Is There a Shot To Bring Mare Into Heat?

Due to the demands placed upon horses that have to work for a living, there is a wealth of information available regarding how to manipulate the mare's cycle in order to keep her out of heat.

Typical signs that a mare is in season include holding the tail elevated, "winking" (opening and closing) the lips of the vulva, and variable amounts of squatting and squirting of urine and mucus. A mare's level of activity usually slows down a bit, and it is often more difficult to get and hold her attention.¹

Undesirable changes in the behavior and temperament of some mares when they come into heat often necessitate some form of intervention, since these can not only render the mare unable to execute their duties, but sometimes present hazards to humans who interact with them.

Occasionally, however, the owner or breeding manager has a need to bring a cycling mare into heat. Reasons for this can include the availability of a stallion, timing of semen deliveries, particulars of the breeding program schedule, as well as others.

Timing and Tricks

The fact that the horse is seasonally polyestrous makes this species a reproductive challenge for horse owners and equine practitioners, since mares will show several estrous cycles during the breeding season, assuming pregnancy does not terminate estrus. There are two primary methods for inducing heat (ovulation) in the mare; these are manipulation of the photoperiod (light cycle) and chemical manipulation of the mare's estrous cycle. Artificially extending the light cycle by using light sources of sufficient brightness and duration can "trick" the mare's system to suggest that Spring is closer than it actually is.

Properly managed stallions do not typically exhibit sexual behavior unless there is a receptive mare around and will never attempt to breed a mare that is not in season.¹ On the other hand, there are horse owners and managers who maintain that having a stallion in close proximity to a cycling mare definitely can't hurt if one is trying to bring the mare into heat.

Managing the Cycle: Art or Science?

Search the online message boards and forums that have to do with equines, and it won't be long before one runs across the question: Is there a shot I can give my mare to bring her into heat?

At the risk of waxing evasive, the answer to this question has some fairly tight qualifications.

Normally healthy mares that are already cycling can be brought into heat with the use of prostaglandins, but the available literature strongly suggests that follicular activity is monitored prior to administering prostaglandins. These sources maintain that this course is only advisable if there are follicles present of sufficient size (usually at least 20 mm diameter), with higher success rates if these criteria are observed.

The use of human chorionic gonadotropin (HCG) in the presence of a follicle greater than 40 mm in diameter in a mare with estrus 3 days or longer in duration and with uterine edema present can also assist in stimulating ovulation. The use of deslorelin (alone or in combination with HCG) in the presence of a follicles greater than 35mm with similar factors has also been shown to be highly effective in bringing on heat and ovulation.

Progesterone and estradiol ("P&E") have been used with success to regulate follicular activity and ovulation in transitional phase mares. There is a higher success rate if there is uterine edema present and follicles present 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.³

Treatment of seasonally anestrus mares with GnRH has been shown to induce cyclicity,² which would then be followed with a light regimen and chemical manipulation of the mare's cycle as above to induce ovulation.