We typically think about supporting joints in our older athletes, those who have spent a career running or jumping and arthritis is starting to creep in. However, joint health begins at an early age, and Thoroughbreds and other horses that are bred and raised to grow quickly are at particular risk of developing several types of orthopedic diseases that can impact their racing career.
Developmental orthopedic diseases include a multitude of growth defects – including flexural and angular limb deformities and physitis (inflammation in the growth plate) but also juvenile osteoarthritis and osteochondrosis (lesions in the joint, articular cartilage). This latter condition can further develop to osteochondritis dessicans where parts of the cartilage or bone can either become loose or break off into the joint capsule.
The term ‘joint’ simply refers to a union between two (or more) bones. To provide cushion between bones, there is a layer of articular cartilage at the end of each bone, and a capsule surrounding the joint flush with viscous synovial fluid to provide additional cushion and decrease friction. Articular cartilage is made of hyaline cartilage, and is composed of water, an extracellular matrix of collagen and glycosaminoglycans, and chondrocytes (cartilage-forming cells). This covers the ends of the bones and ligaments stabilize the joint itself.
Synovial fluid is produced by the lining of the joint capsule and is made of hyaluronic acid and lubricin. The joint is vital to support the weight of the horse and concussion on the limbs as a horse is galloping or jumping. Without a healthy joint capsule, such activities would be very painful (as you could test by knocking your knuckles together).
Long bones develop through a process called endochondral ossificiation. This occurs during fetal development, wherein a cartilage model is ossified (calcium, phosphorus and other minerals are deposited). At birth, long bones continue to grow from the ends (at the epiphysis, or growth plate, between the shaft of the bone and the rounded ends) where new cartilage is formed and then replaced by bone. This process lengthens the bone at both ends until the growth plate closes at maturity.
Most articular cartilage is developed during embryogenesis (development in the womb), but postnatal development continues at a slow rate as additional extracelluar matrix is deposited. Cartilage is further remodeled due to forces upon it (as is bone) which is why young horses need to have access to paddocks for free exercise. However, cartilage has minimal repair capabilities, because chondrocytes are embedded in the cartilage matrix and can’t move to the site of damage, nor is there a blood supply within cartilage.
The development of bone and cartilage requires a fine balance between the provision of nutrients and rate of growth. Often our young horses are fed too much in terms of calories and/or protein and the growth rate exceeds the proper development of bone, and the additional weight gain of the animal can exceed the strength of the developing joints.
Rapid growth rates, as well as jagged growth rates (spurts and lags, often compounded by weaning or moving to a new facility) can increase the risk of a horse developing orthopedic diseases. Therefore, it is important for breeders and managers of young horses to monitor growth rates and adjust nutrient availability accordingly, particularly during that first year. Horses should reach 90% of their mature height and 80% of their mature weight during this time.
The key nutrients for adequate nutrition and bone/joint health include calories, amino acids (building blocks derived from protein digestion), minerals and vitamins. These nutrients should be provided for in a diet rich in forages (pasture and/or hay) as well as concentrates to top up nutrients not provided for in the forages.
Calories (energy) should not be overfed, and monitoring growth rate as well as fat deposition can help identify if needs are being met or exceeded.
Good quality protein sources should be offered to provide sufficient quantities of key amino acids, including lysine, threonine and methionine. Alfalfa hay (in limited amounts) is an excellent source of protein and calcium, and soybean meal (found in most commercial feeds for growing horses) provides an excellent amino acid profile to young horses.
Calcium and phosphorus are vital to building strong bone, and should be provided for in sufficient amounts as well as in a ratio of approximately 2:1. Copper supplementation has been shown to decrease the incidence of physitis ‒ inflammation of the growth plates in the long bones in growing foals ‒ and should be included in the diet. Excess zinc can decrease copper absorption, so the zinc: copper ratio should also be considered, and should be about 4:1, not exceeding 5:1.
Vitamin D is essential for calcium absorption, but really only needs to be added to the diet if horses do not have access to sunlight, as skin exposed to sunlight can synthesize vitamin D. Vitamins E and A are plentiful in pasture, or hay that is not too old.
Oral joint supplements containing glucosamine and chondroitin sulfate (components of glycosaminoglycans) as well as hyaluronic acid appear to have some benefit in older horses suffering from arthritis, and may also be beneficial for young growing horses as well. There is always some concern that orally-fed compounds are not absorbed or may not go directly to the joint, but most of these supplements are relatively inexpensive and likely won’t hurt if you want to be extra cautious.
Feeding management is just as important as the feed itself. Horses should be fed individually to ensure each horse is getting what they need. Horses that are still nursing should have access to creep feed to avoid the post-weaning slump. Feeds should be offered in multiple meals per day, such that each meal size is as small as possible. This can reduce glucose and insulin fluctuations, as altered glucose metabolism has also been implicated in affecting joint development.
Young horses should consume about 2% of their body weight as dry forage (hay or the equivalent amount of dry pasture). This can be a mix of legume and grass, but straight legume (such as alfalfa) might be too rich in some cases.
Lastly, it should be mentioned that some breeds and perhaps even some bloodlines of horses are predisposed to the development of orthopedic disorders. If you notice a mare or stallion tends to have problematic offspring, you may need to consider not using that horse for breeding in the future.
Good quality feeds, monitoring growth rate and working with a nutritionist are steps to set your growing horse – and their joints – in the right direction.