I am currently working on a video series for www.equiuniversity.com that details reproduction in the mare. Reviewing historical and current literature for that topic reminded me of some very important facts in breeding horses.
If we take the horse industry as a whole, the breeding efficiency is about 50%. This means we breed two mares to get one foal on the ground. This is very inefficient.
Although there are multiple reasons for this poor performance, one of the major causes is time of year breeding. Because of the universal birthdate of horses in many breeds we try to breed horses “out of the natural breeding season”. Mares are seasonally polyestrous, meaning they have multiple cycles during certain time of the year. They are long day breeders and are affected by the length of daylight. As the days shorten, melatonin production increases and with that the number of mares in estrus decreases. They enter a period of anestrus. Reports in the scientific literature suggest about 20% of mares will cycle even during the period of short day length. Mares living close to the equator may not experience anestrous. They may be polyestrous; they cycle and are capable of breeding bred year-round. In a natural breeding season, most mares would be in estrus around June 21, summer solstice. Conversely, most mares would be in anestrus, a period of reproductive inactivity, around December 21. As the days lengthen, mares go through a spring transition. Although the mare shows signs of estrus (winking, tail raising, etc.) she may not ovulate.
Below is a graphic representation of day length and percentage of mares cycling. The x-axis represents the month. The y-axis on the left represents the number of hours of daylight and the y-axis on the right represents the percentage of mares cycling. This is based on 20% of the mares cycling during the December and January and 95% of mares cycling during June.
In many breeds, the breeding shed opens February 15. This is not the most opportune time to maximize reproductive efficiency. There are ways to improve breeding at this time and are presented in the upcoming video series.
If we can wait and breed mares in late spring and early summer, we can improve reproductive efficiency in our horses. Foals are born when the weather is usually milder if we wait until spring to breed.