess growth comes at an age when bones and joints might not be ready to bear the amount of weight that can be (and often is) attained through an aggressive feeding program. On the other hand, growth can be stunted or delayed with inadequate energy intake or other deficiencies. Conservative and steady growth should be the goal.

Let's look at some numbers describing normal foal growth for a light breed. The average Thoroughbred newborn weighs between 100 and 120 pounds (45-54 kilograms). Average daily weight gain is highest during the first two to three months of life--during this period, a Thoroughbred foal might gain up to three pounds (1.4 kg) a day, or 90 pounds (41 kg) in a month! Between six and 12 months of age, average daily gain is more modest at 1.1 to 1.8 pounds (0.5 to 0.8 kg) per day. However, illness or stress can suppress growth, while overfeeding has the opposite effect. Genetics also play a role. Average weight for a six-month-old Thoroughbred is 550 pounds (248 kg); at 12 months it is 800 pounds (360 kg). The average horse will have reached 90% of its adult height and 65-70% of its mature weight at 12 months of age.

Horses won't grow any taller than their genetics allows when fed an energy-rich diet during their first year of life, but when overfed can reach their mature height sooner than their more conservatively fed companions (and tend to be overconditioned or fat.)

Many different factors are thought to contribute to DOD, including OCD. Common theories include genetic predisposition, poor nutrition, rapid growth, trauma, and excessive exercise. Although it is likely that all of these factors play some role, rapid growth and over-conditioning rate often are implicated in the development of OCD. Pagan has conducted studies on growth rate and body weight as they affect OCD lesions in specific joints in Thoroughbreds. "When you feed a foal moderately--not as many calories--the skeleton still grows, but the horses don't bulk up," he says.

While a horse is filling out, the growth plates (physes) at the ends of his long bones are producing new bone to increase height. Those growth plates produce cartilage, which later develops into bone (ossifies). The young horse is most vulnerable to bone and joint disease during this period of rapid growth, from weaning--usually at four to six months of age--until 12 months.

Tailor The Ration

As mentioned above, the foal should begin to receive supplemental feed about one month before weaning--about four pounds for an average size foal of 16% protein feed is a sensible starting point. The reasons for supplemental feed for the foal are two-fold. First is the issue of providing nutrients to the foal that might not be in adequate supply in the combination of mare's milk and pasture/hay. The 1989 National Research Council (NRC) text, Nutrient Requirements of Horses, does not give specific feeding recommendations for the suckling foal other than to say that supplemental feed prior to weaning might be desirable in foals nursing mares which are poor milkers. Recent research, both in the United States and Japan, has indicated that foals require supplemental feed to achieve growth rates desired by today's horse owners. These studies revealed that foals not only required supplemental energy, but they also required supplemental protein and minerals.

The second reason for feeding foals prior to weaning is to teach them to eat the feeds on which they will survive once they are weaned. Introduction of grain and hay will teach the foal to eat these feeds and help prevent post-weaning slumps in growth. Post-weaning slumps in growth often are followed by surges in growth once the weanling learns to eat. This slump in growth followed by rapid growth is thought to be a prime opportunity for the foals to get DOD.

The specific make-up of the foal's supplemental feed and the amount to be fed needs to be tailored to the circumstance. You need to evaluate body condition and growth rate and adjust feeding accordingly. These principles are applicable throughout a horse's life, but are especially important during the first 12 months. One way to keep track of growth rate is by weighing the horse regularly. You should also evaluate body condition--weanlings should maintain a thrifty appearance in which the ribs cannot be seen, but can be felt easily. Monitoring weight can be done by weighing the horse on scales, or using a weight tape.

"Something we do with our commercial, primarily Thoroughbred accounts, is monthly growth monitoring, with height and weight measurements," explains Pendleton. "Based on those hard numbers, we can make feed recommendations."

Nutrient composition of rations for any age horse will vary, depending upon the forage available to the horse. Generally, a grain concentrate appropriate for a weanling will contain about 14% crude protein. However, Pagan says that it would be unwise to generalize about a percentage of protein in the grain for a young foal. "In Central Kentucky, where it's a grass-based forage program, 14-16% protein (in the feed) is probably correct. But out on the West Coast, where they may be feeding alfalfa hay, that may be too high," he says. "You have to be sure you make those recommendations with a full knowledge of the forage program." Feed consultants advise that owners have hay analyzed to find out the amount of protein, fiber, energy, and minerals it provides.

Two essential amino acids, lysine and methionine, are vital components of the foal's protein requirements. Lysine and methionine are regarded as "first-limiting" amino acids for growth--meaning an inadequate supply could retard growth and development. "There's not going to be specific information in most instances about the concentration of specific amino acids in a horse feed. It's not required by law," Pagan says. However, soybean meal, a high-quality protein with a lot of lysine and methionine, is commonly used in the formulation of feeds for growing horses. For the most part, a reputable commercial feed that is specifically intended for young, growing horses will contain the right amounts of these amino acids.

Weanlings can be fed fat as a supplement that will supply energy for growth and provide essential fatty acids for hair and skin health. Vegetable oil (e.g. corn or soybean oil) and/or high fat stabilized rice bran are good sources of fat for young horses. Not all fat sources are created equal; some have a balance of other nutrients that's not in keeping with ideal nutrition, and the ration of other feeds must be adjusted to compensate. For example, rice bran, an excellent source of fat, B vitamins, vitamin E, and selenium, serves up an inverted calcium:phosphorus ratio of 1:18. However, some rice bran products do contain added calcium and deliver the correct balance of calcium and phosphorus.

Mineral Requirements

There are nutrients required in much smaller quantities that are as important in the horse's diet as energy and protein. An adequate supply of calcium, phosphorus, and the minerals zinc, copper, and manganese are crucial to normal skeletal development. These minerals also must be provided in the appropriate ratios--in particular, the calcium:phosphorus ratio is ideally 1.2-1.8:1. A diet with more phosphorus than calcium might result in decreased absorption of calcium (which is necessary for bone development, as it makes up about 35% of bone), and excessive phosphorus intake can cause skeletal malformation.

Minerals are necessary for several growth and metabolic functions (see "Nutrient Necessities for Weanlings" below for examples). One consideration when choosing a concentrate is the absorption of trace minerals from the feed by the digestive system. In feed manufacturing, there is a process in which trace minerals can be chelated (chemically bonded to an amino acid to improve uptake by the digestive system). Feed containing chelated trace minerals costs more than feed containing non-chelated mineral sources such as copper sulfate or zinc sulfate. If a feed contains a chelated mineral, it is listed in the ingredients as a proteinate, amino acid complex, or amino acid chelate.

"The whole idea of chelating is that these organically bound trace elements are purported to be more bio-available (absorbable) in the horse's gut," Pagan says.

Older Youngsters

A standard is to feed weanlings a 50:50 ratio of forage to grain. A growing horse is expected to eat the equivalent of 3.0-3.5% of his body weight daily. For the average 550-pound weanling (248 kg) which is expected to mature at 1,100 pounds (495 kg), that is roughly eight to nine pounds (3.7-4.0 kg) of hay and eight to nine pounds of grain per day. Remember, by the time your youngster reaches a year of age, he should look like a gangly teenager, not like a mature horse.

"The bottom line on feeding young, growing horses is balance," Pagan says. "Have a well-balanced diet with adequate minerals for proper skeletal development, and feed for conservative growth. Don't grow them too fast. Those are the key components."

How Big Will He Get?

Most light horse breeds reach 46% of their mature weight at six months of age, 66% at 12 months, and 81% at 18 months of age (from Feeding to Win II, published by Equine Research). The reference also notes that on average, light horse breeds reach 83% of their mature height at six months of age, 91% at 12 months, and 95% at 18 months. (See "Growing Up" in the January 2000 issue of The Horse; http://www.thehorse.com/ViewArticle.aspx?ID=238 for help estimating a foal’s adult size.)

Nutrient Necessities For Weanlings (moderate growth)

Mineral (grams) 4 mo. 6 mo. Function in Horse
Lysine 30 32 First-limiting amino acid for growth, needed for synthesis of proteins in all growing tissues
Calcium 34 29 Makes up 35% of bone structure, involved in muscle contractions and blood clotting
Phosphorus 19 16 Makes up 14-17% of skeleton, required for many energy transfer reactions and synthesis of certain proteins and nucleic acids
Magnesium 3.7 4 Activator of several enzymes
Potassium 11.3 12.7 Involved in maintenance of acid-base balance and osmotic (water) pressure in cells
Data from Nutrient Requirements Of Horses, 5th edition, 1989.
Published by the National Research Council.

Yearling Nutrition Study Underway

Laurie Lawrence, PhD, Professor of Animal Science at the University of Kentucky, is heading up a new study on yearling response to different concentrate feeds. With funding from Cooperative Research Farms (a group of feed cooperatives including Southern States, Land O'Lakes Farmland Feed, Cooperative Federee de Quebec, Co-op Atlantic, Federated Cooperatives Ltd., UCAAB, and Tennessee Farmers Cooperative), they will evaluate glucose and insulin responses of young horses to different types of concentrate feeds provided by Southern States. The study involves mostly Thoroughbreds and will continue through late winter, with results reported shortly thereafter.

"We're trying to get a handle on exactly how these young horses' bodies respond to various feeds," says Lawrence.

--Christy West


Briggs, Karen. Amazing Minerals. The Horse, March 1998, 71-79. http://www.thehorse.com/ViewArticle.aspx?ID=453.

Briggs, Karen. Eating For Two. The Horse, March 1999, 97-104. http://www.thehorse.com/ViewArticle.aspx?ID=288.

Briggs, Karen. Feeding Yearlings. The Horse, May 1998, 75-81. http://www.thehorse.com/ViewArticle.aspx?ID=479.

Briggs, Karen. "Vitamins: Diet Fundamentals". The Horse, February 1998, 67-72. http://www.thehorse.com/ViewArticle.aspx?ID=435.

By Bettina Cohen and Ray Geor, BVSc, PhD, Dipl. ACVIM

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