Young Horse Development Part 2: Six Months to 1.5 Years

Young Horse Development Part 2: Six Months to 1.5 Years

Photo: Anne M. Eberhardt/The Horse

How you manage, feed, and care for your young horse during this transitional phase can affect his future performance and quality of life.

Large enough to do damage, but often untrained, stressed, and in fluctuating stages of growth, weanlings to long yearlings (roughly 6-18 months) are the gangly acne-and-hormone-plagued members of the equine world. As it is with the parents of teenagers, owners of young horses often seem bewildered about managing these metamorphosing charges. Yet, as with the adolescent human, developmental changes in the juvenile horse determine adult outcome.

Management of the horse's environment during this period sets the stage for musculoskeletal development, growth, and trainability. Adverse conditions such as gastric ulcers, developmental orthopedic disease (DOD), stereotypic behaviors (vices), parasitism, and respiratory disease can impair future performance and quality of life.

A juvenile horse's management needs depend on his owner's intended arena/discipline. For instance, a sport horse being prepared for a future eventing career will likely have a dramatically different early life than an Arabian that will be shown at halter. However, despite differences in purpose and varying growth rates and genetic conditions, certain environmental factors are common to all horses.

A horse's environment encompasses housing, nutrition, exercise, preventive care, travel, and socialization--both human and animal. As discussed in the first segment of this series in the February issue, once conception occurs and genetics are set, environment determines outcome. For the juvenile horse, the environmental transition to maturity begins with weaning.

Flying the Nest

In Part I of this series we addressed weaning as the end of early foal development. Here, we will examine weaning from a different angle--as a beginning. Weaning method impacts both physical and behavioral development. There are nearly as many weaning strategies as there are types of horses, and while there is logic to each strategy, not every one is equal.

Preferred weaning age varies greatly (anywhere from 2.5 to 6 months of age) among owners, but the time of weaning seems to be less critical than the method. In a 1997 study Warren et al.1 from the University of Florida found when weaning horses at 4.5 or 6 months that weaning age neither affected foals' bone density or wither height significantly, nor did it impact the formation of abnormal behaviors significantly. However, weaning method and weanling diet together can impact growth and behavior.

In a 2005 study Waters et al.2 evaluated the effect of weaning strategies on the development of abnormal behaviors. The researchers divided their subjects into the following weaning groups:

  • Box weaning (abrupt and in a stall, with complete separation from other foals);
  • Barn weaning (abrupt and kept with small groups of stall- or barn-confined foals);
  • Paddock weaning (abrupt and kept with a group of weanlings in field); and
  • Natural weaning (gradual weaning of foals by mares in a number of pasturing/stabling situations).

The researchers found that box-weaned foals were at a substantially higher risk for developing tracked behaviors (e.g., stall walking, cribbing, wood chewing, weaving) than foals weaned with any other method. This risk was two times higher than for paddock-weaned foals, and barn-weaned or stabled horses also were at an increased risk for these behaviors. Interestingly, they also found that foals born to dominant mares were more likely to develop behavioral problems than those born to low- or mid-ranked dams.

Owners might take these findings into consideration when determining the best weaning method for their foals, but they should generally base each weaning decision on a horse's health status, age, and temperament, as well as available facilities.


Waters and colleagues also examined the connection between diet and behavior in young horses and found that foals fed a fat and fiber diet (before, during, and after weaning) "appeared less distressed immediately after weaning and seemed calmer and more inquisitive during a range of temperament tests" than foals fed a starch and sugar diet.3 Waters et al. also found that weanlings fed concentrates post-weaning were four times more likely to begin cribbing than the foals fed a fat and fiber diet. At the University of California, Davis, School of Veterinary Medicine, Gary Magdesian, DVM, Dipl. ACVIM, ACVECC, ACVCP, advocates feeding young horses a mix of good-quality grass and alfalfa hay as soon as they're weaned or start eating solid food. This blend of forages provides an appropriate mineral balance for the growing horse and should also meet his protein requirements.

Feed intake should be measured by weight, and the bulk of the calories should come from good-quality forage. Generally, growing horses should consume roughly 3% of their body weight in dry matter daily. This might come from both high-quality mixed forage and a concentrate supplement (a pelleted ration balancer). Magdesian believes that complete pelleted diets are "better than grains as far as glycemic index (the measure of a food's effect on blood sugar), bone growth rate relative to soft tissue development, and possibly (avoiding) DOD." Pelleted diets formulated for the growing horse also will provide a better vitamin/mineral balance than grains, he notes.

According to Magdesian, high-grain (sugar/starch) diets might predispose weanlings to developmental orthopedic disease. While researchers haven't completely determined diet's impact on bone development, other studies substantiate the theory that feeds with high glycemic indexes might contribute to DOD. Studies have also indicated that young horses fed high-grain diets have higher insulin levels, leading to concerns that overfeeding these concentrates could lead to metabolic issues such as insulin resistance later in life.

While the perils of underfeeding are obvious to most people, it is also critical not to overfeed the growing horse. Rapid weight gain might place excessive stress on the developing and not yet fully mineralized bones of the young horse.


The dichotomy between the horse's natural range environment and the confinement housing required for maintaining a modern, domesticated horse poses a problem at any age. Beginning from weaning, housing particularly impacts a young horse's growth, social development, trainability, and overall well-being. Therefore, try to keep stall confinement of these young animals to a minimum.

Housing's corollary, exercise, also impacts development. Van den Hoogen et al. examined the effects of housing and exercise on joint health markers in young horses concluded that pasture exercise is best for the development of healthy cartilage that's resistant to injury, and other ¬unnatural exercise protocols (such as longeing) might present harmful long-term effects.4 However, they caution that "extrapolation of the in vitro (in the laboratory) data to the in vivo (in the live horse) situation should always be done with the utmost care."

Depending on your situation, true pasture weaning might not be practical. However, research results and clinical experiences indicate that natural exercise is better for young horses' developing bone. "If foals are allowed some free exercise, are ponied, or led lightly off of a 'quad' in an arena, rather than longed or circled in a round pen, then that is best for orthopedic development," says Magdesian. "Ideally, they would get some free turnout (in a controlled environment with good footing that has been inspected for potential sites of injury or entrapment)."

Growth Patterns

The feeding, housing, and exercise ¬strategies that owners establish during weaning should carry through a horse's long yearling period. As discussed in Part I of this series, the most rapid growth occurs in the first few months of a horse's life--by 60 days, a horse should grow to at least 75% of mature wither height. However, the 6- to 18-month period is still pivotal. By 18 months, the average light horse will reach 90% of his mature height and weight.

Anderson and McIlwraith revealed in their research5 that equine growth is proportional, with long bone growth correlating to wither height at all ages. Also, cannon and pastern bone measurements increase by only 5-7% from weaning until the horse is 3. This suggests that most lower limb growth is complete before the yearling phase; it also suggests that mature height can be predicted by cannon length. They also found that conformation shifts as the foal matures. For instance, realizing a foal can change from being back-at-the-knee as a weanling to over-at-the knee by age 3, or that hoof-pastern axis will shift as the horse grows, can help both owners and veterinarians assess a young horse's development more effectively.

Preventive Care

Owners of young horses might dread the phrase "vaccination series," but there is a reason it's so often used by veterinarians--and it's not that we enjoy jabbing sharp objects into half-broke, three-quarter-ton animals. Immunization triggers the immune system to respond to the presence of a specific foreign material (an antigen) and recognize this antigen as a threat. Just as the horse himself has much to learn, the immune system must be trained. Therefore, multiple exposures to the same vaccine are often necessary to fully prime the immune system against a particular disease.

With financial concerns, logistics of giving multiple injections to a young horse, media hype overinflating some disease risks while overlooking others, and overvaccination risks, it's easy to lose sight of the importance of developing a sensible vaccination strategy for the young horse. Although common equine vaccines with the exception of rabies are available over the counter, owners should work with their veterinarians to develop a vaccine program suited to their farm and geographic area. Magdesian explains, however, that concerns regarding overvaccination are valid. "I try to minimize vaccines to those that are absolutely necessary--tetanus, West Nile virus, rabies, Eastern/Western encephalitis, flu, and then other respiratory vaccines depending upon exposure," he says.

The American Association of Equine Practitioners identifies certain core vaccinations, including the five listed by Magdesian, in their vaccination guidelines.

Parasite control products also are widely available to the horse owner; however, as with vaccination, a deworming strategy is best developed in conjunction with one's veterinarian. This holds especially true with young horses, as they are more susceptible than adults to the depredations of internal parasites. But an ineffective deworming program can contribute to the development of drug-resistant parasites. Establish a comprehensive plan for monitoring fecal egg counts and strategically deworming young horses on your farm.

Dentistry is often considered a maintenance must for older horses. However, young horses' rapid dental transitions, as permanent teeth replace deciduous (shedding, temporary) baby teeth, necessitate frequent monitoring. A veterinarian or licensed veterinary dental provider should examine your young horse for malocclusions, sharp points, retained caps, and other abnormalities every six months.

Hoof care is often overlooked in young horses not yet in training. However, researchers have shown that hoof growth and balance impact skeletal development. A farrier should evaluate your foal's hoof balance from the early weeks on, and at minimum keep the hooves trimmed and balanced as the youngster needs it. This will allow issues such as mild club foot to be addressed early and can prevent strain caused by long toes or poor medial-lateral (side to side) balance.

This period is also a great time to teach the weanling to load in a trailer so that should you need to transport him for an emergency, loading will not be an issue.

Common Concerns

Developmental Orthopedic Disease As mentioned earlier, defects in bone and cartilage development in the growing horse are linked to diet, and they also seem to be more prevalent in rapidly growing horses. Balancing protein, sugar, vitamins, and minerals in a young horse's diet--along with managing growth rate--might reduce the risk of DOD.

Respiratory Disease Magdesian lists respiratory disease, along with DOD, as one of the top health concerns seen at UC Davis' Veterinary Medical Teaching ¬Hospital in horses of this age. Respiratory pathogens such as Rhodococcus equi are endemic (prevalent) on some properties. Additionally, young performance horses are ¬particularly susceptible to such highly contagious agents as equine influenza virus and the bacterium Streptococcus equi (strangles) when stressed from frequent stabling and traveling or when exposed to these pathogens on affected animals from other stables. Magdesian recommends disinfecting stalls at shows before use, not allowing people from other barns/the public to handle or pet horses, vaccinating, avoiding overcrowding, minimizing hauling distance (to reduce stress), and allowing horses to lower their heads while hauling (so they can help clear their airways).

Gastric Ulcers While gastric ulcers exist across the equine population, research has indicated that stressors such as weaning, isolation, hauling, and high-concentrate diets can increase ulcer formation risk in young horses. Providing free-choice forages and minimizing social stresses could reduce this risk, says Magdesian.

Heading into the Homestretch

Although it is easy to overlook the condition of a young horse not in work, pay close attention to weanlings' and long yearlings' nutritional and environmental requirements. At this critical stage, careful management can optimize future performance and trainability.


1. Warren LK, Lawrence LM, Griffin AS, et al. The effect of weaning age on foal growth and bone density, in Proceedings. Equine Nutr Physiol Soc 1997:65-70.

2. Waters AJ, Nicol CJ, and French NP. "Factors influencing the development of stereotypic and redirected behaviours in young horses: findings of a four year prospective epidemiological study. Equine Vet J (2002) 34 (6) 572-579.

3. Nicol CJ, Badnell-Waters AJ, Bice R, Kelland A, Wilson AD, Harris PA. The effects of diet and weaning method on the behaviour of young horses. Appl Anim Behav (December 2005) 95(3)205-221.

4. Van den Hoogen, Bianca M, van den Lest CHA, van Weeren PR, van Golde LMG, and Barneveld A. "Effect of exercise on the proteoglycan metabolism of articular cartilage in foals." Equine Vet J, Suppl. 31 (1999) 62-66.

5. Anderson TM and McIlwraith CW. "Longitudinal development of equine conformation from weanling to age 3 years in the TB." Equine Vet J (2004) 36 (7) 563-570.

About the Author

Christy Corp-Minamiji, DVM

Christy Corp-Minamiji, DVM, practices large animal medicine in Northern California, with particular interests in equine wound management and geriatric equine care. She and her husband have three children, and she writes fiction and creative nonfiction in her spare time.

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