Foaling in Cold Weather

As most folks know, some horse breeders like to have their mares foal just after January 1 each year.  While the reasons for doing this are not part of this article, just recognize that there are good reasons for doing this.  The normal gestation length of a pregnant mare runs from 330 to 345 days, and some mares will foal as early as 320 days or as late as 362 days.  To foal in winter months, mares would need to be bred during a time period when they are normally anestrus – winter months with short day lengths.  While the days are growing longer during mid-winter to early spring, mares usually don’t start to cycle until after the vernal equinox, or late March. If a breeder needs to have foals in early January, then something will need to be done to induce mares to cycle in mid-winter.

According to the Merck Veterinary Manual, mares need to be exposed to naturally decreasing day lengths in the fall.  Then, they can be exposed to 16 hours of light per day for 8 to 10 weeks.  This can be done abruptly or can be done gradually over time.  Starting an abrupt lighting program today (Dec.6) would mean that mares could be ready to cycle by mid- to late February. 

Artificial light should be added starting at 4:30 pm and kept on until 11:00 pm each day.  For the gradual approach, add 3 hours of artificial lighting the first week, and add 30 minutes each week until the total light exposure per day is 16 hours.

The supplemental lighting should be added at dusk.  Research has shown that adding light in the morning before dawn is not effective.  A minimum of 10 foot-candles (107 LUX) of incandescent or fluorescent lighting is necessary – enough to read a newspaper by.  Lighting can be done individually in a stall or for a group of mares in a paddock.

Once the lighting program is complete and mares have been induced to start cycling, estrus can also be manipulated by using hormone therapy under a veterinarian’s guidance.  This can be advantageous to spread out the work load for a popular stallion, geographic considerations, or transportation issues. The Merck Veterinary Manual describes these processes as follows:

Ovulation may be synchronized by administering progesterone in oil and estradiol 17β in oil for 10 days, then administer prostaglandin F_2α (PGF_2α) on the 10^th day.  Mares should come into estrus about 3 days after the end of treatment, with 85% of mares ovulating 9 to 13 days after the end of treatment.  Prostaglandin does not induce estrus in anestrus mares.

 PGF_2α can be used to induce estrus in diestrus mares with a corpus luteum that is 5 or more days post ovulation.  Mares should return to estrus in around 3 days and ovulate 9-10 days after treatment, though time to ovulation is variable.  The mare’s ovaries can be examined by palpation or ultrasonography at the time of treatment to optimize the prediction of ovulation.

Use levels of progesterone, estradiol 17β, PGF_2α, and routes of administration recommended by a veterinarian or equine breeding specialist. Side effects of hormone treatments include lower body temperature, increased heart and respiratory rates, muscle cramping, sweating, colic, weakness, ataxia, and trembling.  Usually these symptoms are transient, seen about 15 minutes after treatment and lasting up to an hour post-treatment.

Folks who want to have foals earlier in the year will need to start adjusting day length right away in order to begin breeding in late February, and there are options to synchronize breeding schedules once mares begin to cycle early.  An equine veterinarian can assist with details of hormone treatment if an owner wants to pursue those options further.