The mare is a seasonal polyestrus animal. This means that during the breeding season -- spring and summer -- a non-pregnant mare will have recurring reproductive, or estrous, cycles.
As daylight hours begin to shorten, the mare will go into an anestrus (non-cycling) period over the winter. When the daylight hours begin to lengthen in late winter/early spring, follicular waves will come and go, causing ovarian follicles to grow and regress without ovulation. This period is called the transitional period, and it can last until the first ovulation occurs in March or April.
Due to man-made time constraints with breed registries, many breeders want to speed up the onset of the breeding season to have foals born as close to January 1 as possible. So how do we match up the natural breeding season with the man-made operational breeding season? Below, I will describe a few ways we are able to manipulate the estrus period of mares in order to hasten the onset of the first ovulation of the season.
The natural (physiologic) breeding season can be made to fit the operational breeding season by using artificial lighting to increase the the period of light exposure for mares. To do this, a combination of artificial and natural light lasting 14 to 16 hours should be used to induce cycling in non-cycling mares. Artificial lighting programs require a minimum of 8 to 10 weeks for the mares to respond, so they should start by mid-November or December 1 in order to establish normal cyclic activity by mid-February.
There are a few lighting techniques that have been used successfully:
- Using a light source that is held steady for 14 to 16 hours a day throughout the entire stimulation period.
Increasing light by small increments (similar to what happens naturally with increasing daylight), usually adding 30 minutes of light stimulation a week until 14 to 16 hours of light is achieved.
Providing a 1-hour pulse of light 18.5 hours after the onset of daylight by using a light on a timer.
In order to set up lighting systems in individual stalls, it's recommended that the mare be within 7 to 8 feet of a 200-watt light bulb. Also, the stall should have a window to allow the same amount of exposure during daylight hours. It should be bright enough in the stall that you can easily read a newspaper. Shadows can inhibit the success of the lighting program, so the source should be positioned to minimize them.
It's also recommended that pregnant mares be exposed to lights, as those that aren't could be at risk for post-partum anestrus. There is no consistency in whether or not a mare enters anestrus after foaling from year to year, so it's a good idea to use lighting programs for all mares that are to be bred.
EquiLume™, a newer product on the market, is a mask that provides a blue light to a single eye and can be used on each mare for an artificial lighting program. This eliminates the need for setting up a system in the barn or paddock area, and it may be more cost-effective for some breeding programs. The mask is automatic and has a battery life of 5 months.
Dopamine D2 Antagonists
Experiments have shown that inhibiting the dopamine activity of mares in late anestrus can shorten the interval to the first ovulation of the year. D2 receptor antagonists such as sulpiride and domperidone have traditionally been used to stimulate prolactin secretion and treat the inability to produce milk (agalactia), but they have also been investigated for the potential to advance the ovulatory season.
Treatment with sulpiride on its own was shown to advance the time of first ovulation by a month, but the average treatment time was 41 days (that’s an intramuscular injection twice daily every day!). Using a combination of artificial lighting with a D2 receptor antagonist has more promising results. With the combination treatment, mares are subjected to 14.5 hours of light and then, if ovarian follicles are 15 mm in size or greater, started on twice daily IM injections of sulpiride two weeks later. This scheme, illustrated below, advanced the first ovulation of the year in a majority of mares 17 days earlier than mares on an artificial lighting program that were not given sulpiride.
Progestogens and Estradiol
Although its main advantage is to help synchronize mares into ovulating around the same time, progestogen treatments, with or without estradiol, are commonly used to shorten the duration of the late transition period. The most common treatment regime consists of daily progesterone administration, whether it is an injection of P4 in oil or oral administration of Regumate®, a synthetic progestogen, for 10 to 15 days. For best results, the mares should be examined to ensure that they are in the late transition period, with multiple follicles of 25 mm or greater in diameter present on the ovaries. Mares with smaller follicles in the early transition period are unlikely to respond. Intervals to estrus after ending treatment vary, but it generally occurs in 4 to 7 days, with ovulation occurring 7 to 12 days later.
The addition of estradiol to a progestogen regime allows for greater suppression of follicular development than with progestogen alone, which results in less variation in the follicular maturation and ovulation process. A progestogen/estradiol regime causes greater synchronicity in mares ovulating on a given day. A general treatment regimen is giving 10 days of progestogen/estradiol (P/E), then giving an injection of prostaglanding F2alpha (Lutalyse® or Estrumate®) on the final day in case some mares in the group have ovulated. An ovulation-inducing drug (Sucromate™) is given to mares once they have grown a dominant follicle of greater than 35 mm. This regimen can also be used at the end of an artificial lighting program and appears to have an additive effect on inducing estrous cycles.
GnRH and GnRH Analogs
Administration of exogenous gonadotropin releasing hormone (GnRH), which is normally made in the hypothalamus, or one if its agonists have been used to help advance the onset of the first ovulation in mares. In reviewing the use of GnRH regimens to induce ovulation, it was found that:
Pulsatile delivery is the most effective method.
The percentage of mares ovulating due to treatment increases as day length increases and the diameter of the largest follicle increases.
Not all mares with small follicles respond to treatment.
Some mares that respond to treatment with initial ovulation may revert to anestrus if they do not become pregnant.
Some mares treated when only small follicles are present may be prone to embryonic loss, most likely due to lower progesterone production from the corpus luteum. This is due to low luteinizing hormone production from the pituitary gland.
Native GnRH and buserelin, a GnRH agonist, seem to have the best results in inducing ovulation in 11 to 14 days after twice daily administration. However, more study in this area is needed.
Purified equine follicle stimulating hormone (eFSH) was first developed for use in producing multiple ovulations in mares for embryo transfer purposes. This has encouraged interest in using eFSH for stimulating follicular growth in anestrous or late transitional mares.
A study conducted in Colorado gave twice daily injections of eFSH to mares with 25 mm follicles in February, after being under an artificial lighting program since December. 80% of these mares ovulated in approximately 8 days, which was an average of one month earlier than the untreated control mares. Similar results were obtained in a Brazilian study using mares that had not been under lights, with the period before the first ovulation being shortened by approximately 2 weeks.
Multiple ovulations can occur in mares that are on an eFSH regimen. Therefore, use in mares that are carrying their own foals can be problematic in that multiple pregnancies can occur, and mares need to be very carefully monitored in order to terminate the extra pregnancies. In mares intended for embryo transfer, however, the multiple ovulations are advantageous in that several embryos can be retrieved and transferred in one flush.
If you are interested in implementing an estrus manipulation strategy for your breeding mares, please call Dr. Kiser for a consultation!