Factors to consider
For every ride you plan, you should take the following into account when determining each animal’s load size, whether it's your trail horse, your pack horse, your saddle mule or your pack mule:
Many of these are self explanatory, but I do want to touch on some important points.
Percentage of body weight
When packers ask me how much weight they can safely load on a their pack horse or pack mule, I give them the basic rule of thumb of 20% of the animal's body weight, depending on all of the factors in the list above. 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 pack animal should be regularly and well exercised.
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 temperature, distance and terrain of your ride 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.
As examples of individual assessments, I once owned a tough, raw-boned pack 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 pack 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.
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 trail saddle or pack saddle to constantly shift as you go down the trail. At best, this is an inconvenience causing you to constantly adjust. At worst, the trail saddle or pack saddle could slip completely and cause a wreck. In another example, a short-backed horse may not be able to carry large horse saddlebags as they will sit uncomfortably too far back on the horse's rump.
Live weight versus dead weight
Additionally, it is important to remember that live weight (i.e. a rider) rides differently than dead weight (i.e. a pack load of any kind) and the 20% rule doesn’t necessarily apply to live weight. A rider can move and shift in the saddle to compensate for rough terrain and can get off and walk. A good rider is also easier for a horse to carry than an inexperienced one. An experienced rider in a good fitting saddle on a fit horse could be fine on a long, tough ride, even if the combined weight of saddle and rider is more than 20% of the animal’s body weight.
Packing the load
Dead weight, on the other hand, does not have the ability to adjust to terrain changes and, therefore, must be carefully packed to stay put and be comfortable for the animal to carry regardless of conditions. Remember, gravity works. Once dead weight begins to slide off to one side, it has the tendency to keep going. This can upset your animal, cause soring or, even worse, cause a wreck.
Unfortunately, there are no hard and fast rules on how to pack a load. It depends upon what you are packing, your equipment, your animal and all of the other conditions listed above. However, there is one rule to always bear in mind…equal size, equal weight and equal weight distribution. If you follow this rule, you should generally have less trouble packing a load.
Saddles, both pack and riding saddles, need to fit well to be effective and not cause additional problems. A poor fitting saddle will not properly distribute weight across the horse's back. If the fit is particularly bad, it can cause sore muscles or even open wounds. Before loading any weight on your animal, be sure to double check the fit and condition of your saddle.
When loading saddlebags, the weight and ability of the rider should be factored with the horse's size and condition as well as with the fit of the saddle. An inexperienced rider can unknowingly throw the horse off balance and too much extra weight will make recovery more difficult. Additionally, poorly designed saddlebags can hang too low or constantly shift, which can irritate your horse and put extra strain on him.
No simple answer
There is never a black and white answer to the question “How much weight can my horse carry?” The answer always has to be found on an individual basis considering the factors mentioned above. As I said, most of these items are simply common sense, but so many of the "horror" stories that I have been told over the years could have easily been prevented if the people involved had just critically and honestly judged their animals against this list.