A consensus remains elusive over the weight-carrying abilities of horses and donkeys, according to the authors of a just-published review.
The overloading of equines has become an important issue among veterinarians, trainers, riders, and welfare advocates. Carrying too much weight can have a range of negative effects — biomechanical, physiological, biochemical, and behavioral. It can cause gait imbalances and lameness.
“It is important to determine how to carefully quantify the load-carrying capacity of both ridden horses and working equids,” Syed Bukhari, Alan McElligott and Rebecca Parkes noted in the journal Animals.
“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.”
In their review, the trio with the City University of Hong Kong set out to summarise current knowledge of the load-carrying ability for horses and donkeys, and the ways used to determine the effect of loading on these equids.
The authors noted that there are about 112 million working equines in developing countries, many associated with brick kilns. Countries that have large working equid populations include China, Mexico, Ethiopia, Pakistan, and India.
“The importance of working equids is well known, but no research has quantified the amount that they are worth to the economies of these countries.
“Although their power has been superseded by machinery in many developed countries, they remain as relevant as technology in some regions of the world because animal power is cheaper and easier to maintain compared to motorized modern power.”
Overworking and overloading have been reported as the most important issue in working horses and donkeys. Overloading, they said, is defined as the weight with which gait rhythm is disrupted, leading to lameness and alteration of behavior.
It is especially a problem for donkeys. For working animals, carrying heavy loads is associated with increased income for their owners, leading to what the authors described as overloading by economic necessity.
For ridden animals used for both work and pleasure, increased human bodyweight is a potential welfare problem because people are getting heavier.
“The effect of a rider’s body weight on the health, performance, and welfare of horses is frequently debated in studies of horse-rider relationships,” the authors noted.
“The weight a horse can carry is important, and it depends upon a number of physical traits, including size, age, body condition score, body conformation, duration of work, third metacarpal bone circumference, type of work, and the intensity of the work to be performed.
“Overall, this subject is poorly studied,” they said.
Donkeys, they said, differ from horses in several biomechanical, physiological, biochemical, and behavioral respects, and they are often undervalued in the equine world.
The review team noted that physiological indicators such as heart rate, rectal temperature, respiratory rate, hematocrit, cost of energy, and muscle factors have been studied in relation to the loading capabilities of horses and donkeys.
However, the number of studies is not sufficient for detailed knowledge of the effects of load on physiological parameters.
Biomechanical assessments have also been employed across a range of studies, assessing gait stability and symmetry, and stride parameters.
They noted that different authors have employed different measures under a variety of conditions for assessing gait changes in response to loads.
“This lack of consistency means that, at present, it is difficult to formulate accurate calculations for weight carrying ability in horses or donkeys based on biomechanical changes.”
Turning to blood biochemistry, the review team observed: “A large number of biochemical and enzymatic factors such as blood lactate, creatinine kinase, malondialdehyde, nitrates, nitrites, glutathione, retinol, tocopherol, cholecalciferol, lactate dehydrogenase, and cortisol level have been studied in relation to the loading abilities of equids.
“However, the number of studies is not adequate for optimum coverage of the effect of load on biochemical parameters of equids.”
They said that behavioral indicators for the assessment of load-carrying capacity in equines are in their infancy.
Biochemical measures have also been employed, such as levels of the hormone cortisol as an indicator of stress. However, the authors noted that cortisol is also strongly linked with psychological and emotional states.
“Generally, it is considered that cortisol level increases with workload and stress, but in recent research on horses, it has been demonstrated that during short term exercise (a 5 minute and 20-second exercise test), an increase in the rider’s weight by 15% to 25% did not result in an increase in cortisol level.”
So, what of the practicalities of weight limits?
“At this stage of scientific research, in this particular field of equine science, answering the questions related to mounted load limitations for horses and especially donkeys is difficult,” they said.
“This is due to the use of different methods and procedures used in different studies under different conditions. Thus, a conclusive comparison for weight carrying ability is currently nearly impossible for equids.”
They said it had been shown that increased rider weight can have a negative effect on biomechanical, physiological, biochemical, and behavioral parameters in horses during work.
However, a more recent study showed that increasing mounted weight from 12% to 23% of the horse’s body weight did not result in significant changes in heart rate, gait symmetry, behavior, and cortisol level during low-intensity work.
“These conflicting recent results could be resolved by doing more detailed research on single horses with different riders with similar weight and exercise intensity to study the effect of riders beyond their weight.”
For donkeys, they said, little research has been done measuring load-associated changes in physiological and biochemical parameters.
The authors noted that, traditionally, the amount of “bone” was used to assess loading capacity, but more recently, gait symmetry has been identified as a potential method.
Assessment of stride parameters and gait kinematics can provide 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, they said.
“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.”
The review team concluded that further research is needed to develop evidence-based guidelines for maximum loading in equids, which could then be used in vulnerable communities to limit overloading and to improve the welfare of working animals.
Similarly, quantified load limits or indicators of overloading could be used by those working with sports and pleasure horses to limit overloading and improve welfare.
They said the body of work in this area has been growing over the last three decades in regard to horses, but less has been done on donkeys.
“Loading affects a large number of biomechanical, physiological, biochemical, and behavioral features of equines, and further research is required to advance our current understanding of these factors.”
Bukhari, S.S.U.H.; McElligott, A.G.; Parkes, R.S.V. Quantifying the Impact of Mounted Load Carrying on Equids: A Review. Animals 2021, 11, 1333. https://doi.org/10.3390/ani11051333