As people get heavier, the danger of overloading our horses looms larger.
But how can we calculate when too much weight poses a welfare risk? Karen Coumbe MRCVS and Anne Bondi, director of the Saddle Research Trust, investigate

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How much weight can a horse carry? It’s never going to be an easy question to answer, as there are many factors to consider when working out the appropriate weight for any one horse to support.

The handicapping system used in racing, where the top-rated horse is given the highest weight to carry, proves the importance of weight-carrying in relation to performance. The British Horseracing Authority employs a team of skilled handicappers whose job it is to study racehorse form and a long list of other variables. They assign specific weights as a handicap to put better horses at a disadvantage over less successful ones, thus working on the well-tested theory that the weight a horse carries affects the speed at which he can gallop. By adjusting this, performance is modified.

The same concept is relevant beyond racing and is not only applicable to horses travelling at speed. The greater the weight carried, the more effort a horse has to put in.

Slim pickings

There is limited scientific evidence available at present to help decide whether a rider is too heavy for a horse.

No straightforward formula provides appropriate weight ratios for horse/rider combinations, simply because horses are individuals, and so many complex variables are involved. The factors linked to how much weight a horse can carry include:

  • A healthy horse will be stronger and more able to carry heavier loads than one with questionable soundness.
  • Fitness and preparation of both horse and rider are crucial to prevent injury.
  • Type and conformation: a horse’s skeletal structure needs to be strong enough to carry extra weight. An Animal Health Trust study showed that thoroughbreds are more commonly seen with thoracolumbar pain associated with impinging spinous processes compared with warmbloods or crossbreds. This may relate to back length and the closeness of the spinous processes.
    Back length becomes important when a large person with a big seat requires a long saddle, which can be too long for a short-backed horse. A rigid saddle tree should not extend beyond the top of the last rib.
    Some conformational issues reduce a horse’s ability to carry weight. For example, a dipped or sway back is a weakness at high risk of strain.
  • Gait and posture are also affected by training and previous injury. Correct head and neck carriage with hind-limb engagement will influence weight-carrying ability, as the whole musculoskeletal system is interlinked.
    A ridden horse has to cope with the static weight of the rider as well as a shifting load when exercised, and will compensate by altering his gait. The effect of the load can increase the extension (hollowing) of the horse’s back, and this may contribute to the onset of kissing spines.
  • Age: if the horse’s musculoskeletal system is immature, such as a young racehorse, or if it is old and frail, such as an ageing pony with a growing child rider, it will find it harder to cope with greater loads and will be more at risk of injury. As with people, older horses generally have stiffer backs.
  • Condition: being overweight significantly reduces the efficiency of movement, whereas an underweight animal may lack strength and energy. A study of endurance horses found that a good condition score was a more important factor for performance than the weight of the rider.
  • Saddle fit and rider skill are critical for efficient load-carrying. If the saddle, rider or horse are asymmetric, it will cause imbalance and make the horse work harder to compensate.
    A rider who cannot follow the horse’s rhythm and sits awkwardly increases the likelihood of back pain, while a heavier, poor rider also compounds the effect of a badly fitting saddle.
    Saddle design should reduce peak pressures by providing as large and soft a bearing surface as possible. Horses change shape throughout the seasons and according to workload and nutrition. Therefore, saddle fit should be checked regularly.
  • Speed: a linear relationship exists between acceleration, mass (rider weight) and the forces exerted by the rider on the horse’s back under the saddle. It is a basic formula that the force is equal to the mass times acceleration, and so the effect of rider weight is likely to be greater at faster speeds compared with the walk. Riding style can alter these forces — the modern jockey position, for example, uncouples rider movements from a horse’s locomotion and allows him to move faster.
  • Type of work: high-impact work such as jumping or the sudden stops and turns seen in polo are harder to sustain with a heavy load, compared with the slow plod of a donkey. Additional weight of up to 15% has been shown to alter jumping technique significantly in horses and is likely to be more challenging with a bigger fence, with a larger drop, or with a rider who lands poorly.
    A well-balanced rider is better than dead weight, which is why the requirement for eventers to carry a minimum of 75kg, regardless of size, was abolished in 1998.
  • Duration and surface type: it is one thing to carry a heavy load for five minutes, another entirely to lug it for several hours — not to mention the obvious difference between working on a hard, dry surface or through deep mud.
    Similarly, going uphill requires harder work than on the flat, because the hind limbs have to carry more weight to provide greater propulsion.

What’s best for welfare?

A recent Saddle Research Trust workshop discussed the challenge of finding appropriate welfare and performance guidelines with the lack of supporting science. As a starting point it was suggested that we consider 15% as average for light work and that loads of 20% might compromise the welfare of the horse.

A 10% ratio was also suggested, but although less weight might help performance, it means a rider would need to weigh less than 50kg to ride an average 500kg thoroughbred. This is restrictive when considering that the average person weighs 70kg.

A survey of weight ratios in riding school clients supported this, showing a variation of between 14.2% and 16.6%. This fits well with the suggestion of 15% for average performance, but the study reviewed only 50 riders and not all were the optimum weight.

Another small study looked at the load-bearing capacity of six native Japanese ponies and how their gait altered when their loads increased up to the extreme 29% of their bodyweight. The ideal is to have horses comfortable to work effectively, and not be overwhelmed as beasts of burden.

The bigger picture

It makes sense that when horses carry a greater load in relation to their bodyweight, performance and safety are compromised and the risk of musculoskeletal damage is increased.

Tests have shown that heart rate and some blood test results increase significantly with an increasing load and changes in gait are also recorded.

Back pain is always a potential issue with ridden horses, and everything possible must be done to minimise this risk. The rider, saddle and their combined weight should be considered. Even so, it is wrong to discourage the heavier rider who is riding slowly and steadily in a well-fitting saddle. Such a rider may be better than an inept, lighter rider with a badly fitting saddle.

This is a situation of horses for courses: instead of relying on a formula it is better to look at the bigger picture — if it looks wrong, it probably is.

Ref: H&H 12 March, 2015