Equine musculoskeletal development and performance: impact of the production system and early training.
The welfare debate around horse racing appears to be focused on musculoskeletal injury and the racing of 2-year-olds. Much of this debate appears contrary to the evolutionary history of the horse as a cursorial animal and the capability of the equine musculoskeletal system to respond to the demands of race training. Epidemiological studies have reported that 2-year-old racehorses have a longer time period from entering training to the first race and a greater number of lost training days than older horses. However, this is, in part, due to the time taken to learn to train and the impact of dorsal metacarpal disease, which is due to loading of naïve as opposed to immature tissue. Across several racing jurisdictions and codes, it has been demonstrated that horses that train and race as 2-year-olds have longer, more successful, careers than those that start racing later in life. This positive trend has also been observed with horses starting in equestrian sport at an early age. The literature on the growth and development of the horse indicates that the musculoskeletal system is primed for activity and loading from an early age. Additional exercise for the young horse has a positive rather the negative effect, with many tissues having a sensitive period for 'priming' when the horse is a juvenile. This implies that under many modern management systems, the challenge to horse welfare is not 'too much exercise too soon' but 'too little too late'. The current limitation in our understanding is the lack of knowledge of what is the correct exercise dose to optimise the musculoskeletal system. Modern management systems invariably provide too little exercise, but is the exercise data from feral horses the 'gold standard', or more a reflection of what the horse is capable of if resources such as food and water are limited? Further research is required to refine our understanding of the optimal exercise levels required and development of greater precision in identifying the sensitive periods for priming the musculoskeletal system.