Broodmare Retirement Age

[paisjpq]

Please say a prayer today for one of my favorite old mares, at 21 she is pregnant with her 13th foal. And it is suspected that she is hemorrhaging internally. As I write this she is on her way to Hagyard's, a trip which is likely only going to save the life of her foal.

My question is this...how old is too old? I know it has been debated regarding racing, but at what point does it become cruel to ask a mare to produce 'just one more foal'... time and time again older mares die foaling and I have to say I am tired of seeing it happen to the one's that I love...
older mares have lower rates of conception,
higher rates of fetal loss,
higher rates of complications such as retained placenta, placentitis, and uterine artery rupture.
Increased scarring of the uterus from multiple foalings can lead to poor placental development and attachment, which can in turn lead to problems with the fetus.
Many older mares have lower quality colostrum

and all of this doesn't even take into account that older mares cost more...the vet bills to get an old mare pregnant and keep her that way are far greater than those of a relatively young mare.

thoughts?