Join Date: Dec 2010
Location: southern Arizona
I have a short-backed gelding that used to carry 200 lb of rider plus 40 lb of saddle for 8-12 hours at a time (30%). He weighs around 800-830, so that would be 29% or higher. I weigh 175, and he has no trouble with me - but his back is very short.
My mare is around 880, so I'm at 24% with her. Another Arabian, so another short back - short enough that finding a saddle that isn't too long is a real challenge.
I had a mare who weighed in at 750-800, and when I first started riding her, it was a challenge for her to carry my weight...partly because it affected her balance, but some of it was a lack of muscle. I sold her after a couple of years, but she had gained weight and learned to carry it...that would have been around 800-850 lbs.
That mare was hurt by a too heavy rider...the guy who sold her to us (unbroken) got on her back for picture taking. His 300+ lbs on a 750 lb horse left her back sore for a month, but that was, with saddle, around 45% of her weight.
A western saddle will distribute the weight over more area. Sitting a trot puts more pressure on their back than posting, and trotting more than walking, so some of it depends on what you plan to do, how long, and how well they can ride.
I'd watch the horse when the rider gets on, and then let the horse walk some with the rider. If it hollows its back, have the rider get off...