Is AERC a reputable source of information? Another weight debate!
Interesting reading from the American Endurance Riders Conference, taken from their handbook, Chapter 3 selecting the endurance horse.
When it comes to breeds, itís obvious that as a group Arabians do the best. On the other hand, individuals from any breed can excel in endurance, and not all Arabians are suitable candidates. Arabians are usually easier than other breeds to keep fit once they are in shape. In fact one is looking for a type of horse, not a breed, I.e., a sound, efficient mover with staying power. What does seem to be true is a higher percentage of Arabians are the right type than most other breeds.
The most preferred age of a prospect is 3-8 years. If you like to start with a horse who has never been ridden and are willing to spend the two years for basic equitation training then 3-4 is a good age, otherwise look for the 5-8 year old. A younger horse may take many years before you will have any idea whether or not he is suitable for the sport, and an older horse is fine if you are well aware of his history. (Endurance horses often perform competitively well into their late teens.) This history should include regular exercise, preferably trail riding. It will normally take two to three years to have the horse in top fitness if they have not been exercised a lot before your purchase but while you are conditioning you may enter many endurance rides on horses that are this age when you purchase them, provided you have the self-restraint to ride these early rides as conditioning rides and not races. Many a good prospect has been ruined by racing too soon Remember, if your horse obviously has shown you that he is not a good endurance horse prospect, there are many others out there that will fill the bill.
There is no ideal size for an endurance horse. Ponies can do very well, as can horses of 16 hands or so. Many good endurance horses are between 14 and 15 hands. The size of the horse should be appropriate for the size of the rider, however. No one could expect a pony to be competitive carrying a 200 pound man. Small, lightweight riders obviously have an advantage in that they have a greater range of sizes from which to choose. (As a rule, the horse can carry up to 30% of his body weight, depending on his bone size, I.e., a 900 pound horse should be able to carry approximately 250 pounds on his back.)
I am interested a discussion on the bolded part there, so often we see the 20% rule taken as gospel, but here is a group who oversee a gruelling discipline saying that 30% should be the rule.
Not in any way is this a fat bashing or promoting thread, but hopefully a meaningful discussion of the 20% or 30% rule.