I own a 10 year old 15.2hh Foxtrotter gelding with stout legs & big feet.  My riding consists of about 10 hours a month, spread over two or three weekends, on very easy trails.  I ride in a 22lb saddle.  I've been riding a little over 10 years, I'm not a couch potato, but I'm surely not in shape, and my weight has been shifting up and down with a steady overall increase.    This spring I've hit maximum density, 265 lbs.  Am I too heavy to ride this horse?

What's the opinion on my breaking out a 3 year old, currently 15hh Foxtrotter gelding this fall?

When deciding on whether a young horse is ready to be put under saddle I have been pointed to their knees by all I have ever been taught by, so how do you determine whether a youngsters back is ready for him to be under saddle? X-ray?

I have to disagree with the knee's not being an issue.  Try lifting something sitting down and then tell me if your knees are not involved! 

Horses rely primarily on their legs to support their back.  It is a culmination of ALL muscles and ligaments.

If you don't consider the knees then you lose your support.  If you put too much weight and too much stress too soon you are going to blow out the ligaments, tendons, the tendon sheaths, and get shin splints. 

At 3 years old the horse is a tweenie...between a youngster and a seasoned horse.  Lots of muscle work, coordination skills need to be done.   Size isn't the problem, it is the age, condition, and coordination of the horse.

I have seen some ten year olds in just as bad of condition muscle tone and coordination wise as a youngster, due to languishing in a pasture.  The problem is folks will let their horses sit all winter and then in the spring expect them to be ready to go.  We start round penning our horses at the beginning of spring to get their muscles used to being worked again, we also work on backing, and lots of trotting. 

You want good strong legs, strong hind quarters, and lots of backing and trotting excersises to strenghthen the back.  All this can be done from the ground. 

Get your horses in shape and you will find yourself getting in shape right alongside them!  I had gained over 25 pounds this winter, I have lost all of it and then some!  If you work your horses you will get in shape yourself.  As long as we stay away from the beer, snacks and other tempting morsels!

Whether your too heavy for your horse, YOU will be much healthier and a better rider if you can continue your weight lose.  Congratulation's on what you have lost and continued success!

As for the 3 y/o...yes, your too heavy.  He's still growing. 

I come from a CTR background and the rules don't allow anything younger than 4 to compete in the Novice class. They have to be 5 years old to compete in the Open. So I've always used that has my guidelines.

4-5 very easy rides on a 2 year old. Just get them used to some one on their back. Nothing longer 20-30 mintures on flat ground.

During the winter between 2 & 3 years old. I pony my coming 3 year olds along side my riding horse. Empty saddle, no weight.  Just get them used to crossing streams, mud puddles, crossing logs, bridges and other obsticles, seeing deer etc. These are usually 5-10 mile rides. Helps leg them up and get them in shape for training.

3 years olds I start riding IF their knees have tightened up. If they are still loose, we wait.  I'll put a 10-12 easy rides on my 3 years olds. No hills, no rough country.  If the horse is not blowing up, I'll let my daughters start riding them along side me on evening trail rides. Nothing faster than a trot, and rarely that. Mostly just long brisk walks. By end of summer they are doing all day rides and I take them elk & deer hunting in September & October. Here they get some serious hill climbing. But usually with a panier carry 100lbs of camp.

4 years old, I get serious about me riding them.

The general rule of weight bearing for the horse is:

They should not carry more than 1/5th of their body weight...in other words... your horse should weigh between 1325-1400#.

A modified low-carb diet would help you lose rapidly and gain lots of energy....Keep riding!!!stop eating things that are bad for you... and remember the time when you were at your ideal weight... it was too good then, wasn't it??? GOOD LUCK

I read an article once about a study that was done on weight and riding. 

From what I remember in general they mentioned it wasn't always the weight as much as the rider's riding skills.

They had a mechnical horse that measured the pressure on the back.  It was found that a lighter bad rider was worse on the back than a heavier good rider.

I do agree with most that you're probably too heavy for a young horse. 

My horse is my best walking partner.  I like to get off and walk during my rides and my horse just follows.  I take off weight during the summer just with the extra walking with my partner Sonny.