Well, the downsides would be the mares would be using calories to work instead of developing the foal. How would you feel working hard and doing something un natural while you were half way or more through a pregnancy? I have seen people use their mares as far as 7 months but quite frankly just about everything is over with by the end of October anyway. The big key here is to be able to keep them in shape on a regular basis so that means riding at least every other day, this is the way that you can keep from hurting them. Also remember their hormones change a lot during preg. I think that it is best to have something else to ride if you are going to breed a mare. Like I said I have done it but don't do it any more, once bred here they just stay that way unless they are recovering from an injury and then sometimes I never get back to riding them as was the case with Shot.