Rider to horse weight ratio typically depends on the size and strength of the horse, and the type of work that the horse is expected to do. However, from a vet’s point of view how much weight is too much weight for a horse to bear? Are we exposing horses to orthopaedic problems or even pain by asking them to carry more weight than they can reasonably bear?

How to choose an appropriate weight limit.

When we're deciding appropriate weight limits for horses we need to consider how tall the horse is and the type of build as well as age. Two horses that are both 15 hands high will not necessarily have the same build and will therefore not necessarily be able to cope with the same level of weight. A racehorse could carry significantly less than a draft horse but also, an overweight horse cannot necessarily manage a heavier load. We also need to look at the rider. How competent are they? Will they be handling the horse elegantly or will the horse struggle to manage not only the weight but the low level of skill? A more skilled rider means that the horse won't need to expend as much energy due to the riders practised balance. We need to consider what we're asking the horse to do. Is it a gentle trot around an easy track or is the horse being asked to climb hills, to gallop or to exercise for long periods of time? It will also depend on the horse’s state of health, if they have already showed signs of orthopaedic problems then the load that they carry should be adjusted accordingly.

What are the current rules?

As a rule of thumb, in the UK a horse should not be asked to carry a rider that weighs any more than 10% of its body weight. Interestingly in America a rider is allowed to be 20% of the horse’s weight. The two scenarios are distinctly different with regard to what we're asking the horse to do. For a horse that weighs 500kg, the rider could weigh up to a relatively light 50kg in the UK but in the States, the rider is permitted to weigh up to 100kg which equates to 15 stones 10 pounds.

There isn't any solid guidance about the maximum weight that a horse should carry but riding schools tend to limit rider weight to somewhere between 76kg to 100kg. It has long been assumed amongst riders that the maximum weight should not exceed 20% but the Journal of Veterinary Behaviour indicates that anything over 15% puts a horse's health at risk. 

Prior to 2008 there was no limit as to how much donkeys working on seaside beaches should carry. Since 2008 a limit of 50.8 kg has been put in place. The limit was decided following consultation with the Donkey sanctuary. Although the 50.8kg seems relatively light, donkeys are quite a bit smaller than horses and usually don't weigh any more than 180kg. Therefore, if they're carrying the maximum weight, this could equate to 28% or more of their body weight. They are however only offering short, slow rides along the beach.

Load carrying horses working abroad in developing countries can be asked to carry a huge amount more, even up to as much as 70% of their body weight. Their load can include heavy items such as bricks or produce and they may be required to do a number of trips in one day. Charities are doing their best to maintain the wellbeing of horses and donkeys in this scenario.

What health impact can a heavy rider have on a horse? 

A recent study suggests that up to a third of riders may be too heavy for their horse and this can lead to a number of health problems including pain, behavioural problems and lameness. The risk of biomechanical failure increases if a horse is overloaded. The statistic is unsurprising because the weight of an average human continues to rise. 

The condition and overall health of a horse has a significant impact on how much weight it can reasonably carry. In particular the bone density of the horse and width of its loin seem to be most significant. A study in Iceland found that the more muscle or fat a horse carries on its back, the greater weight it can withstand from its rider, and can even carry up to 30% before there is any evidence of damaged muscle. This conclusion was reached by looking at the horse's creatinine kinase (ck) and aspartame amino transferase (AST) levels after exercise with a rider 30% of their body weight.

Where do we go from here?

The take home message from this is that giving a horse too much weight to carry can lead to medical conditions and welfare issues. This isn't a surprising conclusion. I'd does seem odd however that there are no official guidelines to help establish some common ground. It would be useful to have some clear figures that vets could communicate to owners.

The good news is that there is plenty of dialogue amongst horse owners about appropriate weight of rider, which is encouraging. As vets, the onus is on us to ensure the welfare of the animal, but it is tricky to balance this with avoiding offence to the clients and owner. The fact that the horses condition and discipline as well as the rider's ability and size have to be taken into consideration makes it very tricky to define a rule as these factors are not easily quantifiable. However, an absolute upper limit of 30% of the horse's body weight seems reasonable. Communication with the client is key and making owners aware of these considerations is a good start to ensuring equine wellbeing.