Questions and Myths

Horse Breeding Myth-Information

– Some commonly heard myths and errata about horse breeding (equine reproduction). Some funny, some frustrating – all incorrect!! Myth-informtion at it’s best!

By Jos Mottershead & Kathy St. Martin

We often hear breeding-related “theories” that make us chuckle. We term them “myth-information”! Included here are some of the more common ones with explanations!

X-rays kill sperm when shipping transported semen

My mare had a reaction to the semen extender

  • As semen extenders themselves are most commonly composed of non-fat dried milk solids and sugars (all comparatively inert), the likelihood of a reaction to the extender is minuscule;
    • It should be remembered that a uterine inflammatory response is a perfectly normal sequela to any form of breeding (fresh, cooled or frozen semen);
    • “The mare reacted to the extender” is a reason often heard when there seems little other plausible explanation for pregnancy failure, or the situation has not been fully explored;
      • A reaction to the antibiotic in the extender is a possibility, albeit a remote one;
      • The use of an alternative antibiotic is suggested, providing the stallion has sperm that are compatible with it;
      • Alternatively an extender without antibiotics can be used, or no extender at all if the semen use is on-farm.
    • It is strongly recommended that all other areas be evaluated before moves to a different extender are requested or performed.

“Knocking off” a follicle

  • May be commented on as occurring during transport of a mare to a breeding facility;
    • The mare is seen to be in estrus at home and transported to the stallion’s location with a report that she has just come into estrus, but when breeding is attempted 2-3 days later she is found to not be in estrus;
    • This is actually because the mare was not entering estrus when seen to display at home, but was close to, or in the process of ovulating;
      • She hadn’t displayed estrus behaviour until she was extremely ready to be bred.
    • There is no reduction of the follicle as a result of the transport or any other stress.

Old Mares experience menopause

  • While it is true that older mares tend towards being less fertile, there is no standard age of “menopause” such as is seen in humans.
    • Some mares do “shut down” when they are older (typically greater than 20 years or so), but it is not universal!

Old mares have eggs that are not as good and will produce foals with problems such as bad legs, deformities etc.

  • It is true that the oocytes that are present to be fertilized in a 20+ year old mare are also 20+ years old (mares are born with all the oocytes they will ever have);
  • It is true that some of those oocytes will not be as viable as a result of age-related deterioration;
  • It most decidedly is not true that if fertilised those occytes will result in “bad legs, deformities”!
    • DNA is DNA and it is DNA which causes the presence of bad (or good!) legs etc.
    • You breed 2 crooked legged horses that are 5 years old and you will stand as much chance of a crooked legged foal as you will if you were to breed them at 25
    • Genetics and environmental issues causes problems such as that, not “old” oocytes!

Mares have a 28 day estrous cycle

  • This confusion arises as a result of the human cycle being 28 days;
  • The typical mare’s estrous cycle is 21 days from ovulation to ovulation (for more information “click” here).
    • It should be noted that not all mares experience the same regularity, so the 28-day cycle may exist – but not routinely!

Having a mare around a stallion will cause a mare to go into estrus

  • This myth stems from a mare not displaying estrus without the presence of the required stimulus (a stallion).
    • The mare is undergoing all the normal hormonal changes associated with estrus – including ovulation – but does not display normal estrus behaviour;
    • When the stimulus in the form of the stallion is introduced, the previously absent estrus behaviour becomes apparent, hence the origin of the myth.
    • It is also possible that a mare may be showing submissive behaviour to the stallion in the form of estrus-like responses, but that does not mean she is in fact in estrus!

My mare can’t be bred by AI and must be bred live cover to become pregnant
(or the further extrapolation – “the only way we can get old Bessie pregnant is to turn her out with old Joe in the pasture”)

  • There can be a variety of reasons why this may apparently be the case, but the most likely is that the mare has a delayed uterine clearance or uterine fluid problem, and that during teasing prior to or following breeding, when endogenous oxytocin is released, the fluid is cleared.
    • The same results could be achieved using AI and the oxytocin protocol that can be found at this location on this site.

Pregnancy rates in the wild are better than those in managed breeding programs

  • While of course in a poorly managed program this may be true, but in well managed programs it most decidedly is not!
    • One would do well to consider that in managed programs, mares with all manners of reproductive problems are bred, but in the wild, the poor reproductive quality mares are driven from the herd and not bred – hence the overall greater pregnancy rate;
      • Those mares driven from the herd in the wild are usually eaten by predators!
    • It is worth considering that in the wild, reproductive conformation will tend to be healthier as there is a natural selection for the reproductively sound mare;
      • In domesticated breeding programs there is a greater likelihood of reproducing poor reproductive conformation!

In the wild stallions often breed their own daughters, so “inbreeding” is natural

  • In the semi-feral pony herd used in research at The University of Pennsylvania, New Bolton Center in all the years of research into herd-situations there has never once been an “inbred” foal born;
    • When fillies become sexually active, they leave the herd to seek another herd or bachelor stallion with whom to breed.
  • “Inbreeding” is therefore not natural, but rather a man-made phenomena.

Impending foaling can be predicted by evidence of “waxing” on the udder

  • This is a totally unreliable predictor!! Do not use it – instead evaluate the milk as outlined in this article.
    • Some mares may wax up as much as 3 or 4 weeks prior to foaling;
    • Others may not wax up at all!

Mares normally have a 340 (or 341) day gestation

  • There is no such thing as a “due date” in the equine!

    • The range of “normal” pregnancy is 320 – 370 days;
    • There can still be healthy foals born outside this time frame, although prior to 320 days, the foal must be considered premature.
      • Foals born before 300 days will not be viable as the lungs will not be formed;
      • The longest pregnancy duration (with a live foal) on record is 417 days.
        • Long-term foals often have a tendency to be small rather than large!
        • This is often thought to be as a result of retarded uterine growth.

It is dangerous for a menstruating woman to be in the presence of a stallion

  • There is no reason why this should be the case, and there is no supporting evidence that is it true;
  • It is suspected that this is a relic of the “male dominated” breeding sheds of the past quoted to prevent the presence of women in such an “ungenteel” environment.

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