" I found a number of articles, in a very superficial googling, including statements about the US Cavalry guidelines,
which backed up my memory that they too had also advised 20%
Bogus, as best I can find out. I've tried finding a US Cavalry publication with such a guideline and have never found it. I looked thru about a dozen that I had access to. A poster on another forum a few years back posted the results of his looking...he never found it either. He DID find it in a commercial publication that was supposed to be an abridgment of a Cavalry manual, but he couldn't find it in the actual manual itself.
However, as he noted, there WERE multiple guides telling the cavalry what sort of horses to buy and what sort of use they needed to be capable of - hence the 230-250 lbs load carried by a horse weighing 900-1100 lbs. Do the math, and you do NOT get under 20%.
That is also why, when the Army tried some endurance races, the Remount folks used a weight in that range:
"The second Cavalry Endurance Ride was held in 1920. The U.S. Remount Service, representing the Army, became much more involved in the ride this year. The Army wanted to increase the weight carried to 245 pounds and the Arabian owners agreed. The horses traveled sixty miles a day for five days with a minimum time of nine hours each day. The highest average points of any breed entered went to Arabians, although a grade Thoroughbred entered by the Army won first."
Welcome to Arabian Horses.org - Education
There are also manuals discussing what a trooper needed to carry with him on his horse...and the total load ran upwards of 100 lbs.
"When carrying 15 and 20% of their body weight, the horses showed relatively little indication of stress. It's when they were packing weights of 25% that physical signs changed markedly, and these became accentuated under 30% loads.
I've addressed this badly flawed study many times. They put the horses on 4 months of pasture rest before starting, so the horses were NOT in riding shape. They then used them for 45 minutes every 2 weeks, enduring they stayed out of riding shape. The blood work showed signs of increased stress at 30%. Equine massagers rated horses as more sensitive after riding at 25%, and thus the level at which no stress was caused was the 20% mark.
But they used horses who were deliberately kept out of shape, and still the blood work didn't support a 20% limit. It took humans to rub backs and find signs of stress at 25%. Given the lack of support in the bloodwork and the biased structure of the test, I don't trust the equine massagers to have been totally unbiased in their assessment.
Worse, it made no attempt to look at stress where it counts: THE LEGS. Horses don't get put down very often for back problems. It is usually the legs that deteriorate under heavy use.
However, when it comes to back stress, based on my own observations, how well a saddle fits and how well it distributes weight is a bigger factor in 'back stress' than rider weight. So is rider ability. My horses indicated more discomfort to their back ridden in a jump saddle than a much heavier western saddle. Wonder why?
They also showed more back stress when my 100 lb DIL first rode than when I did at 180. Hmmm....
Meanwhile, I'm still waiting to find a study that shows a 20% limit or an explanation for how it would work. Studies at the Tevis Cup found this:
"Rider weight independent of the animal BW had no effect on completion rate, or on overall placing. Among disqualified horses, rider weight had no effect on miles completed prior to elimination. This is in contrast to traditionally held beliefs, but agrees with previously published data collected at this same event (Garlinghouse and Burrill). Although work by Pagan and Hintz (1986) demonstrated that energy requirements increase with weight load, the relatively low intensity of sustained exercise during endurance competition may mitigate the substrate depletion and lactate accumulation observed in high intensity exercise. The results of this study would suggest that horses in good condition are capable of carrying relatively heavy loads, whether as rider weight or in their own body weight, over a 160-km course without the deleterious physiological effects seen in maximal exercise.
Body weight of the horse had an effect in that as body weight increased, failure due to lameness increased. Mean cannon bone circumference measurements of 19.25±.71 cm were similar to values of 18.83±.66 cm reported in Garlinghouse and Burrill. Circumference did not increase proportionately as body mass increased. These results suggest that increased body weight without a proportionate increase in the cross sectional area of the metacarpus increase the incidence of exercise-induced trauma and biomechanical failure.
The RW/BW for animals disqualified for metabolic failure was higher than those that completed the race. This would appear to support traditionally held beliefs that horses cannot successfully carry rider weights in excess of a given percentage of the horse BW, yet this is not supported by RW or BW results. There was also no effect of RW/BW on overall placing. Therefore, it would appear that the effect of RW/BW on metabolic failure is a function of decreasing CS in some animals, rather than an inability to carry heavier weights relative to BW.
The results of this study confirm that rider weight, either independent of, or relative to the animal BW is not a critical factor in predicting performance during a 160-km endurance competition. BW was also not a factor in horses disqualified for metabolic failure, but did have an effect on lameness."
If one thinks about it, it makes sense. The horse's joints are not stressed by the rider's weight, but by the total weight of horse and rider. A big, stocky horse adds its weight to the rider. That is why drafts are not really better at carrying a heavy person than smaller horses - they are already closer to the limits of their legs. And it is the LEGS that are most likely to go bad on a horse, not the back.
Further, increasing weight does increase metabolic stress. Think heat. But a stocky, heavy animal is less efficient at getting rid of heat. That is probably why big quarter horses don't do well in endurance racing. That is also why Siberian tigers are bigger than jungle tigers...retaining heat is a good thing if one lives in Siberia.
If you want to figure out what contributes to a long and useful life in a horse, the first thing to think about is what breaks down under heavy use. Why are horses likely to be put down? Leg problems cause a much bigger percentage of problems than back problems. Thus we need to look at what stresses the LEGS, not the back. We also need to use normal horses, not pasture puffs. And we need to try to eliminate the human element in the evaluation.
We also need to stop citing a mythical Army manual that, even if it did exist somewhere, contradicts what the US Cavalry did in buying, equipping and training horses. BTW - the guy who did more research on it found the British and German armies used a higher weigh requirement than the US.
"As with most things horse, there are too few actual hard & fast studies done, but and mostly my ideas on that note come from workshops I've done with bodyworkers, anatomists, chiro's etc, who's examinations of damages to horse's bodies and who's anecdotal studies on client's horses have led to this guideline being advised. I would call it ignorance presumption to say it's based on myth.
Sorry, but your anecdotal evidence doesn't match the anecdotal evidence I've seen on ranches. Too many old horses still being ridden long miles regularly, without signs of stress, by men who were way over a 20% limit.