My trainer has a mare that she wants us to consider. My daughter would be using it for shows. Should I be concerned that the mare is likely to be in heat for some fraction of the shows?
There are products out there that calm there heat cycles, I heard people using a product called 'Mare Magic' with great success. There's also a shot a vet can give. I have three mares and two you can't tell and one will show, just not to other horses strangely, I have no problem with any and I let them roam free throughout the barn when I'm there. I've been camping/trail rides with my mares in-heat without issues.
Talk with the trainer to find out if the horse has been showed before, even bring up her heat cycles and how she acts during them. I personally haven't hand any issues with mares in heat(matter of fact I find them more lovey and cuddly like during that time), but I know people out the have.