Breeding season is almost here. Why? What makes mares cycle and deliver foals from August till December? It seems that evolution has helped mares deliver foals at a time when feed is maximal and weather conditions are favourable. It we look at the number of mares that are cycling at any one time of the year it varies from around 25% in the winter up to 100% in late December and January. This is represented by the pink bars. The breeding season for the Thoroughbreds (and most other breeds) is represented by the yellow bars and the Standardbred breeding season is represented by the blue bars. Clearly there is something wrong here. Why are the times we like to breed so out of line with the number of mares that are actually cycling?

The reason appears to be driven by breed registries trying to prevent breeders from having foals too early and thus gaining an unfair advantage over others that are producing horses for performance based pursuits.

What then makes mares cycle? Put simply it is the interplay of increasing light and nutrition on the brain and the production of a hormone named GnRH (gonadotropin releasing hormone) that is released from the hypothalamus (bottom of the brain) and has an action on the pituitary (a gland at the base of the brain). The pituitary releases hormones that stimulate the mare’s reproductive tract (FSH- follicle stimulating hormone and LH-luteinizing hormone). These two hormones control the growth and release of follicles and eggs from the ovary. The ovary produces hormones that control behaviour of the mare. Oestrogen (from the follicle) causes the mare to show heat (oestrous) and progesterone (from the corpus luteum) stops the mare from showing heat (dioestrous) and prepares the mare for uterus for possible pregnancy.

Can we influence the time of year when non-pregnant mares cycle? The answer is yes and it is just as well or we would be waiting and waiting to start breeding if nature had her way. The most physiological method of hastening mares into the breeding season is to use an increase in the amount of light she is exposed too. In general if mares are exposed to 16 hours of light from July 1, they will be ready to breed in the first few weeks of September. Breeders use this trick all the time, however it will not work if the mare is not well looked after and by that we mean rugged when cold and given good nutrition, especially a grain based diet and removed from as many stresses as possible.

What about pregnant mares, can we influence when they cycle after foaling?
Yes again, although nature has a marvellous ability of allowing mares cycle in the first two weeks after foaling, if mares are not looked after they will not cycling and enter into a lactational anoestrous (ovarian quiescence).

When do mares foal?  The gestation for mares averages about 340 days, however we have seen live foals born as early as 292 days and as late as 418 days after the last service.

The foaling mare
Great excitement grips most breeders as the time the foal is due draws near. The mare should be sent away to foal at least 30 days prior to the expected delivery date. There are two reasons for this.  Firstly, to prevent a stress induced foaling on the transport truck and secondly, to give the mare time to produce antibodies to the bacteria and viruses on the farm where she will deliver the foal. Those antibodies are sent to the milk and absorbed by the foal as colostrum in the first few hours after birth.

The process of foaling will be posted at a later date. However a few words on the foaling process would appear useful at this time. When the mare is close to foaling she will start to fill her udder with milk (bagging up). When foaling is imminent she secretes colostrum into the udder (waxing). The day she foals she will often not eat her morning of evening meal. However, remember that none of theses signs are as reliable for maiden mares and not all mares seem to have read this discussion and so don’t follow it.

Most mares foal without problems and probably many mares have problems that can be directly related to our intervention, such as bring them in to foal and separating them from their friends etc. Despite this we all continue to monitor mares round the time of foaling and watch with alarms and full time dedicated foaling staff, as none of us could bear to loose the foal or even worse the mare if there are problems. The real problem with foaling mares (apart from the long legs of the foal) is that she is very strong and quite explosive in her delivery. The foal will not survive most abnormal presentations for more than around 15-30 minutes as the placenta will separate quite quickly and leave the foal without enough oxygen to absorb from its umbilicus. So, if not monitored it is possible to get up in the morning and have not only a foal that succumbed to an abnormal birth process but a mare who is in a bad way as well

Once the mare has foaled it is essential that the foal receive its colostrum. Failure to absorb colostrum is associated with septicaemia (infection and toxins in the blood), which can cause death or localisation of bacteria in places such as the joints (joint ill). The transfer of antibodies from the colostrum to the foal is so important that we recommend that all foals are tested for antibody levels. There are a variety of commercial tests available and all rely on either serum or whole blood to be collected from the foal between 12-24 hours after birth. Ask your veterinarian.

In general mares will cycle around 7-14 days after foaling (foal heat). We do not recommend breeding on the foal heat as it is associated with lower fertility and a higher rate of early embryonic death. Fancy a mare a being able to deliver a foal after 340 days of gestation and then be pregnant again with two weeks. No wonder they don’t hold the pregnancies very well. The mare needs time to get over her pregnancy. If the gestation wasn’t so long and was more like a cow or human
(~9 months instead of 11 months) then it would be no problem to ask the mare to become pregnant and have a foal every year. Perhaps we are all a bit greedy in expecting mares to have a foal every year.

Our strong recommendations are to recycle the mare by administering the drug called prostaglandin (PGF₂a for those that really need to know) on about day 18
after foaling or at least 6 days after the foal heat ovulation (release of the egg and end of the cycle) has occurred. That way if they become pregnant on that cycle they can maintain an ~ yearly inter-foaling interval.

The maiden and dry (empty) mare
These mares need to be bred as soon as they cycle. The difficulty is often to get them to cycle at a time that suits us. If they arrive at the farm in September then it is already too late to put them under lights if they don’t have much activity on their ovaries. In these instances we may resort to the use of drugs to artificially manipulate the transition into the breeding season. In general the harder we have to push these mares the worse the result.

The normal cycle
This starts with oestrous- the period of sexual receptivity (heat). Oestrous last for a variable length but approximates 5 days. It is longer early in the season (September) and shorter later in the season (December). Oestrous occurs as a result of oestrogen released by a large follicle. Oestrous finishes after ovulation occurs. In general it takes around 24 hours for mares to stop showing heat after ovulation occurs and the mares enters dioestrous. This period is when the uterus is preparing for pregnancy and is always ~ 16 days long. On occasion the mare may not get the signal to return to heat and may not be pregnant either. Theses mares are in prolonged dioestrous and can be given prostaglandin to return them to heat once it has been established that they are not pregnant.

Breeding the mare
Mares are restrained after they have been shown to be sexually receptive (teased). The restraint is necessary to avoid accidental injury to either the mare (or her foal), the stallion or to farm personnel that are involved with breeding process. Quite commonly mares are given drugs such as Ovuplant (artificial GnRH) or hCG (artificial LH) to hasten ovulation and prevent re-breeding. In general if mares have not ovulated within 48 hours after breeding they are rebred, however many stallions have semen that is capable of lasting 3 or 4 days (or longer).

After breeding the mares are often examined in the next 24-48 hours. This examination is used to detect if there are any problems such as infection or failure to eliminate post service inflammatoryproducts. In addition this is the time where minor reproductive surgical procedures may be carried out, such as suturing of the vulval lips together (Caslicks’ procedure).

Pregnancy testing
Most commonly mares are tested around 14 - 16 days from service using an ultrasound. Ultrasound at this age is also useful for detecting and managing twins. Later on the foetal heart beat can be seen (from around 23 days onwards) and is used to asses foetal well-being. On most breeding farms the day of final pregnancy test is 45 days and service fees are billed at that time.