Originally Posted by NittanyEquestrian
That depends on if she has ever foaled before. A mare that has foaled out prior tends to have a much easier time becoming pregnant and maintaining a pregnancy even into their 20s than a maiden mare. The older they are as a maiden the more problems you should expect. 16 in horse years is like the 40-60 range in human years. So some 16 year old horses foal fine, some are already starting to get irregular heats and their reproductive systems are starting to slow down it is very individualized and many factors come into play besides age like health, fitness and genetics. A foal from a 16 year old mare is going to be more expensive than an under 10 mare and a foal from a 20 year old mare will be even more expensive. You will run into irregular heats, decreased fertility, decreased chance of sustaining a pregnancy and possible morphological problems of the reproductive tract that will make breeding and maintaining a pregnancy in an older mare difficult. That's not to say that it can't be done but breeding anything over 15 especially a maiden is going to increase your chances of injuring the mare, having problems with the pregnancy and getting a foal with physical or mental problems either from the pregnancy itself or harder labor/birthing problems. If you do decide to breed then I would consult with a qualified veterinarian and/or a local breeding expert for your horse's breed and make a wise and informed choice. Internet searches are good as well but make sure you get information from reputable and sourced sites as anyone can post anything online. Good luck!
Very well said! Especially performance mares this is applicable. For some reason they have a higher incidence of endometriosis so this may be another obstacle to breeding the mare already. I definitely second the vet visit and have a full exam done on her before breeding.