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21 February 2019   
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Horse Breeding myths

Do mares absorb embrios?

Although many stud vets use terms such as 'absorb' , 'resorb' and 'reabsorb', this has been shown not to occur. Fluid associated with the embryo is absorbed by the uterus, but the embryo itself is aborted and is ejected through the cervix during the next estrus cycle, as the uterus is simply not capable of breaking down and absorbing embryonic tissue.

*Mares have a 28 day oestrus cycle

Humans have a 28 day cycle. The typical mare's cycle lasts for 21 days, although every mare is different and some mares may have shorter or longer than average cycles.

*Old mares experience menopause

While it is true that some older mares are less fertile, there is no equine menopause, as such. Some mares may 'shut down' after the age of 20, but this is not a universal occurrence.

*Old mares have old, damaged eggs, and will produce foals with problems such as bad legs, deformities etc.

Eggs produced by a 20 year old mare, are themselves 20 years old, as each mare is born with a all the eggs they will ever have. These older eggs may not be as viable, and so may be less likely to produce pregnancies. However, DNA is DNA, and the genes in each egg remain unchanged, whatever the age of the mare (and the egg). Environmental factors and mare/stallion genetics cause these problems, not old eggs!

*My mare cannot be successfully bred using AI, and must be covered naturally to become pregnant

There may be a variety of reasons why this may appear to be the case, but generally it is because the mare has delayed uterine clearance issues. During the teasing stages of natural covering, the mare releases oxytocin which helps to clear the uterus of any post breeding debris and fluid. Exactly the same results can be achieved with judicious use of AI and oxytocin administration.

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

This myth stems from the fact that mares show signs of being in season when they are presented with the required stimulus (i.e. a stallion). Some mares cycle silently, and will only show any sign of being in estrus when a stallion is nearby. Stallion presence does NOT cause estrus.

*My mare had a reaction to the semen extender

Extender is extremely unlikely to cause a reaction in the uterus of the mare. It is possible however, that if the mare is sensitive to a particular antibiotic, and that antibiotic is present in the extender, that it may cause excessive uterine inflammation. However, blaming the extender for failure to achieve a pregnancy is short-sighted and it is strongly suggested that all the other possibilities are investigated first.

*I drove my mare to the stud and knocked off the follicle!

No you didn't. Mares continue to show estrus signs for 2-3 days after ovulation in some cases. What probably happened is that the mare was taken to stud after ovulation, but while still showing this post-ovulatory estrus behaviour. It is not physically possible to "knock off" a follicle - otherwise everyone would drive their mares around in order to perfectly time ovulations.

*X-rays kill sperm when they are scanned in airport security

There is no research that supports this theory. In fact, studies that have been undertaken show no reduced fertility of sperm exposed to x-rays.

*Pregnancy rates are higher in the wild than they are managed breeding programmes

While this may be true in a poorly managed programme, in a well managed programme it is totally false. We manage to breed mares which, in the wild, could not be bred and would be driven out of the herd.

*Wild stallions breed their own daughters, so inbreeding is totally natural

In New Bolton, Pennsylvania, the equine behaviour centre has studied a feral herd of ponies for over 9 years now. They have never had an inbred foal. When fillies become sexually active, they leave their birth herd, and seek new stallions. Inbreeding is man made, and is likely to be a significant factor in reduced fertility of modern animals.

*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 no evidence to support the hypothesis. It is likely to be a relic of the male-dominated breeding set-ups of the late 19th and early 20th century.

*The foal's umbilical stump should be treated with iodine after birth

Iodine has been shown to be far too astringent on neonatal umbilical tissue. It should in fact be treated with a 0.5% chlorohexidine solution, four times a day for the first 3 days after birth.

*Foal heat scours are caused by the mare coming back into estrus

This was thought to be the case until one farm began worming the mares shortly after birth with an ivermectin based wormer - this almost completely eliminated the scours. It is now thought that the scours might be due to the expanding diet for the foal, as there are no detectable changes in the composition of the mare's milk around the time of scours. The method of action of ivermectin on foal scours is currently unknown. Please note: Ivermectin-based wormers should NEVER be administered directly to foals. Please follow your vet's advice, and read and follow the instructions provided with your wormer.

This article is based on the Breeding Myths article from Jos Mottershead's Equine Reproduction LLC of OK, USA. Please visit their website at for more great articles!


Jamie Anderson of Equine Reproduction Ltd

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