Protected light for barn use

Inducing Early Cyclicity in Mares

– What practical solutions are there to start mares cycling earlier in the year? This article discusses some of the options.

By Jos Mottershead
(Originally Printed in: “The Reproduction Forum” in the CSHA Newsletter, 1996; updated 2006, 2015 & 2021)

Q: I want my mare to have an early foal, but she doesn’t cycle early in the breeding season. What can I do to start her cycling earlier in the year, and when should I do it?

A: The mare is seasonally polyestrous. That is, she has multiple heat cycles during certain seasons of the year. In the northern hemisphere, most mares have regular estrous cycles only from late spring until, if she is not impregnated, mid fall. At other times of the year, she may demonstrate no estrus activity at all, or if any activity is shown, it may be erratic and is often anovulatory. It is estimated that 75-85% of mares become anestrus in the winter months, with many of the remaining mares – although showing estrus behaviour – in fact being anovulatory. Year-round estrus behaviour is more commonly seen closer to the equator; in fatter mares; and in mares that are exposed to prolonged artificial lighting. It is not however restricted to those situations1.

As the age of a horse is calculated from January 1st in many breeds and disciplines, the absence of estrus in the mare in the winter and early spring creates a problem for those who wish to breed the mare in February to maximize foal growth in a given year. One solution that has long been recognized by experts as being a sensible and straightforward one is to consider the birthday of all horses in the northern hemisphere as either April or May 1st. This appears, regrettably, unlikely to happen. The alternatives therefore are not to breed earlier than the mare would be naturally receptive, or to manipulate nature, and thereby the mare, into being receptive. So what praxtical solutions are there to start mares cycling earlier?

Phototropic Stimulation

Protected light for barn use to start mares cycling earlier

Incandescent lighting can be adequate to start mares cycling earlier, however the bulbs should be protected from equine access

The major external factor causing the mare to start cycling regularly is the amount of daylight to which she is exposed. This can be artificially extended very simply by using a light source of sufficient power and duration to suggest to the mare that spring is closer than it actually is. It is believed however, from observation of other species, that the animal must go through some period of “short days” prior to entering the photostimulatory period, although what duration is unknown. It should be noted that ambient temperature may play a significant secondary role – not in promoting onset of regular cyclicity, but rather in suppression of same. Mares in a colder climate that are exposed to lengthening daylight duration may be later to start regular ovulatory cycles than mares similarly exposed in a warmer climate2. The form of artificial lighting (incandescent, fluorescent etc.) appears to be immaterial.

How much light is required? The generally accepted amount is that which would be emitted from a 100 watt light bulb, or between about 10 to 12 foot-candles or 107 to 129 lux. You may wish to test your light source and, if you have a single lens reflex camera – either digital or film, you can do so. Set the DIN (ASA) reading to 400 and the shutter speed to one-quarter of a second, and place a plain white Styrofoam cup over the lens. The aperture reading should be f4 or f5.6 if you have sufficient light available. F8 would indicate more than enough, f3.5 insufficient. As a rough rule of thumb though, if there is sufficient light anywhere in the mare’s stall to read the small-print classified advertisements in the newspaper, there is sufficient lighting present.

How long does the light have to be on? Formerly it was believed that up to 16 hours of light (including natural light) was required per day to stimulate the mare. Although it is accepted that a regime of 16 hours of light followed by 8 hours of darkness will indeed work, more recent research has indicated that the duration required may be as little as 14½ hours. It should be noted that with this “extended” (artificial) light, the extension can occur either prior to or after the daylight portion. The critical factor is that there be no reduction of light between the two sources, such as might occur if a horse is put out in the field before full onset of daylight, or brought in late in the day after the onset of sunset. One of two other plans may also be utilized, neither of which require 16 hours of light, and may prove more convenient to the manager.

The first plan consists of extending the natural daylight by 2 to 2½ hours of artificial lighting immediately following sunset. The time that the artificial light is turned on should be adjusted each week so that it keeps pace with the sunset. The second plan is to subject the mares to a 2 hour pulse of artificial light starting 9 hours after the onset of darkness2. Either of these plans can be achieved by the use of a timer on the light or manual management.

If the more standard routine of increasing light duration is to be used, the total light to which the mare is to be exposed should total 14½-16 hours per day. The lighting must be continuous, so if the mare is to be turned out in a paddock through the day, she must be brought in and placed under lights before the natural daylight dims.

When should the lighting program be started to start mares cycling earlier? As the use of an artificial lighting program will not shorten the transitional phase of the mare’s cycle, only advance it’s timing in the calendar, it is essential that the program is implemented sufficiently early to bring about normal estrus when wanted. At least 60 to 90 days of phototropic stimulation are needed prior to the onset of normal cycling. In some mares longer is required. Therefore, if breeding is desired in February, the lighting program should be commenced no later than December 1st. It should also be considered that good breeding practices dictate that any pre-breeding work up is carried out on the heat cycle prior to that upon which the mare is to be bred. If that is the intention, then the lighting program should be started in November.

What happens if the light is turned off in the middle of the treatment period? All indications suggest that as few as three consecutive days of interrupted lighting program can “reset” that program to the “start” position at best, or even cause the mare to lapse back into winter anestrus. It is therefore recommended not only to ensure maintenance of the lighting program once started, but also to maintain the mare on the program until about 40 days after the ovulation upon which she is bred and which results in her being checked in foal, or May 1st – whichever happens first. By maintaining the lighting program in this manner, in the event that the mare does not become pregnant on the first cycle bred, or loses the pregnancy to EED, she is more likely to return to estrus.

It is also interesting to note that stallions are receptive to photostimulation. Although, of course, a fertile stallion remains fertile all year, libido and sperm concentrations are commonly lower in the winter months, increasing in the spring to a maximal level in the height of the natural breeding season (May and June). Caution should be used in subjecting a stallion to lighting program if he usually has a long breeding season, as it has been noted that, unlike the mare, if a stallion successfully undergoes phototropic stimulation, he may be inclined to show a decrease in libido and sperm concentrations earlier in the year than if he is allowed to follow a natural day lengthening cycle. One should therefore be conscious of possible implications to the stallion of a mare’s lighting program is the stallion is in the same barn or location as the mare.

An Equilume hood on a mare being used to start mares cycling earlier

The blue-light Equilume hood can be beneficial to start mares cycling earlier

In 2015, a new lighting product became available – the “Equilume”. This is a device which provides blue light to one eye mounted from within a hood which is worn on the mare’s head. Well-received in some quarters and viewed as potentially money-saving, in other areas its value has been questioned. The cost (about US$500 in 2015) and single-season usability are seen as potentially a downfall by many small operations, however for large-scale farms where daily retrieval and stabling of horses to achieve the adequate light duration involves larger numbers of staff and facility use, or any operation where daily stabling during the dark period is not practical, there are clear benefit potentials. More recently reusable units have become available on the market.

Hormonal Manipulation

Treatment of seasonally anestrus mares with GnRH has been shown to start mares cycling earlier3, but cost is prohibitive and practicality outside a research environment minimal.

The use in anestrus mares of Dopamine antagonists such as Domperidone and Sulpiride has been shown to assist in inducing cyclicity4. Mares must undergo 2 weeks of lighting program followed by 3 more weeks of lighting program, plus receiving the drugs daily. This tends to work more successfully in locations with milder winter climates.

Equine FSH (not currently available in the USA) has shown promise for getting mares to start mares cycling earlier, with twice daily treatment with 12.5 mg eFSH for 15 days resulting in ovulations an average of 11.8 days after commencement of treatment and following hCG treatment. None of the control group animals ovulated during that time period. Animals producing this result were already in transitional phase5. Other (unpublished) research by the same group suggested similar treatment for an average of 5 days (11.3 treatments average) resulted in and average of 2 ovulations at an average of 7.6 days after initial treatment. This compared to a single ovulation an average of 34.9 days after onset of treatment in the control group.

Gonadotropins can be used with some success to promote earlier onset of estrus, or in mares which are not cycling during the regular breeding season. Possible protocols for use in mares with follicles less than 10 mm in diameter and in winter anestrus include:

  • 50 µg Histrelin (Low Dose) BID for 7 to 14 days until ovulation occurs
  • BioRelease Histrelin + P4 LA Injection (1cc IM) SID for 10 days
  • Use BioRelease™ Histrelin 0.5 mg when follicle is 35 mm or greater to induce ovulation

Altrenogest – in the commercial preparation “Regumate™” (Intervet – Millsboro, DE, USA; Whitby, ON, Canada) or biorelease compounded formulation (BET Pharm, Lexington, KY, USA); injectable progesterone; progesterone impregnated intra-vaginal (CIDR) devices; and the compounded hormonal combination of progesterone and estradiol (“P&E”) have been used with success to regulate follicular activity and promote earlier ovulation in transitional phase mares. There is a higher success rate if there is uterine edema present and follicles are 20 mm or greater in diameter, or if there are more than 6 follicles of 10 mm diameter present at the time of treatment. With the BET Pharm Biorelease products (“P4 LA” and Biorelease Altrenogest) follicular diameter equal to or in excess of 15 mm is recommended. These should all therefore be used in conjunction with, not instead of, one of the lighting regimes noted above. Treatment is carried out daily for 15 days, at regular dosage rates, after the mare has been subject to photostimulation for the requisite period. Some practitioners recommend following the final progesterone treatment with a dose of prostaglandin F2 alpha or one of its analogues suitable for equine use as some mares may ovulate while receiving the progestin therapy. Estrus should appear in 3½ to five days. It has been observed in early estrus cycles, in mares that have undergone progesterone treatment, that follicles tend to be somewhat smaller, and estrogen and luteinizing hormone levels tend to be lower than those in mares ovulating naturally later in the season. Consequently, estrus in these early progesterone stimulated cycles is liable to be longer in duration, with ovulation occurring after more days of standing heat than is seen later in the year.

It should be noted that the addition of estrogen to the progestin therapy (i.e. use of P&E) during transitional phase may in rare cases result in an extended anestrous condition. Although such use has been seen to be successful in induction of a timely ovulation during regular seasonal estrus, in our opinion the risk of promoting continued anestrous outweighs the benefits for use in the transitional mare, and we consider the use of P&E prior to the first ovulation of the season to be contra-indicated.

The use of hCG in the presence of a follicle >40 mm in diameter in a mare with estrus >3 days in duration and with uterine edema present may also assist in promoting ovulation. Other ovulation-promoting agents such as Deslorelin, Histrelin or Buserelin may be equally efficacious with a slightly smaller-sized follicle (35 mm). It is important to ensure that such a follicle is indeed growing in size and not regressing, as in that situation the use of hCG or other agents will fail to promote ovulation.


Although a lighting program may easily and cheaply start mares cycling earlier, it should be noted that other factors (lack of sufficient lighting; interruption of lighting; significantly low ambient temperatures etc.) may impact the efficacy. The lighting program may be further enhanced with the suitable use of progestins. A good lighting program is effective in alamost all cases, but must be well-managed to be successful. Other protocols may also be effective, but as with the lighting program, it seems that for maximal success a combination of factors must be reviewed with suitable management in place to achieve a successful outcome.

An additional point that must be considered is that not only will photostimulation and some hormonal treatments hasten the onset of estrus, but they will also hasten the loss of hair associated with the spring moult. This may well make the promotion of early cycling impractical for horses that are kept outside for a major portion of the day, especially in colder climates, if adequate additional blanketing is not available.


1: Ginther O.J., Gastal E.L., Gastal M.O., Beg M.A. 2004. Seasonal influence on equine follicle dynamics. Anim. Reprod. 1;1:31-44
2: Daels P.F. 2006. Management of Spring Transition. Proc. 8th AAEP Annual Resort Symposium
3: Harrison L.A., Squires E.L., Nett T.M. and McKinnon A.O. 1990. Use of Gonadotropin-releasing hormone for Hastening Ovulation in Transitional Mares. J. Animal. Sci. 68:690-699
4: Brendemuehl J.P. and Cross D.L., 2000. Influence Of the Dopamine Antagonist Domperidone On the Vernal Transition In Seasonally Anoestrous Mares. Journal of Reproduction and Fertility Supplement. 56:185-193
5: Squires E.L., McCue P., Niswender K., Alvarenga M. 2003. A Review on the Use of eFSH to Enhance Reproduction Performance. Proc. 49th AAEP