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weefoal 09-02-2009 09:13 AM

Mares Controlling the Size Of A Foal
I dont want to highjack the other thread so thought I would start a new one. I have been breeding American Shetlands and American Miniatures for 9 years. I have unfortunately seen first hand what happens when a much bigger stallion is bred to a smaller mare. Its not pretty and usually you lose both the mare and the foal. If you are lucky you will only lose the foal.

Over the years I keep hearing over and over of this study done were shetland mares were bred to draft stallions and the foals were fine. I have never been able to locate this study. If someone has it would you please post it??? Im beginning to think its an urban myth.

The bad thing is so many people believe this and use it as just reason to breed a big stallion to a small mare. You can get lucky and have it work, or you might not.

Heres an excerpt from on this issue


In regards to the mare size vs stallion size problem, Carla L. Carleton, DVM, MS, Dipl. ACT, an associate professor in equine theriogenology at Michigan State University, says that although a mare has the internal ability to modulate the size of the fetus that's developing in her to some degree, "When you get a discrepancy and the stallion is quite a bit bigger than the mare, the mare's internal control over fetal size may be insufficient to the task. If the fetus' size exceeds that which may easily pass through the mare's birth canal, it will present as a dystocia due to fetal/maternal disproportion.
And NO this is not just an issue in miniatures and ponies. My good friend down the corner runs a state of the art foaling facility. This spring I went to chat and she told me how she lost her favorite mare this year due to the foal being way to big for the mare to foal out. They did cut the foal up to get it out but the mare died anyway. This was a standabred to standabred cross. The stallion was only a few inches taller then the mare. But unfortunately this was the stallions first foal crop and they found that for whatever reason this stallion produces huge foals. He was gelded.

No way will I risk the life of my mare to breed a big stallion to a small mare. Just not worth the risk.

close2prfct 09-02-2009 10:14 AM

This past June our buckskin mare foaled a gorgeous bay tobiano colt which we lost due to his size. Misty was unexpectedly in foal when I bought her so I had no knowledge of the stud. Misty is 13.1 hh the colt was not much smaller his legs were as long as hers already and because of the that according to my vet it caused him to be severely overextended he could not stand very well and walking his hooves flopped around in front of him. He could not nurse because his legs were so bad and his head already came to withers he had to be spread eagle to even try to nurse. He was the size you would expect a much larger QH or TB to produce not a small pony. Although Misty appeared to have foaled without complications and she was an exceptional mother he also appeared to have been in the birth canal too long, he just wasn't right. I followed the vets instructions however he didn't make it through the night. Thankfully we did not lose Misty in the process but this is something that taught me to be very wary of buying mares or if I decide to ever breed choosing an appropriate size stud and have knowledge of his background and the size of horses produced in his line and not go by just his size alone.

MN Tigerstripes 09-02-2009 10:18 AM

I just emailed my professor for the name of the study and what journal it's published in. I know when we talked about it in class (Animal Growth & Development) I was very suprised that they would do something like that... I mean it seems like you're opening a door to lots of trouble in terms of the mare's health. Anyways, I'll post as soon as she emails me back.

MN Tigerstripes 09-02-2009 10:35 AM

Here is a quick pull from an article (from the first link)

The concept of maternal constraint on fetal growth has a long history. Walton and Hammond's classic 1938 paper12 reported the results of crossing large Shire horses with small Shetland ponies. They found that offspring of the crosses delivered to Shire dams were heavier than that of pure Shetland ponies, but below that of pure Shire offspring. In contrast, the reciprocal cross-delivered to the Shetland dam was of the same weight at birth as the Shetland purebred foal. Thus, the Shetland mother was able to down-regulate the in utero growth of her foal sired by the much larger Shire horse, while the in utero environment provided by the larger Shire mother facilitated enhanced growth. As on average, crosses delivered to the Shetland and Shire mare would have the same genetic complement, they inferred the existence of a maternal regulatory mechanism linked to maternal size. Although based on very small numbers, Walton and Hammond's paper is a model of elegant analysis and interpretation. Their conclusion was that the maternal regulation may be brought about by one or more of the following mechanisms: ‘(i) maternal regulation of fetal nutrition; (ii) maternal hormonal control; and (iii) cytoplasmic inheritance.’ Many of their key findings have been confirmed over the past decade in horses and cattle,13–15 and their paper continues to be widely cited both by animal researchers as well as those interested in human growth. Moreover, their idea of ‘cytoplasmic inheritance’ anticipates by half a century the relatively recent interest in non-Mendelian modes of inheritance through mitochondrial DNA16 and imprinted genes.17

Commentary: The development of the Ounsteds' theory of maternal constraint--a critical perspective -- Leon 37 (2): 255 -- International Journal of Epidemiology

I cannot get access to the full Walton and Hammond paper as I am not a member of the scientific journal... annoying. Although I may be able to do so next week when I'm on campus. But this is the full citation.

The Maternal Effects on Growth and Conformation in Shire Horse-Shetland Pony Crosses
Arthur Walton and John Hammond
Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 125, No. 840 (Jun. 16, 1938, pp. 311-335 (article consists of 27 pages)
Published by: The Royal Society
Stable URL: JSTOR: An Error Occurred Setting Your User Cookie

MacabreMikolaj 09-02-2009 09:52 PM

Can it be proven without a doubt though that those mares experienced difficulty simply because of the size of the foal? I've known plenty of big foals causing problems when the parents are the same size.

Now, I don't think I'd chance fate and go breed a pony to a Shire, but is it even possible to put a size factor of "danger" zone? I spoke to a lot of breeders and vets before breeding my 14hh Arab to a 17hh Hanoverian with huge bone substance and everyone seemed to agree it was a myth that a big stud could cause problems. In this case, they were right. However, the foal remained small even as an adult. I'm not sure if that had anything to do with the small stature of the dam.

I think it's good to take precautions but I've known a lot of small mare X big stud breedings and never heard of a problem. Maybe the stud wasn't TOO big?

weefoal 09-03-2009 07:35 AM

I agree that sometimes the stallions size doesnt predict as case in point is my neighbor. That stallion was barely bigger then the mare but like I said all his first foals were huge. Was it something further back in his pedigree or ???

But my point is that mares dont control the size of the foal or you wouldnt have all these foals that are so big they have to be cut out of the mare. The most horrible experience a breeder can have is watching a vet take a foal out in pieces. When it happened to my friend I held her for an hour while the vet worked to take that foal out piece by piece and like I said they still lost the mare. In that case the stallion was bigger then the mare but so many people had told her that the mare controls the size and unfortunatley in this case it was wrong. My friend was so devastated that she never bred another mare after that. It really is a traumatizing thing to go through.

No matter what we do things can and do go wrong. For me I feel so responsible for my mares and putting them in foal that I have to take any precaution I can to do no harm to them.

I would really love to read the entire study. I think the clue here is this statement


Although based on very small numbers,
Really wish we knew what the exact number was. If they only tried it on say 2 mares then to me that is not a "study" Also have to take into account that was 1938. A study done now Im sure would have to take a sampling of say 30 mares to be considered a real study in how mares control fetal size

MN Tigerstripes 09-03-2009 08:00 AM

I will try to get the study next week (a lot of times we have access to these journals on campus but not off because the school subscribes to the journal or something). I don't know anything about breeding horses and only a little about breeding dogs, but I would hazard to guess that this is just like anything else in the world and some animals would be better at regulating size then others. It's entirely possible that there are genes that enable females to do this and some females just don't have them. I too wonder how large the study was, hopefully we can find out next week.

I guess if it was me I probably wouldn't take the chance with an animal by breeding it to a large stud or to one who has a history of large foals. Which as a side note, calf size is heritable in cows (from the sire, though I don't believe highly heritable) so in the Holstein industry they will oftentimes intentionally breed to bulls with a family line of smaller calves to reduce chances of problems. So again it's probably a combination of factors and genetics that have to line up in just the right way to produce the desired result... again leading to the I probably wouldn't risk it statement.

weefoal 09-03-2009 08:26 AM

Thanks so much for trying to locate that study. I am just dying to read it!


upsidedown 09-05-2009 11:58 AM

Honestly I don't know more about this than what I have read and heard, I'm not a breeder or a vet of any kind. But weefoal yours is the first that I've heard of something going terribly wrong from it. I'm going to be watching this thread, since I'm assuming it was my statement that sparked it, but I don't really have anything to say that hasn't been said.

MN Tigerstripes 09-11-2009 12:40 PM

See if this link works for you guys... Otherwise the article is like 28 pages long, so I don't want to post it here.

JSTOR: An Error Occurred Setting Your User Cookie

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