Bones, joint cartilage, ligaments, and tendons make up your horse’s skeletal system, which provides structural support for your horse’s body.

Most horse owners pay close attention to the role of joints and soft tissue health in preserving soundness and comfort. But bone strength is just as critical for supporting mobility and performance in horses.

Bone is a dynamic tissue that responds to impact and loading forces. Research shows that exercise and nutrition influence bone density in horses.

This article will review bone development, remodelling, and adaptation to training. Keep reading to learn how to support equine bone health with proper feeding and training.

Bone Development in Horses

Skeletal growth occurs rapidly during the first two years of a horse’s life. Studies show the average thoroughbred reaches 98% of its mature height by 24 months. [2]

Horses evolved for early locomotion to elude predators. As a result, newborn foals have distal limb bones similar in size and structure to mature horses. [2]

However, the skeleton of newborn foals only contains 17% of the bone mineral content (BMC) of adults. BMC is the concentration of calcium and other minerals in bone. Mineral content is responsible for 70% of bone strength. [3]

While skeletal growth slows significantly by age two, maximum BMC is not reached until the horse is six years old. Bone is a dynamic tissue and BMC constantly changes throughout the horse’s life through remodelling. [3]

Bone Formation

The bone formation of the appendicular skeleton, including the limbs, occurs through endochondral ossification. This process transforms cartilage cells into bone cells and occurs primarily in utero before birth. [1][2]

Longitudinal growth after birth occurs at the physis, or growth plate. Cartilage cells remain in growth plates to allow the bones to continue to grow until the growth plate ossifies. Some foals can suffer from a developmental disease of the physis called physitis. [4]

Mature bone contains three types of cells and an extracellular matrix. This matrix has inorganic and organic components. Collagen comprises the organic portion, while crystalline mineral salts and calcium comprise the inorganic part. [5]

Types of Bone Cells

Bone cells include osteoblasts, osteoclasts, and osteocytes: [5]

  • Osteoblasts are responsible for hardening bone by laying down the extracellular matrix.
  • Osteoclasts break down old bone so osteoblasts can replace it with stronger bone.
  • Osteocytes maintain bone strength while modelling or remodelling occurs.

Bone Remodeling

Bone remodelling happens when minor damage occurs to mature bone due to aging or stress. Several hormones regulate this complex process. [6]

In a healthy horse, osteoclasts remove the old or damaged bone tissue and trigger the other cells to repair it. Osteoblasts rebuild bone by laying minerals and collagen over the area to strengthen it.  [6]

Scientists estimate that horses replace 5% of their total bone mass through remodeling each year. Bone tissue is in a weakened state during this process. Injuries can occur if the horse’s bones are subject to excessive loads that damage the bone quicker than it can be replaced. [6]

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How Exercise Affects Bone Strength

The remodelling process allows the bone to respond to the forces acting upon it. Exercise is one form of stress that can cause minor damage and trigger remodelling. The body responds to increased strain by replacing the damaged bone with stronger tissue. [7]

When horses are fed a well-balanced diet, exercise is the most significant factor affecting bone strength. Research shows free exercise and appropriate conditioning programs can increase bone density in horses of all ages, but it is especially important for young horses. [7]

Horse owners must be careful not to overload the skeletal system. Exercise only improves bone strength if horses have adequate rest periods to lay down stronger bone tissue. [8]

Impact and Loading

During motion, the horse’s stride has a contact phase with the ground, which can be further divided into the impact, loading, and break-over phases.

The impact phase occurs immediately when the hoof hits the ground. This contact causes rapid deceleration that sends a force through the limb. The hoof initially absorbs the force and transfers it to the bone and joints. [9]

Bones are under constant load while supporting the horse’s body weight. But the impact during exercise increases that load.

Hoof balance, conformation, and speed affect the impact and loading forces on the horse’s limb. The horse’s limbs must endure a force equal to three times his body weight at a gallop. The greater the load, the more bone remodeling takes place. [9]

Bone Adaptation

Adaptive responses to exercise can modify bone mineralization and density to decrease the risk of injury. Tolerable levels of microdamage trigger an adaptive response, but an inadequate repair can lead to damage accumulation. [8]

Balance is key when designing an exercise program for your horse. Bone requires rest periods for tissue repair to adapt to increased loads. [10]

Repetitive stress increases the risk of injuries when bone is repeatedly subjected to the same forces within a short period. [11]

Studies show short durations of high-speed exercise increase bone density more than repetitive workouts. Alternating more challenging efforts with lower-intensity exercise and rest periods allows the bone to remodel. [12]

Initial conditioning programs should begin with low-speed, long-distance exercise. This type of exercise develops cardiovascular fitness without overloading the skeletal system. Walking on different surfaces can also stimulate adaptive bone changes. [13]

Conditioning Young Horses

Exercise at a young age can significantly influence bone strength as the horse matures due to dynamic structural changes to the skeletal system.

Research supports the benefits of appropriate exercise programs for young horses and describes the negative effects of restricting free exercise in growing horses. [14]

One study found that weanlings raised in full or partial pasture turnout had less bone mineral loss than those confined to stalls. [15]

Another study observed that young horses subjected to additional imposed exercise as juveniles had stronger bones than horses raised on pasture turnout without extra training. [16]

This research suggests that pasture turnout and careful conditioning from a young age could reduce the risks of future injuries in performance horses. [14]

Bone Loss During Stall Rest

Sometimes, horses must be confined for extended periods due to an illness or injury. But research links stall rest to significant decreases in bone density. [17]

One study found that bone mineral content decreased by approximately 0.45% per week in highly conditioned horses on stall rest. Increased dietary calcium did not prevent bone loss during deconditioning. [17]

This research suggests that even relatively short periods of limited activity can predispose horses to injury when returning to work. Horse owners should slowly introduce exercise to their rehabilitation program to avoid maladaptive bone disorders.

Non-Adaptive Bone Remodelling

Skeletal injuries usually do not occur because of a sudden abnormal force on a healthy bone. Instead, many bone injuries result from the accumulation of chronic damage, fatiguing the bone over time to the point of failure.

The highest joint loads occur at the fetlock, the most common site of subchondral bone disorders. Subchondral bone refers to the bone tissue underlying the cartilage of a joint. [24]

Repetitive training without appropriate conditioning and rest can cause microcracks in the subchondral bone. Fatigue injuries occur when this microdamage accumulates faster than the horse can repair with remodelling.

Young horses in training are often susceptible to nonadaptive bone remodelling. Horses who are still growing experience bone remodelling from both growth and exercise. [24]

Sometimes, repetitive trauma can lead to bone edema visible on x-rays. Regular lameness exams by a veterinarian are the best way to catch signs of nonadaptive bone remodeling before issues occur.

Nutrition and Bone Health

Bone supplements cannot reverse the negative effects of confinement or overtraining on bone strength. But a balanced diet that provides adequate vitamins and minerals is just as important for bone health as exercise. [19]

Nutrition also plays a role in preventing developmental orthopedic diseases such as OCD in young horses. Diets that are deficient in minerals and too high in energy can affect bone formation during growth and lead to abnormalities with lifelong impacts. [18]

Proper nutrition can also decrease the likelihood of skeletal injury throughout the horse’s performance career. [19]

Protein

Collagen is a protein that makes up 30% of your horse’s bones. Collagen forms the matrix where osteoblasts deposit minerals to strengthen the bone, providing structure to resist mechanical forces. [20]

Amino acids are the building blocks of all proteins, including collagen. Horses must obtain essential amino acids from their diet because they cannot synthesize these compounds in their body.

Collagen is high in the amino acids glycine, proline, and lysine. Lysine is the most limiting amino acid in the equine diet. Horses can make glycine from threonine, another essential amino acid. [21]

One study found lysine and threonine supplementation increased bone mineral content in yearling horses. [21]

For horses on low-quality hay, an essential amino acid supplement such as Mad Barn’s Three Amigos can help support collagen production and bone health.

Minerals

Calcium and phosphorous are macrominerals that form the foundation of strong bones. These minerals make up 70% of the BMC.

An optimal dietary ratio of around 2:1 calcium to phosphorous is key for proper bone mineralization. Imbalances in the Ca:P ratio can lead to demineralization.  [2]

Copper and Zinc are trace minerals that are also crucial for bone formation. Copper is required by the enzyme responsible for forming the collagen matrix, while zinc plays a role in cartilage turnover. [2]

Research suggests horses supplemented with balanced trace minerals, including zinc and copper, have higher bone mineral content. Zinc and copper should be included in the diet at around a 3:1 ratio. [22]

Vitamins

Several vitamins are involved in bone formation and remodeling in the horse’s body.

Vitamin A supports the development of osteoblasts responsible for laying down new bone, while vitamin D is required for calcium absorption by bone. [2]

Some research suggests that feeding vitamin K supports the production of osteocalcin, a hormone that facilitates bone metabolism and mineralization. More research is needed to understand how vitamin K affects bone mineral density. [23]

The best way to support your horse’s bone health is to feed a complete equine vitamin and mineral supplement, such as Mad Barn’s Omneity. Omneity is formulated to balance most equine diets and will ensure that your horse doesn’t have nutrient deficiencies that contribute to poor bone strength.

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Summary

  • Bone is a dynamic tissue that adapts to progressive stress by remodeling areas of minor damage with stronger tissue.
  • Skeletal growth occurs rapidly during the first two years of your horse’s life, but bones don’t reach their maximum mineral content until horses are about six years old.
  • Exercise programs that cause minor damage to the bone with adequate time for repair increase bone density and reduce the risk of skeletal injury.
  • Pasture turnout and appropriate exercise promote strong bone development in young horses.
  • Repetitive stress on bones can lead to non-adaptive remodeling, while stall rest can cause mineral loss that weakens bones.
  • Balanced nutrition provides the essential nutrients necessary to build strong bones but can’t compensate for inadequate exercise.

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References

  1. Huntington, P. et al. Growth and development of Thoroughbred horses. Anim Product Sci. 2020.
  2. Rogers, C. et al. Growth and Bone Development in the Horse: When Is a Horse Skeletally Mature? Animals. 2021.
  3. Lawrence, L. et al. The mechanical properties of equine third metacarpals as affected by age. J Anim Sci. 1994.
  4. Strand, E. et al. Radiographic closure time of appendicular growth plates in the Icelandic horse. Acta Vet Scand. 2007.
  5. Florencio-Silva, R. et al. Biology of Bone Tissue: Structure, Function, and Factors That Influence Bone Cells. Biomed Res Int. 2015.
  6. Lanyon, L. Functional strain in bone tissue as an objective, and controlling stimulus for adaptive bone remodelling. J Biomech. 1987.
  7. Firth, E. The response of bone, articular cartilage and tendon to exercise in the horse. J Anat. 2006.
  8. Norrdin, R. et al. Subchondral Bone Failure in an Equine Model of Overload Arthrosis. Bone. 1998.
  9. Davies, Z. et al. Ground reaction forces of overground galloping in ridden Thoroughbred racehorses. J Exp Biol. 2019.
  10. Evans, D. Cardiovascular Adaptations to Exercise and Training. Vet Clin North Am Equine Pract. 1985.
  11. Morgan, R. et al. Incomplete longitudinal fractures and fatigue injury of the proximopalmarmedial aspect of the third metacarpal bone in 55 horses. Equine Vet J. 2011.
  12. Bramlage, L. Response of Bone Necessitated by High-Speed Exercise. AAEP Proceed. 2013.
  13. Gibbs, P. et al. Scientific Principles for Conditioning Race and Performance Horses. Prof Anim Sci. 1995.
  14. Logan, A. et al. Training Young Horses: The Science behind the Benefits. Animals. 2021.
  15. Bell, R. et al. Daily access to pasture turnout prevents loss of mineral in the third metacarpus of Arabian weanlings. J Anim Sci. 2001.
  16. Firth, E. et al. The effect of previous conditioning exercise on diaphyseal and metaphyseal bone to imposition and withdrawal of training in young Thoroughbred horses. Vet J. 2012.
  17. Porr, C. et al. Deconditioning reduces mineral content of the third metacarpal bone in horses. J Anim Sci.1998.
  18. Lepeule, J.et al.  Association of growth, feeding practices and exercise conditions with the prevalence of Developmental Orthopaedic Disease in limbs of French foals at weaning. Prev Vet Med. 2009.
  19. Nielsen, B. et al. Small changes in exercise, not nutrition, often result in measurable changes in bone. Compar Exerc Physiol. 2008.
  20. Carrin, S. et al. The role of collagen in bone strength. Osteoporos Int. 2006.
  21. Graham, P. et al. The effect of supplemental lysine and threonine on growth and development of yearling horses. J Anim Sci. 1994.
  22. Ott, E. et al. The Influence of Mineral Supplementation on Growth and Skeletal Development of Yearling Horses. J Anim Sci. 1989.
  23. Tetachi, T. et al. Plasma vitamin K concentration in horses supplemented with several vitamin K homologs. J Anim Sci. 2011.
  24. Martig, S. et al. Bone fatigue and its implications for injuries in racehorses. Equine Vet J. 2014.