I hear this question often in reference to all manner of weight: the rider, western saddlebags, horn bags, pack loads, etc. There is no simple answer. Just like humans, some horses will be able to comfortably carry more weight than others and each animal needs to be evaluated individually.
Factors to consider
For every ride you plan, you should take the following into account when determining each animal’s load size.
- Size and weight of animal
- Health and general state of the animal
- Conditioning and fitness of the animal
- Conformation of the animal
- Attitude of the animal
- Age of animal
- Size, fit and weight of the riding/pack saddle
- Ability of the riding/pack saddle to distribute weight across the animal’s back
- Weight of the rider or pack load
- Ability of the rider
- Design of the packs or horse saddlebags
- Distance of the ride
- Type of terrain
- Temperature and weather conditions
Many of these are self explanatory, but I do want to touch on some important points.
I can not stress enough that you have to know your animal and for every trip you need to evaluate at least the animal’s condition as well as the distance, temperature and terrain and base your load weight on those factors.
A long ride on uneven terrain at the height of summer requires animals in peak condition. An animal should also be given time to acclimate to a change in altitude. Humans are not the only ones who can suffer from altitude sickness. If your animal is not up to the task you are asking of him, you may be endangering not only his life, but yours as well.
To pack a heavy load, an animal needs to be in good health. This doesn't just refer to whether or not he has a cold, but whether his feet are in good condition and properly shod or trimmed, whether he has any bites or sores in spots where they could be irritated by the gear and whether he is well rested and prepared for the trip ahead. Good fitness means your animal should be regularly and well-exercised.
As examples of individual assessments, I once owned a tough, raw-boned mule named Henry. Henry only weighed about 1100 pounds, but he could pack a 250 pound load for 15 miles in hot weather and dance the whole way. However, I currently have a mule, Daisy, who is pushing 35 and would be retired if she didn’t pitch such a fit when she gets left behind. Daisy’s loads typically weigh in at maybe 15% of her body weight. We all walk a little slower to accommodate her and I keep her in mind when deciding how far we’ll go each day.
Percentage of body weight
When packers ask me how much weight they can safely pack on a horse or mule, I give them the basic rule of thumb of 20% of the animal's body weight. Once again, this depends on all of the factors in the list above.
The animal's conformation can be a factor in how well your pack load or horse saddlebags ride. For instance, a low withered animal will need to be packed carefully and evenly because even a minor difference from one side to the other can cause the saddle to constantly shift as you go down the trail. At best, this is an inconvenience causing you to constantly adjust. At worst, the saddle could slip completely and cause a wreck. In another example, a short-backed horse may not be able to carry large western saddlebags as they will sit uncomfortably too far back on the horse's rump.