The physiologic breeding season can be successfully manipulated to fit into the operational breeding season by artificially increasing the photoperiod. The minimal length of light exposure necessary has not been critically established, but field experience indicates that 14 to 16 hours of light stimulus (artificial plus natural) per day is adequate. A recent French study provided evidence that high light intensities may not require as many days of lighting to induce cyclicity in anestrous mares and that low light intensities may not be as efficacious in stimulating desired responses. Nevertheless, because lighting programs have traditionally been thought to require a minimum of 8 to 10 weeks for response, mares in the northern hemisphere are exposed to the lighting system by December 1 to establish normal cyclic activity by mid-February. Various methods of light administration have been used successfully, the most common of which are (1) use of a light source that is held steady at 14 to 16 hours/day throughout the entire stimulation period; or alternatively, (2) increasing light by small increments (similar to that which occurs naturally), usually by adding 30 minutes of daily light stimulation at weekly intervals until 14 to 16 hours of light exposure is achieved (e.g., 10 hours December 1, 10.5 hours December 8, and so on). After summarizing results of various lighting programs, Sharp et al. (1993) suggested best results are obtained when the supplemental light is either added to the end of the day or split and added to both the beginning and the end of the day, instead of adding the supplemental light only at the beginning of the day. Palmer and coworkers (1982) have described a technique for providing a 1-hour pulse of light 18.5 hours after the onset of daylight. This dark-phase light pulse has reportedly resulted in resumption of reproductive cyclicity similar to that observed with the more traditional lighting techniques. The efficacy of this lighting technique should probably be studied further before being applied to commercial operations.

For individual stall-lighting systems, Kenney et al. (1975) recommended the mare should be within 7 to 8 feet of a 200-watt incandescent light bulb to provide adequate light exposure and the stall should have sufficient window space to permit the same exposure during daylight. Minimal light intensities have not been adequately determined. Sharp et al. (1993) recommend a minimum intensity of 10 foot-candles at mare eye level. The presence of shadows can prevent achievement of desired results, so care should be taken to eliminate them. Paddock lighting systems are also successful if light exposure is sufficient in all areas of the paddock. Guidelines for ensuring adequate light exposure in paddock lighting programs have been reviewed by Ginther (1992). A practical method for measuring light intensity has been described by Sharp et al. (1993) wherein the American Standards Association (ASA) reading of a 35-mm single-lens reflex camera is set to 400 and the shutter speed to 1/4 second. The bottom of a Styrofoam cup is cut off, and the cup is fitted over the lens to gather diffused light. If the aperture reading is f4, light intensity is 10 foot-candles.

With artificial lighting systems, a widely held belief is that pregnant mares should also be exposed to lights because early-foaling mares that are not exposed to lights are thought to be at risk for entering seasonal anestrus. Kentucky workers recently showed that no consistency exists regarding when mares enter anestrus (e.g., an individual mare may enter seasonal anestrus in November of one year and in December or January of the next year; and an individual mare that cycled throughout one year may enter seasonal anestrus the next year), so provision of artificial lighting to all mares to be bred on the farm may be critical if consistent responses to lighting are to be expected.

Although the mechanism of ovarian stimulation remains unclear, evidence shows that inhibition of dopamine activity in mares in late anestrus can shorten the interval to first ovulation of the year. Cornell workers have postulated that dopamine exerts a tonic inhibition of reproductive activity during the anovulatory season of the mare. Dopamine may mediate its effects through prolactin (PRL) secretion (PRL increases during the ovulatory season in relation to increasing day length), which could in turn stimulate expression of ovarian gonadotropin receptors. D2 receptor antagonists (sulpiride, domperidone) have been used to stimulate PRL secretion and are of particular use in the treatment of some forms of agalactia. D2 receptor antagonists have also been investigated for their potential to advance the ovulatory season in mares. Their success when used by themselves has been equivocal except in mares in mid to late transition. For example, Cornell University workers administered 200 mg sulpride once daily to anestrous mares beginning February 5 and were able to advance the time of the first ovulation of the year by more than a month (compared with untreated control mares), but mean treatment length was 41 days. However, recent results from the use of D2 receptor antagonists in combination with a lighting program have been encouraging. Duchamp and Daels (2002) reported that subjecting anestrous mares to 14.5 hours of light on January 10, with administration of sulpiride (1 mg/kg intramuscularly [IM] twice daily) beginning 2 weeks later, resulted in advancement of the first ovulation of the season in 86% of mares (17 days earlier than lighted mares not given sulpiride). One of the authors has used this program for noncyclic race-trained maiden mares arriving at the breeding farm in February or March (Figure 3-1). Mares are immediately placed on an artificial lighting program and are examined for ovarian activity 2 weeks later. If follicles 15 mm in diameter or more are present at this time, twice-daily intramuscular injections of sulpiride (200 mg) are begun. Beginning 5 days later, mares are examined every other day until a follicle achieves 30 mm in diameter. Examinations are daily thereafter, and once the dominant follicle reaches at least 35 mm in diameter and endometrial edema is present, an ovulation-inducing drug (hCG or deslorelin) is given and the mare is bred. For a group of 16 maiden mares treated in this manner during February and March 2006, 10 responded (64% response rate) to treatment and ovulated 13.4 ± 4.2 days (range, 8 to 22 days) after beginning sulpiride administration (treatment was discontinued in the six mares that showed no significant follicular growth after at least 10 days of treatment). Pregnancy rate was 50% (5 of 10) in the mares that responded to treatment, and all of the mares that did not become pregnant continued to have regular estrous cycles thereafter. Of the six mares that did not respond to sulpiride treatment, ovulation did not occur for 2 to 8 weeks.