REPRODUCTIVE CYCLE IN MARES

Some mare owners who do not have a thorough understanding of mare’s physiology may become frustrated that their mares can not be served as soon as they arrive on stud, or otherwise do not comply with owner expectations.

Mares cycle during the late spring and summer period. Unfortunately, mare’s natural period of cyclicity does not coincide with the season imposed by the stud book and our desire to produce an early foal. The majority of mares “shut down” during winter and do not cycle normally if at all. Most will cycle from October/November through to March/April.
Mares go through a transitional period in the spring where they may behave as though they are in season but do not have a mature “egg” ready for release and will not go in foal if served. While some owners suggest that we “give these mares a needle” to get them in season, injection of prostaglandin (PG) only works to short cycle a mare and bring her in to season if she is already cycling. It will not work on mares that have not yet cycled this season.

As mares are long day breeders, the most effective way to ensure mares are cycling and ready for service in early September is to put them under lights. By exposing mares to artificially increased day length they are “tricked” into initiating their transition from non-cycling to cycling, earlier. This process takes about six weeks, so should be commenced in early to mid-July in order to encourage start of September cyclicity. There is no value in commencing this program in late August/ early September as it will still take around six weeks and mares will not be cycling normally until mid- October.

Once cycling normally, mares will usually be “in season” for 6 – 10 days, and out of season for 12 – 15 days, to make up an average cycle length of 21 days (range17 – 28 days). Once mares are cycling normally and in season, we examine their reproductive tracts regularly by transrectal ultrasonography (follicle testing).

Ovulation (“releasing of the ‘egg’”) occurs 12 to 48 hours before the mare goes out of season. Service after ovulation produces very poor conception (early pregnancy) rates, so much time and effort is devoted to predicting ovulation time in order to asses optimum time of service. Many commercial thoroughbred stallions have very “full books” and the aim is to have the stallion serve each mare once at the ideal time to give her the best chance of going in foal. As time of ovulation is quite variable we medicate mares to increase the reliability of ovulation occurring soon after service. We are extremely motivated to ensure mares are only covered when they are most likely to go in foal.

Mares that have foaled will normally come into season between 6 and 12 days after foaling. We routinely examine all post foaling mares to ensure that their reproductive tract is recovering well. Some mares recover rapidly enough to be served on this season (“foal heat”), while others require extra time. These mares that are not served on foal heat, along with cycling mares who arrive at stud not in season can be “short cycled” with an injection of prostaglandin. That is, the “out of season” phase of the cycle can be shortened in order to get the mare back in season ready to be served more quickly. Injection of prostaglandin will not work if given within 5 days of ovulating. Response to prostaglandin injection can be as quick as 1 day if given at the right stage of her cycle, however it is usually a couple of days and any mare that has not responded in a few days can be retreated

After service, mares are treated with antibiotics in order to help clean up any contamination at service and increase pregnancy rates. They are also scanned to see that they have ovulated in a timely manner and don’t require reservice

Mares are then examined for pregnancy at around 15 days post service, at this time any twins seen can be reduced to a single pregnancy. If the mare is not in foal she can be short cycled if necessary and will normally be soon served again. Horses are not infallible, and even if everything went well, not every mare will go in foal first serve.

Mares that are pregnant at the first test are re-examined at 30 days post service, again if they have lost their early pregnancy they can soon be re-served.

Mares are checked again at 45 days pregnant. Mares that carry a pregnancy beyond 35 days and then lose it, will generally not cycle again that season. Forty five days is traditionally the final test and the time when service fees are payable. This is also the time when it is regarded as “safest” to transport non-resident mares, as moving them before pregnancy is well established may lead to increased losses.
The above treatments and procedures are included in the Riverdene Veterinary Contract, making this excellent value for money.