Aspects of Breeding in Mares

by Eryl V. Davies BVSc MRCVS

by Eryl V. Davies BVSc MRCVS

The scanning season is upon us once more; it is wonderful to see mares with foals at foot come into the surgery for pregnancy scanning and leave with happy owners. The latest ultrasound scanning equipment can detect a foetus at 15-16 days and a heart beat at 22-24 days of pregnancy. We can even sex the foetus now at about 60 days! Much has changed in the type of equipment used over the last 25 years but we can be more confident now with our diagnoses as the quality of digital screens keeps improving.

Pre-1985 vets could detect pregnancy by rectal examination from about 42 days post service. When mares were not pregnant we used to advise about the size of the follicles (eggs) on the ovary and treat accordingly. With the advent of scanning we have been able to widen our knowledge of breeding. Here are some examples:-

– Early detection of pregnancy

A 35 day embrio

A 35 day embrio

Now that we can be confident of declaring a mare pregnant at 18-20 days we can advise an owner that they can take the mare home and not have to return to stud. This has two advantages: the mare will be in her own environment (less stress) and the owner saves on ‘keep’. However a second scan is advisable at the 28-30 day stage to ensure that the embryo has implanted properly in the womb and to confirm the absence of twins. At this stage a healthy heart beat is a reassuring sight on the screen.

– Detection of twins

Twins in mares are, thankfully, rare. However when they go undetected they very rarely survive to full term. The mare’s womb is incapable of carrying twins and feeding them successfully through the pregnancy so one foetus usually dies. This results in an abortion and usually the second foetus (although viable) is pushed out at the same time.

If twins are detected early on in the pregnancy then there are two options:-

(a) Inject the mare with prostaglandin to induce the next season (this has the effect of altering the hormonal balance in the womb) and lead to an artificial abortion.

or (b) If the twins are at a sufficient distance apart in the womb, one can be terminated gently under the guidance of the ultrasound scanner.

– Artificial Insemination

MARE-&-FOAL-3-copyTiming is everything when breeding mares artificially. With the help of ultrasound we can determine when the optimum time for inseminating is likely by watching the eggs grow and develop. By hospitalising mares at the surgery we can scan the ovaries several times during the 24-48 hours prior to insemination and record the changes in the ovary. Careful use of hormone injections can help ovulation, but no amount of interference will increase the fertilisation rate – that is another topic entirely! Chilled semen is used in the majority of cases but the trend over the last five years has been increasingly towards the use of frozen semen.

– Sexing the foetus

As mentioned earlier the improvement in the quality of scanning equipment, along with the constant research, has enabled us to ‘sex’ a foal at an early stage. The best ‘window’ to determine a colt from a filly is between 58 and 64 days. After this time the foal grows so rapidly so that the weight of the womb takes the foetus too far away from the scanner head to diagnose confidently.

 – Sub-fertility

Rectal examination with an ultrasound scanner can reveal abnormalities which would otherwise go undetected. In the older mare endometrial cysts can reduce the chances of a pregnancy surviving if the embryo implants too near to the cyst. As the embryo grows it needs increasing amounts of nutrients and the cyst may be large enough to prevent this from happening. Certain abnormalities of the ovaries can be detected also; a granulosa cell tumour is quite a striking feature when seen on an ultrasound screen.

If you have a particular interest in breeding and want to discuss this further, please feel free to telephone us at Putlands. Our advice is free!

If you have any questions about any aspects of your horse’s health, or indeed your pet, farm or smallholding animals, please do not hesitate to contact us on 01892 835456. Putlands Veterinary Surgery is based in Paddock Wood in Kent, and offers a friendly, professional and personal service for all species of animal. We have four dedicated large animal vets with a wealth of equine experience between us, and are always happy to help.

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