A 2023 paper entitled “A review of Three Decades of Research Dedicated to Making Equine Bone Stronger: Implications for Horses and Humans” describes the experience of the author, Professor Brian Nielsen (Michigan State University) and his mission to decrease injuries in athletic horses.
Bone loss in Early Training Associated with Stabling
Radiographs were taken of the third metacarpus from 53 Quarter Horses during a study to examine differences in bone mineral content during training. The horses had a 9-week "education" period (longeing, driving and riding in a round pen, familiarisation with track/barriers) followed by an 8-week conditioning period (walk/trot/gallop/sprints) prior to racing. Bone mineral content was found to decrease during the first 8 weeks of the program remaining low through Week 14 before increasing by the conclusion of the study at Week 34. Most bone-related injuries occurred in this group between Week 8 and 17 - when bone was at its weakest and when horses were beginning to race. The fact that bone mass was lower when horses were starting fast work, than what it was at commencement of training was alarming.
There was an immediate concern that levels of calcium were not adequate in the diet of these horses to support bone turnover. To address this, the researchers placed a further 10 Quarter Horses into race training who were offered diets that met published requirements for calcium and other nutrients. Similar to the previous study, radiographs revealed a decrease in bone mineral content of the third metacarpus by Week 8 of training, prior to the initiation of speedwork. In the second half of the study, bone mass increased and was accompanied by greater calcium retention.
By moving horses from pasture into stables and having no fast exercise during the early part of training, it was hypothesised that the loss of bone may have been caused by the lack of loading on the skeleton. To test this theory, 16 Arabian yearlings previously housed together on pasture were randomly divided into two groups: half remained on pasture while the other half were moved into box-stall housing with one hour of walking daily. By Week 4, bone mineral content of the third metacarpus of box-stalled horses had decreased and remained low through the 20 week study. Even when horses were started under saddle and began race training after 12 weeks, no increase in bone mass was seen in the stalled horses during 8 weeks of slow work without speed. Blood markers suggested bone formation decreased and bone resorption increased in the confined horses, thus explaining the loss of bone. Horses kept in stalls had lower bone mass at the end of the five-month study than they did when they started it. Even horses in another study that had been previously conditioned, placed on a walker in two 30-minute bouts daily and fed twice the dietary calcium requirements still lost bone mass when boxed for 12 weeks.
The researchers tested the theory that allowing young horses access to pasture some of the time might offset the loss in bone mass. Seventeen Arabian weanlings were either housed on pasture; housed in stables or stabled for 12 hours and allowed access to pasture for 12 hours. After 8 weeks, greater increases in bone mass of the third metacarpal were observed in both groups allowed access to pasture as opposed to the group stabled. Thus, even partial turnout could prevent bone loss. At the end of the study, all weanlings were returned to pasture and radiographed at 12 months. All horses had similar bone mineral content of their third metacarpus suggesting short-term stall housing of young horses does not necessarily result in lower bone mass throughout life, assuming the return to pasture occurs while the horses are young and experiences relatively fast bone growth. It is unclear if the same holds true for mature horses.
Many skeletal injuries that develop are the result of bone being ill-prepared for high loads due to sedentary periods without loading. Future studies showed that pasture turn-out prevented bone loss associated with confinement and as little as one short sprint per week made a dramatic difference to bone strength. This work provides us with food for thought on how it might be best to manage yearlings, young horses in early training and spellers to promote bone health.