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Will the size of the uterus or genes effecrt foal size more?

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    Will the size of the uterus or genes effecrt foal size more?

    I got an unbelieveable email and request to breed a 14.2 pony to CTF(16.1 1/2H). It is a VERY special pony, not only in it's own right....but to it's family. I'll share the rest of the story at a later date.
    However, my question is this........they are contemplating an ET to be sure the mare has no problems. I wondered how the size of the recipient mare may effect the total size of the foal. If they're trying to have another pony from this match , do they need to use a small recipient?? Will a larger mare cause the foal to be bigger?

    I know many of you have bred HUGE stallions to small ponies and gotten ponies. It always made me nervous, but from wht I've seen here it's been successful (Escapade and Art Deco come to mind)

    Anyone have any knowledge on how the size of the uterus effects the size of the foal when it's born, as well as how big it'll grow. I've heard that a foal will only grow to a safe size in it's dam (so for the sake of discussion, what would happen if you bred a draft stallion to a small pony???????.......not that I EVER would!!!)

    Thoughts???? Experiences???
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    #2
    Many years ago NB bred 10 shetland mares to a Shire stallion. None of the 10 mares requried assistance at foaling, none of the foals got stuck, all presented normally as well. All matured to bigger than their dam and smaller than their sire ... ... but NO idea if they were conformational nightmares or not!

    A client bred her small Welsh pony mare (12.2hh) to my TB stallion (16.1hh) and he appears to be heading towards a 14-14.2hh large pony which is exactly what she was hoping for. She also had no problems foaling out this colt either. The ONLY issue she found is that once born, the colt grew rapidly (leaning towards the genetic component of your question) and he started to have problems nursing as he couldnt get "under there" quite so easily anymore ... .. and the mare also didnt appear to have enough milk to satisfy his appetite so he was supplemented as well

    I am no scientist, but I believe that in your situation the size of the donor mare / uterus will dictate the in utero size of the foal coming out to a degree, but it will NOT affect the final genetic height it will achieve no matter who carried the embryo and what size she was
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      #3
      I personally don't think 14.2 is too small... unless she's a super, super dainty 14.2. We bred 14h +/- QH types to bigger stallions all the time at the Ranch & the APHA farm. Never an issue. UNLESS you know the stallion throws particularly large birthweight babies...

      Uterus size will determine birth size, genetics will determine eventual size... EXCEPT uterine *competence* can have an effect on diminishing the genetic potential size.
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        #4
        I have heard as TC stated above, that the mare determines the size of the foal. Most dystocias have no correlation to the sire's size.

        Now in cattle, the size of the bull absolutely determines the size of the calf. Of course for beef farmers want the largest sized calf they can get, so they breed to the biggest bulls - hence all the problems with calving and so forth.

        Our vet breeds cattle a TON as well as horses and she said while you can never say never, general research indicates it's not typically a problem as it would be in cattle. We asked because we got a fabulous imported GRP mare (medium) and were looking at what to breed her to, so we asked this very question.
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          #5
          Wait, they're hoping for a pony from this match? I really like your boy, don't get me wrong, but I strongly doubt they're going to get a pony from 14.2 mare and 16.1 1/2 stallion!

          Physically, it shouldn't be a problem even if the pony carried it (at least, any more so than a normal pregnancy's problems) but ET-ing into a smaller recipient mare is NOT going to change the final height of the foal - I'd guesstimate it'll be 15.1 or so unless CTF has 'short' in his pedigree behind him?
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            #6
            Originally posted by sniplover View Post
            Wait, they're hoping for a pony from this match? I really like your boy, don't get me wrong, but I strongly doubt they're going to get a pony from 14.2 mare and 16.1 1/2 stallion!

            Unless you didn't want one. I got a pony from a 15.3 hh mare and a 16.2 hh stallion. Perfect foal. Born on due date. Healthy and looked like parents. I dare not complain as at least she wasn't 14.3.

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              #7
              The issue is not whether a 14.2 hand mare can safely carry a foal from a 16.2 stallion, but rather whether, by using a small ET recipient pony mare, you can effectively downsize the product of a cross between the 14.2 large pony mare and 16.2 stallion.

              I would say you are very unlikely to get a pony from that cross. EVEN if the smaller pony recipient mare manages to produce a tiny foal (due to small size of uterus), the foal is still likely to grow to its genetic potential (i.e., whatever its biological parents would produce). In addition, I know from experience that the notion that the in utero foal will only grow as large as the particular mare can accomodate is BS; just this year I had a very large foal out of a maiden that was so large relative to the mare that we would have lost both had it not been for the intervention of my vet. So I think it borders on the unethical to try to downsize a cross by deliberately choosing a smaller embryo recipient mare.
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                #8
                The following scientific studies have found that the size of the foal is determined primarily by the size of the mare and not the stallion:
                • Walton A, Hammond J. (1938) The maternal effects on growth and conformation in the shire horse-shetland pony crosses. Proc Royal Soc. B, 125, 311.
                • Hammond J (1955) Progress in the Physiology of Farm Animals, Vol 2. Butterworth Scientific Publications, London, England.
                • Influence of maternal size on placental, fetal and postnatal growth in the horse. I. Development in utero WR Allen, S Wilsher, C Turnbull, F Stewart, J Ousey, PD Rossdale, and AL Fowden; Reproduction (2002) 123 445-453

                This latter - and most recent study - says in the summation: "Thus, the results indicate that in equids, maternal size interacts with both the maternal and fetal genotypes to control the rate and extent of fetal growth by influencing the gross area of the diffuse allantochorion, and the density, complexity and depth of the microcotyledons on its surface" - and hence does not produce a foal "too large" for the mare.

                From the above, it can be clearly seen that for many years, it has indeed been scientifically proven that the mare size controls the foal size at delivery.

                There are many causes for dystocia not related to fetal size. One area where we probably can agree is that depth of chest, and width of hips and shoulders in the foal can cause foaling problems, and in this regards, one would be well advised to review the pelvic opening diameter of the mare, and the type of build of the foal that the stallion typically throws. Again though, this is still very different from the concept that a small mare bred to a tall stallion will result in a foaling problem - which (unless there are other areas of disparity) it will not.

                The primary concern in crossing a large pony mare on a full-sized horse is that it is most likely to produce a foal that will ultimately exceed pony height, regardless of the size of the mare that carries the pregnancy. Although the foaling size may be smaller, as noted by others, genetics will result in a realization of (at least close to, if not) full growth once mature.
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                  #9
                  From the above, it can be clearly seen that for many years, it has indeed been scientifically proven that the mare size controls the foal size at delivery.
                  Kathy - can you please clarify on this point as for me it seems a little "murky". As well - these studies would all have been done well before the advent of any ET breedings ...

                  If a 12hh pony mare is bred to a 16hh stallion and the embryo is implanted into a 17hh draft mare, surely to goodness the draft mare size wont have one iota of correlation as to what the birth size of the foal will be, nor the mature size either??? Correct?

                  I am assuming (and please correct me if I am wrong on this one) if that same foal was left in its maternal dam it would emerge at birth the same size as a genetically full sibling that was implanted into a 17hh draft mare

                  Am I right or wrong on that one?

                  Thanks Kathy!
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                    #10
                    I think what she (and the studies) imply is that the mare determines the size of foal at birth, but not final mature height. ie) breeding to a large stallion may result in a large adult offspring, but the fetus will not grow to a size that is detrimental to the mare.

                    In the case of "upsizing" I don't think a fetus will grow to larger then genetically determined, though once born the quality of a mare's milk may make a foal grow too fast etc, which is a problem I think a lot of people doing ETs into draft horses are figuring out.

                    But basically, mare's won't allow a fetus to grow to a point that it will cause harm to the mare during birth, YET the mare's uterus size will also not effect the final size of the foal beyond the genetics of sire and dam.

                    So in your case, breeding a 14.2h mare to your 16.1h stallion will most likely result in a foal in the 14.2-16.1h range, probably closer to the 15.2h range, but it also depends on the genetics the parents might both carry (for example, my mare bred to 2 smaller stallions has produced 2 colts bigger then herself, most likely due to having height somewhere in her pedigree (or theirs also))
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                      #11
                      This is a previous thread of a similar topic:
                      http://www.chronicleforums.com/Forum...d.php?t=204927

                      Before I bred my maiden mare last year I looked up everything I could find on foal birth size. The papers I saw were all in agreement with Kathy's post. (I just needed a Kathy and I could have saved myself a bunch of time).

                      One paper I read did use ET. They had taken pony draft crosses and allowed the ponies to carry one and they ET the rest to larger horses. The results were the ponies carrying the crosses, delivered a foal that was proportionate to the mare's body size. The larger ET mares delivered larger foals, but still a bit smaller than when they had been crossed to a full size stud. At the end of 3 years (I can't find study) all the offspring were, within statistical range, the same/similar heights.
                      I was talking to my vet when my mare foaled and was noticing the placenta was very different in horses and cows. The cows cotyledons are like buttons, and the horses placenta it is the entire surface area. I thought this might be a mechanism for foal size determination??
                      My mare's foal was smaller and super skinny, while the mare was fat and a large horse.

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                        #12
                        My ex used among others, an arabian, and a small grade mare as ET recip mares, and they foaled out big warmbloods. They would be born small, and then grow and grow.
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                          #13
                          Oh... I misread the original post. I thought the main concern was the foal might be too big for the 14.2 mare.
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                            #14
                            Basically I have found that uterine size affects the size of the foal at birth, genes affect the size of the foal at maturity.
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                              #15
                              I don't believe that they will get a foal that will stay pony sized regardless of the size of the recipient mare. I am sure most pony breeders will agree. Breeding a 14.2hh mare to a 16.1 1/2hh stallion is extremely unlikely to result in a pony sized offspring. Depending on the genetics of the mare and I am assuming she is a crossbred due to her size, the mare owners should probably breed her to a stallion no larger than 14hh. And they should be very sure that the stallion does not have a lot of big size in his pedigree.
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                                Original Poster

                                #16
                                I don't necessairly think they are hoping to get a pony. I would have advised them that it needed to be a SMALL pony bred to CTF to have a good shot at a 14.2 and under pony. I think they're hoping to size up, but not too much.

                                The trainer asked my opinion on ET, and it was ME who was wondering if a different sized uterus would effect the size of the foal. (why are first foals mostly smaller with the same dam and stallion???).the uterus hasn't stretched out as much I would suppose???

                                Thanks for all the info above.......keep it coming. I am always interested to learn more......
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                                  #17
                                  Originally posted by TrueColours View Post
                                  Kathy - can you please clarify on this point as for me it seems a little "murky". As well - these studies would all have been done well before the advent of any ET breedings ...
                                  Actually, the last one (Allen et. al. - performed as you will see in 2002) looked at exactly that - ET's of ponies into TB's and vice versa. You can view the abstract at, and download the entire paper from this location.

                                  If a 12hh pony mare is bred to a 16hh stallion and the embryo is implanted into a 17hh draft mare, surely to goodness the draft mare size wont have one iota of correlation as to what the birth size of the foal will be, nor the mature size either??? Correct?
                                  No. As the cited research demonstrates, the larger (recip) mare with the smaller (pony) fetus/foal will have a larger birth-sized foal than if that same foal was maintained in the original donor mare for the entire pregnancy. The mature size of the foal however will be related to the genetics behind the original sire and dam, not the recipient mare.

                                  I am assuming (and please correct me if I am wrong on this one) if that same foal was left in its maternal dam it would emerge at birth the same size as a genetically full sibling that was implanted into a 17hh draft mare

                                  Am I right or wrong on that one?
                                  No, you're wrong.

                                  Take a look at the cited and linked research paper. It's interesting. The "discussion" in that paper observes (in part):

                                  The results of the present study confirm and extend the
                                  results of earlier studies (Walton and Hammond, 1938;
                                  Tischner, 1985, 1987) to show that foal birth weight is
                                  determined primarily by the total microscopic area of the
                                  allantochorion, independently of the maternal or fetal genotype.
                                  However, development of the total area of microscopic
                                  fetomaternal contact was itself dependent on both
                                  the maternal and fetal genotypes so that fetal growth was
                                  indirectly affected by both genotypes. Pony embryos transferred
                                  to Thoroughbred mares produced larger placentae
                                  and, hence, were larger foals at birth compared with their
                                  P-in-P counterparts. Nevertheless, the placenta mass and
                                  birth weights of these P-in-Tb foals were still lower than
                                  those of the Tb-in-Tb control animals. Conversely, transferring
                                  a Thoroughbred foal into a Pony uterus restricted
                                  placental, and hence fetal, growth compared with that in
                                  the Tb-in-Tb pregnancies, but it also gave rise to a larger
                                  total microscopic area of fetomaternal contact and larger
                                  birth weight than in the Pony foals born to similarly sized
                                  Pony mares. Therefore, genetic factors enhance placental
                                  growth in Tb-in-P pregnancies but restricted it in the P-in-Tb
                                  pregnancies. The cellular and molecular mechanisms
                                  whereby the fetal and maternal genotypes control placental
                                  development remain unknown.


                                  This research used pony ("P") and Thoroughbred ("Tb") mares.
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                                    #18
                                    Originally posted by camohn View Post
                                    Basically I have found that uterine size affects the size of the foal at birth, genes affect the size of the foal at maturity.
                                    Not entirely. At least one study using ET showed that reduced birth size due to the dam being a pony influenced mature height. In that study, none of the foals born to ponies grew to the same size as genetically similar foals born to horses.
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