Quantifying the Impact of Mounted Load Carrying on Equids: A Review

Simple Summary The overloading of equids has become an important issue among veterinarians, trainers, riders, and welfare advocates. Increased weight carrying may have negative effects on biomechanical, physiological, biochemical, and behavioral parameters of equids during exercise, including causing gait asymmetry or lameness. It is important to determine how to carefully quantify the load-carrying capacity of both ridden horses and working equids. There are many options to assess the effect of loading on an animal's body, but these have been inconsistently applied, making it difficult to reach a consensus, even for horses. This review summarises current knowledge of the load-carrying ability for horses and donkeys and the different parameters used to determine the effect of loading on these equids. Further research is needed to develop evidence-based guidelines for maximum loading in equids. Quantified loading limits or indicators of overloading could be used by stakeholders working with sports and pleasure horses and working equids to limit overloading and to improve the welfare of these animals. There are approximately 112 million working equids in developing countries, many of which are associated with brick kilns. Brick kilns and overloading are associated with welfare problems in working equids. Understanding equids' abilities and influencing factors are important for both effective performance and welfare. Traditionally, measurement of the amount of 'bone' was used, and more recently, gait symmetry has been identified as a potential marker for loading capacity. Assessment of stride parameters and gait kinematics provides insights into adaptations to loading and may help determine cut-off loads. Physiological factors such as the ability to regain normal heart rates shortly after work is an important tool for equine fitness assessment and a more accurate measure of load-carrying capacity than absolute heart rate. Oxidative stress, plasma lactate, and serum creatine kinase activity are reliable biochemical indicators of loading ability. For monitoring stress, salivary cortisol is superior to serum cortisol level for assessment of hypothalamus-pituitary-adrenal axis and is related to eye temperatures, but this has yet to be interpreted in terms of load-carrying ability in equids. Further research is needed to standardize the evidence-based load-carrying capacity of working horses and donkeys.