Wednesday, February 7, 2018

Old Data and New Research On Gestation Length: KER Article

A recent article in the Kentucky Equine Research Equinews library of articles points out new research on what the "normal" length of gestation is for mares. I put normal in quotes because - as they mention in paragraph 1 - the foal will only come when it's ready! Mares are not known for reading the same book you did.

The point of the article is that our expectation of normal - 340 days - was determined over 50 years ago. Only with Thoroughbreds. And only in northern European countries. But pregnancy lengths are different all the time, even for the same mare bred to the same stallion with other obvious factors the same. According to Kathy St. Martin of Avalon Equine and Equine Reproduction, "I've bred mares to the same stallion, year after year after year and had completely different gestational length. I had one mare some years ago that went 367 days, despite having foaled at 332 the previous year when bred to the same stallion, at the same time of year. One can only assume that for whatever reason, things cooked slower.

In some ways this is an example of the research finally catching up with what the breeders in the barn already knew. But it should be helpful to many breeders - especially novices - to know that the official word is no longer that mares "should" foal at 340 days. 

It would be simpler for humans if a horse's gestation period were more predictable - the rest of the article talks about how knowing what the gestation "should" be will help determine whether a foal is truly premature or not, and how that's important to know - for example. But it's not clear how that could ever be possible, since the reality is that there is a whole range of normal. 

If you have an opinion, please leave a comment.

To read the KER article, click here.

Click here to read a related article by Jos Mottershead: Is My Mare Overdue?

Kathy St. Martin is well-known and respected in the breeding community, not only as the owner of Avalon Equine, an established and successful breeding farm, but also as part of Equine Reproduction. is operated by Kathy and her husband Jos Mottershead, and provides extensive repro services from collecting and freezing semen to teaching breeding short courses all over the US and Canada.

Avalon Equine has several stallions with Profiles on Click each name to view that stallion's Profile:




Belafonte d'Avalon

Colorado Skrødstrup

Dracula d'Avalon


Hertog Jan v.d. Paddensteeg


Silver Creek's Validation

Smoke Tree Poetry in Motion

ES Toronto

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