How Much Weight Can a Horse Carry?
Generally speaking most horses can carry 20% of their body weight before showing any signs of stress (elevated heart rate, and so forth). If we consider the average horse to be around 1,000 pounds this would mean that the rider and saddle should have a combined weight of no more than 200 pounds. A person riding western, for example, would then be considered too heavy for their horse if they weighed over 170 pounds, noting that the typical western saddle itself can weigh 30 pounds.
Consider the Horse Factor
The horse itself must be considered. You can have 2 different 1,000 pound horses, one of whom will show signs of stress after being ridden by a 170 pound rider, and one which wont. As well there are risk factors to the horse that wont show up in stress tests but may show up as arthritis in the joints years later, and the general risk of injury if a horse is consistently ridden by a rider that is too heavy for the horse.
|Lightweight jockeys racing on undeveloped 2 year olds.|
The horse's physical condition is important to note here as well. An underweight horse, or an overweight one, will already have health factors that put it at risk for stress related problems after being ridden. A horse that is not well conditioned (one that is seldom ridden) will also have problems with a heavier rider.
Horses with less "bone" and narrow loins are more likely to show signs of stress when ridden by an overweight rider. To compare how much bone a horse has a measurement around the middle of the cannon bone is taken. Two horses of the same weight may have different amounts of bone, for example compare the thick and solid legs of a Fjord to those of an Arabian.
Horses with long backs will suffer more from a rider that is too heavy.
The amount of time that an overweight rider rides the horse will also impact the horse and have a part in determining if they cause stress to the horse or put it at risk for long term concerns.
The Whole Picture
Just because a 250 pound cowboy rides a 900 pound cowpony doesn't make it right and should never be used to justify that is okay for an overweight person to ride a horse. Riders must consider the health and well being of their horse overall. Again, an important thing to realize is that some problems related to carrying an overweight rider might not show up until later in life, and may be the kind of things that shorten the horses career.