Spring Heat - Transitions In The Mare
by Drs Allan Gunn, John Chopin, David Hanlon and Jim Rodger

The natural reproduction cycle dictated by nature has mares producing foals at the time of the year most suitable to them surviving, and this time frame is in the best interests of a given species. The time of most reliable and abundant food for horses is usually towards the end of spring and in the summer, enhancing the foal’s chances of surviving to produce offspring of its own.

To achieve this late spring foal the breeding of the mare to the stallion is required in spring or summer, as the development of the foal in the uterus is about 330 days, or 11-12 months.

Spring is a time of increasing day length, temperature and food availability. As a result of the longer daylight hours in the days when the mare is cycling, or ‘in season’, she is referred to as a ‘long day breeder’. This is in contrast to another farm animal, the sheep, that ‘cycles’ in the autumn and winter, and is known as a ‘short day breeder’. Due to the shorter gestation period of a sheep, both species give birth to their offspring during the spring and summer months. That seasonal cycling occurs in order for the mare or sheep to have their offspring at the optimum time of year, when there is a greater chance of them surviving, is an important concept to remember for those intending to breed horses.

‘Seasonally Polyoestrous’ is the phrase used to describe the reproductive cycle of the mare, as detailed in Hoofbeats Oct/Nov 2011issue, Vol 33. While this may sound like some exotic disease it simply means that the mare cycles - has periods of being ‘in heat’ or ‘in season’ (oestrus) and no oestrus (dioestrus) - during a particular season of the year. It also means that there are times in the year when she does not show any signs of being in season (anoestrus).

Simply put, mares have many cycles and are reproductively active through summer, (polyoestrus), during which they will come into season/oestrus/heat at regular intervals approximately 21-23 days apart. There is a period of time in the winter, when the daylight hours are at their shortest, where approximately 80% of mares will stop cycling (anoestrus). In between these two periods are transitional periods in autumn and spring when the mare is going from having regular cycles to when she is not cycling.

Effects on Mare Breeding
Because most mares do not cycle regularly during the winter and early spring - it is not possible to breed them at this time of the year. This can be particularly frustrating to owners hoping to breed a mare in autumn or winter to have an autumn or winter born foal, or if breeding the mare early in the spring is desired to obtain the so called ‘early foal’. It can also be very frustrating for those importing a mare from the Northern hemisphere to breed, or vice versa.

Most of the incentive in the Thoroughbred industry has been to have foals born early in the spring, rather than later in the breeding season. As a result more is known about early breeding than is known about the later breeding of mares. There are still large gaps in the knowledge and treatment of attempting to breed mares ‘out of season’.

Spring ‘Transition’ Period
The reproductive cycle is regulated by hormones, which are chemical messengers in the horse’s body. These hormones allow communication between the brain - mainly the hypothalamus and the pituitary, and the reproductive organs - the ovaries and to some extent the uterus.

During the winter, there is very little stimulation of the ovary from these hormones and therefore the mare does not cycle. In summer, however, there is regular stimulation of the ovaries by these hormones from the brain. As a result the mare cycles, producing follicles and eggs from the ovary, as well as hormones that affect the brain, and these are known as ‘feed back loops’. The transition times are the periods between no cycling in winter (anoestrus) and the regular cycling patterns in summer (seasonally polyoestrus).

Mares have developed this mechanism during their evolutionary adaptations to enhance their survival as a species. Due to the importance of programming their cycles, they have used the most consistent environmental cue for seasonality, and that is LIGHT. Regardless of the type of season, the day length at a given time of year has been, and will be, a consistent feature of the different seasons.

Once the days start to lengthen, the mare starts to prepare herself for the breeding season, ovulating about 60-90 days from the time the days start lengthening. Ovulation occurs, and signals the end of transition.

The length of day is detected by the horse’s eye and registered by a small part of the brain, known as the pineal gland, that produces a hormone called melatonin when the eye is not detecting any light. So when the days get longer, the amount of melatonin produced is decreased. The change in melatonin production is detected by the hypothalamus in the brain in seasonal breeders. This starts the process of releasing the hormone GnRH (the ‘driver’), which signals to the pituitary gland to produce and release the hormones FSH (Follicle Stimulating Hormone ‘the stimulator’) and LH (Luteinising Hormone ‘the ovulator’). These hormones travel through the blood and stimulate the follicles, which have been lying dormant in the ovary over winter, to start growing. Once these follicles start to grow more than about 15 mm in diameter, it takes another 40 days for a few ‘waves’ of these follicles to grow, mature, and finally to ovulate, which means the mare is ‘in heat’ and ready to breed. It is during this later period of spring transition that the mare shows irregular signs of being in season. This is a particularly frustrating time for mare owners, brood mare managers and veterinarians. The signs vary markedly between mares and within the same mare at various stages during the transition period. The frustration is more evident the later in the season that a particular mare shows these varying signs of oestrus. Not until they finally ovulate, are they really cycling properly.

From this explanation, it can be seen that ‘Spring Transition’ is NOT a sudden event, and that it is triggered by a change in day length, or period of light.

‘Controlling’ Transition
As transition is a process and not a single event, in order to change it, the most important consideration is to ‘start it early’. To do this, the single most effective ‘treatment’ is to artificially increase the length of the day. This is best done by ensuring there is enough light to read a newspaper - either by natural or artificial light - for at least 14.5 hours each day. Some reports suggest a length of 16 hours of light, however, it is important to remember two things: the extra light must be added at the end of the day; and there must be a period of darkness. The lights must go off, probably for at least 6-8 hours. There is also a ‘pulse’ lighting treatment whereby the lights are turned on for an hour after about 9 hours of darkness. This has the same effect of adding light at the end of the day. It is another form of ‘light treatment’ that is available, and while it is more energy efficient, it is a lot more difficult to instigate.

Because the period of time for transition to occur is about 80 days it is important to ensure that ‘light treatment’ starts as soon as possible after the shortest day in the Southern hemisphere, or Winter solstice, that occurs on about the 22nd June. Typically this means that light treatment, or putting mares ‘under lights’, starts on or before the first Monday in July.

Factors Affecting Transition
Although the main environmental signal is light, there are undoubtedly others that have important roles to play in ‘tricking’ the mare to start cycling. These include temperature, so keeping the mare warm, such as rugging, appears to assist in making her cycle earlier. Ensuring that she is in good body condition, not too thin and not too fat, helps in getting her to cycle sooner in the year. There is also likely to be a ‘green grass effect’, so feeding her good grass in a paddock if possible may help. Factors that decrease stress, such as not continually mixing her with different mares and not travelling her during this time, will help her to cycle sooner.

Drug Treatments
There are a number of drugs that can and have been used to try and ‘trick’ the mare into shortening her transition period, although these have usually had very varied results.

Most treatments appear to be more effective when the mare has already started to show signs of cycling.

Although it may seem frustrating for those wishing to tamper with nature, the reason for the mare not cycling all year is an inherent programming to maximise survival of the species. This means that should a breeder wish to change this evolutionary breeding pattern, it is essential to use nature’s cues that mares normally use to ‘trick’ them into cycling sooner.

Remember that transition is a process, and not a single event, and while drugs may be beneficial in some mares they are probably best thought of as just an aid to dealing with transition.