Ask the Vet: Reproduction

Courtesy of AAEP

Question: Is it true you can breed (or inseminate) a small mare to a much bigger stallion? Some say it is dangerous and others say the size of the mare’s uterus will limit the size of the foal to a safe delivery size. If there is a limit in difference of size what is that limit? .

Answer: There was a very interesting study done by Dr. Ginther years back, where he put “large” mare embryos into pony mares and vice versa. There was not any appreciable amount of problems with those mares foaling on either side. They then followed the foals out to their 3-year-old years and measured bone sizes, wither height, etc. There were some small changes, but the author felt that they were insignificant in the grand scheme. There are not any “rules” per say on sizes or limits. I would say that common sense would prevail in choices of mare size (probably wouldn’t put a draft embryo in a mini!) but it shouldn’t be a major problem as long as they are standard sized mares. I have bred many standard Quarter horse sized mares to Friesian studs and not had issues with those foals.


Question: My new broodmare, I hope to breed, has very large mammary glands from past breedings. Will this be a problem for a new born foal?

Answer: Sounds like you have a nice broodmare on your hands! Previous pregnancies can alter the maiden mare’s mammary glands over the years, causing them to stay in a somewhat enlarged state. As long as she doesn’t have signs of mastitis or infection of the mammary glands, such as heat, pain, or foul discharge, she should be just fine. Happy breeding season!


Question: My 10-year-old mare has MILK and as far as I know is NOT pregnant…she doesn’t look pregnant either. She has had two foals before I purchased her 6 years ago. Should I be concerned?

Answer: There are several reasons your mare could have discharge from her mammary glands, including pregnancy, mastitis (infection in the gland), or some produce fluid for unknown reasons that are not harmful to the mare. If the mare is at your place with NO intact males, including young colts around in the last year, then pregnancy is probably low on the list. Mastitis can look as simple as just an increase in size of the mammary gland with some fluid discharge all the way to highly increased in size, hot, painful and include foul discharge. Although some mares can normally have fluid discharge without disease or pregnancy, a simple check-up with your veterinarian can rule out these reasons and give you an answer to let you rest easy about your mare.