I would ask your vet. As long as she has had proper nutrients and at a healthy weight (not over or under weight), she should be fine. I bred a 14 yr old maiden mare and it took three times. Pretty sure she's expecting since she hasn't been coming into heat, but will be getting an ultrasound :)
If the mare is healthy and comes into heat I do not see a problem with her carrying a foal.
I have bred a 24 YO mare and we did have a 27 YO mare foal.
Having a foal is really not that stressful on a mare.
We the owners are the ones who try and make it so. Shalom
I get worried when people start talking about breeding maiden horses in their late teens or early twenties. A pregnancy can be really difficult on a horse particularly if that horse is not an "easy keeper". Its not so much the pregnancy but the nursing that can really suck the weight off a mare. There are also issues with older mares have arthritis based issues that the extra poundage of a foal can affect. Older mares have older eggs and a harder time conceiving and carrying to full term a foal. Just because a mare is in heat at an advanced age does not mean she should or could carry a foal. Our 32 year old mare would go into heat but would have never survived a pregnancy. No one had told her body and she went through the motions. Sadly no equine menopause appears to exist.
Maiden mares can have a harder time with conception and with the whole idea of a stallion can sort of blow their mind. This is particularly true if they are being pasture bred (I don't recommend that at any age due to the danger to the stud).
I would check with your vet. Its more about the condition of the mare than anything else. Some horses are just conformed in such a way that its easier for them to get a uterine infection. Their anus can be in a position where fecal contamination is a risk. This does not seem that bad but keep in mind a uterine infection will kill any foal and cause a problem for the mare. A breeding examination would allow you to know if this would be a potential issue.
My I ask why you are looking to breed your older mare?
With respect the question was about middle aged, rather than aged mare, so I would be assuming (and we all know where that gets us) that we are talking teenage rather than in her twenties?
I would be happy enough to get a mare in her teems checked out be a vet and if deemed to be breeding sound then to try her. Once they turn 20 then I would be more reluctant. As ever though so much depends on the mare, all are different so be guided by your own common sense and the vets advise.
Breeding a maiden mare at any age without a breeding soundness exam is a bad idea.
Breeding a maiden mare after 20, with or without a breeding soundess exam, is risky, and I would only undertake it if there was a really compelling reason - a mare of a extraordinary quality who carried a rare bloodline in her pedigree and breeding her was the only way to perpetuate it. Even then, the mare would have to get a thumbs up from the vet, and still be ovulating without chemical assistance.
Maura I have never had any complications with breeding an older mare past the age of 20.
A healthy sound mare can conceive and carry a foal with little difficulty well into their 20's.
I have bred a mare that was 24 successfully and she is now 29 and going strong.
We had a mare that was 28 when she foaled last she is now 32. That one was not planned and she did foal a very correct 3/4 arabian filly. I was visiting Israel at the time and the hands here thought she was too old to breed.
She fooled everyone
In the end it depends on the mare. Shalom