I have owned 2 mares that I bred. After the foal was weaned we went on to compete them for many years. (Everything from beginner walk-trot to 3'3" hunter equitation) There were NO issues in doing so. Perhaps in some mares there is an issue but not in the situations I have encountered.

Buncha lies. Mares are just as good to ride after they foaled, but don't be cruel and ride her immediately after she was in labor, give her probably 5 to 7 months to start riding again.

It's complete bull.
Anyone I know that has bred has started riding the mare about a month after foaling. Obviously something extremely strenuous such as roping, gaming, etc isn't the best idea for a couple months, but just a bit of riding to get her back into shape (and her foal can follow and get exercise as well) isn't going to hurt her in the least.

Total bull! I have a wonderful mare that had a foal before I got her, and she is going through a re-training process and she is an absolutely amazing horse, physically and mentally. I think it's probably only if there were problems during or after pregnancy, or they have countered lameness or back problems.

Bull. The horse that gave me most of my confidence was a 13.2 hand pony who had a foal about 8 months before. I was a really bad rider at that time(all hand, no leg or seat) but she went anywhere and did everything you could ask for.

Also, my mom took lessons growing up on pregnant mares and mares who had foaled a couple of months before. Some were great and others were not, but they were all good!

Absolutely untrue! Of all the mares I've ridden, all have foaled several times. Of course you should wait until they've recovered from the birth, but after that, they're the same horse they were before.
Just try telling a human mother that she's no good after birth, and see how she takes it.

I agree with everyone it's a bunch of BS. We bred my friends mare and after weaning the foal, she was anxious to compete(barrels )

it really depends on the attitude of the mare, some are no good for riding after a few foals, usually after just one a horse will be fine. It just makes them more cranky and irritable. So if a horse was trained to ride when it was younger and was a great riding horse, then was turned into a broodmare (or a mare who is used only for breeding) she probably wont make a good riding horse.

ok, I see that everyone thinks its a bunch of bull, but let me make it clear that it really has to do with the number of times you breed her. yea, you breed her once she will most likely be fine with a few months of rest (light exercise/riding) I only meant that a horse that has had many foals probably wont be the best for riding, not just bred once or twice.

The mare that I ride most often for my lessons has had a foal. So they're definitely still good for riding.

I have a 13yr old that had a foal when she was 4. Since then she's run barrels and is now being trained for hunters

Yeah, pretty much....it's all bull. We've had OTTB mares that have foaled two or three times and they are wonderful to ride.

Its a bunch of bull. I had mares with foals and have had no problems.