I think your vet is the best to weight up what your mare can handle
I’ve known horses to have foals til 20
But a vet check and opinion is best
Every horse is different
I know someone whose mare had her first at 18 or 19, but I'm not sure I'd want to do that with a maiden mare. My understanding is it doesn't matter as much if they've had foals when they're younger.
The older she is the greater your risk of losing her and leaving you with an orphan foal, which (leaving out how sad you would be to lose your mare) can be a complete nightmare.
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It really depends on the mare.
Your best bet is to get a good repro vet to check her out .
I’ve seen some very fit and well mares over 20 have nice foals.
In my experience I wouldn’t bother if mare is over 18 . I tried with my old girl purely because she has the most wonderful foals and some great old fashioned lines . She cost me thousands and just didn’t catch again . ( she had 2 foals when a bit younger )
My vet is very much against covering 18+ mares , he says it’s like covering your granny .
I also tried with another one on breeding loan , again it cost a fortune with nothing to show .
Breeding isn’t cheap , so I now only use younger mares
Their fertility drops fast after 12 unless they have been kept in foal but a mare has evolved to be in foal every year nd their endometrium stays younger if they are in foal every year so if its a mare who has not been in foal before you may well struggle to get them in foal after 14 and declining fertility every year - for brood mares it depends on the individual horse… some are very healthy and can go on having foals til 20 and some not so much… its like riding horses , some are out competing into their 20s , some have to retire earlier. there is no magic number
It depends on the mare’s health and breed as well. A nice fit pony mare I would be happier breeding a lot later in life than a big warmblood or TB made showing signs of age at 15.
I’ve seen a few early 20s mares foal easily and well, and saw it go wrong in the worst way with a lovely 10 year old.
As above, it varies with breed, because the breeds age differently.
The Highland pony stud next door to me lets hers foal until 20. (Also they are living a very natural life on a steep hillside with hundreds of acres to wander in so are very fit.)