Advancing the Start of the Breeding Season
As described in the last published article, "Physiology of the non-pregnant mare's reproductive system", mares are seasonal polyoestrians and go through a transition period before starting to cycle regularly.
This transition period takes place in early spring and can last from 2 to 8 weeks. Indeed, after the winter anoestrus, the mare's reproductive system is slowly recovering. During this phase, the mare may show signs of erratic heat without ovulation; but without ovulation, the cycle is not fertile.
During anoestrus, the follicles present on the ovaries measure on average less than 10 to 20mm in diameter. When the follicles grow to 20-25mm, the mare is in transition. In most mares, several follicular growth waves will occur during this transition period until a follicle becomes dominant and eventually ovulates.
For many reasons, many breeders want to start the breeding season as early as possible in the year, ideally starting in February. To do this, there are several methods to shorten the anoestrus and the transition period in the mare.
Artificial photoperiod
The major external factor regulating the reproductive system in the mare is the amount of light she is exposed to (photoperiod). It is actually possible to extend this duration by exposing the mare to artificial light.
Artificial lighting is used to increase the clarity of the day up to 16h. The mare will be exposed to 16h of light then 8h of darkness, that is to say, lights on at 7am and then extinguished at 11pm. Clarity must be continuous so if the mare is out during the day, she must be inside and put under the lights before dusk.
In general, 8 to 10 weeks (60 to 70 days) of continuous artificial photoperiod are required to induce ovulation in the mare. A mare placed under artificial light on December 1st should start cycling normally by February 15th. A fact to note is that the ambient temperature also plays an important role. The colder the climate (like our beloved Quebec winters), the longer it takes for the mare to cycle regularly, even if she is exposed to artificial photoperiod.
A gradual increase in the duration of clarity is not necessary. The use of an electric timer is recommended to ensure optimal control of the lights. The type of light (fluorescent, incandescent, etc.) is not important. On the other hand, the intensity of the light is: 100 Lux (incandescent lamps of 200W or 2 neon tubes per stall). It is advisable to check the light intensity at eye level of the mare with a lux meter. The rule of thumb is that if you are able to read a printed newspaper advertisement anywhere in the mare's stall then the light intensity is adequate. In addition, it is necessary to make sure that the mare can not get her head outside the stall in order to avoid the light.
Once the mare has her first ovulation, it is essential to keep her under artificial lights for at least 40 more days because otherwise she could fall back into anoestrus. In fact, it seems that an interruption of the artificial photoperiod for 3 consecutive days can make the mare fall back into anoestrus.
A new product developed in Ireland has been on the market since 2015: Equilume. It is a mask with an eye cap that produces a blue light with an intensity of 50 Lux on one eye. The mask is placed on the head of the mare, under her halter, and the mare can be kept outside. Its price (between 530 $ and 925 $ US) is high but this product can be worth it to breeders that do not have the necessary facilities in their stables to put in place the artificial lighting.
Hormonal manipulation
In general, no hormonal treatment is as effective as artificial photoperiod to bring an anestrous mare to the transition period and then oestrus. Some treatments are helpful in shortening the transition period and are ideally used in conjunction with artificial photoperiod.
Altrenogest (commercially known as Regu-Mate) is a synthetic progesterone often used in this case. This product is administered orally once a day for about 15 days and a dose of prostaglandin in the form of intramuscular injection is often given in conjunction on the last day of treatment. The mare then falls in oestrus usually 3 to 5 days later. Attention, this treatment is not miraculous: the mare must already be at the end of the transition period so that it can be effective.
An ovulation-inducing drug, such as hCG or deslorelin, can be used in a mare that has a dominant follicle measuring more than 35mm in diameter and uterine edema on ultrasound, so again at the end of the transition period. These products will cause ovulation of the follicle and thus the beginning of regular cycles in the mare.
