The mare is a seasonal breeder with a series of estrous cycle normally occurring the breeding season which coincides with long day lengths. The longest day of the year is June 22 and the shortest is December 22. Therefore mares should reach a peak in their reproductive activity around June 22 and be the least reproductively active in late December.

Anestrous period

The period in which the mare is the least reproductively active is termed the “anestrus” period and typically occurs during the winter months. Generally speaking, during this period, the mare will show no signs of estrus with the ovaries being very small and inactive. Some mares will show signs of estrus, but few will ovulate during this period.

Transitional period

As day length increases in the spring, the mare will begin to show signs of estrus beginning around March. However, regular ovulations do not begin until about 30 days after the first period of estrus. These early heats are often erratic and unpredictable lasting for two weeks or longer. During this period, the mare is changing from reproductive inactivity to maximal activity of the breeding season. This change is called the “transitional period”. Ovarian follicles may develop and grow, then regress (decrease in size) without ovulation. Therefore, attempts to breed mares during this transition period can be extremely frustrating and conception rates are low.

Breeding season

The natural breeding season typically begins around mid-April.

Fall transition period

Beginning in late summer and early fall, the mare again will go into a transitional time similar to that seen in the spring. The periods of estrus and ovulations become more erratic and irregular. As winter approaches, the estrous cycle is complete as the mare once again enters into anestrus.

Hair coat and reproductive seasonality

There is a positive correlation between the shedding if the hair in tufts and the first ovulation of the breeding season. The first ovulation os a mare generally occurs two months after she has begun to shed her winter hair in tufts. A smooth, “summer type” hair coat typically does not appear until after the first ovulation. Thus the shedding of hair can be used as an indicator that the ovulatory or breeding season is approaching. Furthermore, the horse appears to use photoperiod, or day length – not temperature, as the signal to change its hair coat from winter to summer and vice versa.

Management of transitional mares

All mares must pass through a transitional phase and anyone breeding mares through this phase realizes how frustrating this can be. Transitional mares typically will have long, erratic periods of estrus (heat) with low conception rates. One management practice that enhance normal estrus cycles is to place mares on exogenous progesterone for 10 to 15 days. Once removed from progesterone, most mares will return to estrus within 3 to 4 days and ovulate 9 to 10 days following progesterone withdrawal. For this practice to work, mares must have sufficient activity on their ovaries (several follicles of at least 20-25mm or larger). Most commonly, progesterone is given orally, marketed under the name – Regumate (altrenogest). Oral progesterone is given at a dosage of .044mg/kg body weight or approximately 1 ml per 110 pounds of body weight.

Kathy Anderson, Extension Horse Specialist, University of Nebraska