If you're like me, you've been seeing news of a study out of Devon, England published in The Journal of Veterinary Behaviour, on the "proper" ratio of rider weight to horse weight. According to its authors, the optimal ratio between a horse's and rider's weight is 10%. They also stated that 10-15% was satisfactory; that only after 15% did the weight become problematic. Unfortunately, most of the news stories quoted 10% as the only ratio which was not detrimental to the horse, not even mentioning the higher ratios which were deemed acceptable. And one horse health site (which I had thought to be very reliable), noted the higher percentages, but put the word "satisfactory" in quotation marks, inferring that the weights were perhaps not safe.
This is very troubling to me for several reasons. In the first place, a 10% ratio between a rider's weight and the horse's is completely unrealistic. Jockeys weigh about 10% of their mounts' weights in thoroughbred flat racing. Quarter horse jockeys (and I believe, jump jockeys) weigh slightly more. These men and women have to just about starve themselves in order to maintain their weight. So what kind of message is the news media sending out to the many young girls (some of them already troubled) who love to ride horses. Anorexia Nervosa is already a problem for many teens, and articles like these just add fuel to the fire.
Quite frankly, the way these stories have been reported seems to be nothing more than bashing overweight people. They have very little to do with the welfare of the horses. They're just sensationalized stories which get everyone all stirred up and give people who know nothing about horses a reason to point fingers and cry "abuse".
Now don't get me wrong. Weight does affect the horse. Weights are assigned to racehorses to "handicap" the race. In other words, the older, more successful horses are assigned more weight than the younger, less successful ones. This is done in order to even the chance of each horse being able to win. The higher weights slow the horses down, but they don't injure them. So if you're riding in a race, or if you're doing Grand Prix jumping, or have a barrel horse in the National Finals Rodeo, perhaps a 10% ratio would be a good number to aim for, but otherwise it's ridiculous.
And that's what's so troubling about this study. It does not differentiate between a high level performance horse and a horse used for recreational trail riding done at a walk. In the authors' own words, they did not take into account "age, breed, style of riding, or experience of the rider". There is also no mention of the horses' builds or their fitness levels- all of which are hugely important to the amount of weight they can safely carry. There is also no mention as to whether the horses' saddles fitted correctly. An ill-fitting saddle will magnify the effects of any weight the horse is bearing and is by far the most likely thing to cause a sore back.
So let's look at some of those other factors. The first consideration is rider ability. People who know how to ride well enough to stay in balance with their horse, and distribute their weight evenly between their seat and their stirrups will be much easier on a horse (no matter what they weigh) than riders who are unsteady in the saddle. There have been studies done with infrared thermal photography, documenting that a heavier well-balanced rider puts less stress on the horse's back than a lighter, unbalanced rider. This makes perfect sense if you stop and think about it. A person can carry a heavier weight in a well balanced backpack than they can in an unsecured sack thrown over one shoulder. The balance of the weight makes all the difference.
The next thing to consider is the horse's fitness level. A 15 hand horse who weighs 1200 lbs. because he's obese is not going to be able to carry any weight as easily as a 16 hand horse that weighs 1200 lbs from size and muscle weight, just like a 220 lb. Marine can carry weight much more easily than a 220 lb. sedentary office worker.
The age of the horse is another huge consideration. A young horse's bones have not finished growing, and his tendons, ligaments, and muscles are all more prone to injury. Again this is just common sense. A large child is still not as strong as a healthy adult.
The build of the horse also plays a part in how much weight a he can carry. Dr. Deb Bennet, Ph.D., who is an expert in the biomechanics of horses, says that horses who are wide across the loins are more able to carry weight, than their thinner counterparts. (Loins are the part of the horse's back located between their last rib and their croup.) So a stouter, wider horse is more able to carry weight than a taller, slimmer horse.
So, taking all that into consideration, what is a healthy ratio of weight? Another study was done at Ohio State University. This study found that mature horses who were trotted and cantered, carrying between 15 and 20% of their body weight showed relatively little indication of stress. At 25% they started showing marked increases in their stress levels, and these increases became even more accentuated after 30%. The day after the experiment, the horses carrying 25% of their weight displayed some soreness, while those carrying 30% showed even more. The study also found that horses with larger cannon bones and (in agreement with Dr. Bennet) wider loins could bear the most weight without distress. The authors of this study concluded that riders should not weigh more than 20% of their horses' weight, which I think is a much more realistic number. The U.S. Calvary recommended this same ratio in their Manual of Horse Management published in 1920. And since the calvalry had thousands of men who practically lived on their horses to study, I think their findings are probably the most reliable of all.