Making Equine Ultrasound Safe and Affordable For Breeders

Hormonal Influences in Equine Reproduction

For the intent of this presentation, it is not critical to have an understanding of the intricate control of the reproductive processes in the mare. However, a basic understanding of the major changes will be beneficial in that it allows for the understanding of where the mare is within her cycle.

From a simplistic view, the mare’s reproductive behavior, as well as predictive changes in her reproductive tract, are influenced by circulating concentrations of estrogen and progesterone. During estrus the mare is under the influence of estrogen, while during diestrus she is under the influence of progesterone.


Estrogen is produced by follicles located on the mare’s ovary. As the preovulatory follicle grows and increases in size, increasing amounts of estrogen are secreted. The increase in estrogen is responsible for the onset of estrous behavior (exhibited when the mare is exposed to a stallion). As estrogen concentrations continue to increase, estrous behavior intensifies. As estrogen concentrations begin to decline, estrous behavior “intensity” also declines.

The preovulatory follicle grows steadily at a rate of approx 3 – 5 mm per day, and then slows or decreases in size just prior to ovulation. For most stock type mares, the preovulatory follicle will be 40 – 45 mm at time of ovulation. This varies with season with larger preovulatory follicles early, and smaller sizes later in the season. An important relationship in the mare that should be understood, is that circulating concentrations of estrogen peak just prior to, or at the time of ovulation. Most report that peak estrogen concentrations occur 24 – 48 hours prior to ovulation. This relationship allows managers to better predict ovulation because estrous behavior and reproductive tract changes will mirror the decrease in estrogen. That is, predictable changes occur prior to ovulation.

(Note: follicles on especially large breeds of horse can prove substantially wider in diameter, ultimately increasing in size up into the 50 – 65 mm range.)


Note on “Tease Scores”:
0 – Antagonistic towards stallion, not receptive.
1 – Indifferent to stallion, accepts presence without aggressive behavior.
2 – Receptive to stallion, displays typical signs of estrous behavior. May be slow to react to stallion.
3 – Quick to show signs of being receptive to the stallion
4 – Strongest signs of being in estrus, quick to react, intense, leans into stallion.

Thus, most mares will display a lower intensity of estrous behavior just prior to ovulation. Mares will continue to tease for 1 -2 days post-ovulation. A similar relationship is also seen between uterine edema and estrogen, and actually provides a better indication of impending ovulation. As the mare comes into estrus (as estrogen begins to increase) the uterus exhibits little edema when viewed on ultrasound exam. As she progresses “deeper” into estrus (as estrogen increases) uterine edema increases and peaks at time of peak estrogen. Peak edema is seen as a “wagon wheel” or “star fish” pattern on the ultrasound screen due to the prominence of the uterine folds. These folds begin to diminish 24 – 48 hours prior to ovulation.

Uterine Edema Scores

Uterine edema scores have been developed (Samper et al.,1997) and are listed here:

0 – No edema evident. Very homogeneous appearance across uterine body/horns.
1 – Minor edema.
2 – Moderate edema seen, especially in uterine body.
3 – Obvious edematous folds that extend through body and horns, but lacking extreme definition.
4 – Maximal edema displaying typical wagon wheel /starfish pattern. May see free fluid within lumen.
5 – Abnormal edema pattern. Seen as “unorganized swirls” may appear “turbulent”


If one is artificially inseminating mares or monitoring cervical tone, the same pattern exists in most mares: maximal relaxation of the cervix will coincide with peak estrogen. As estrogen begins to decline, the cervix begins to tighten up. Therefore it is not uncommon for inseminators to feel a “more closed” cervix when breeding a second time closer to time of ovulation.

It becomes apparent that there are number of “informational changes” that can assist managers in determining when to breed:

Day of estrus. Although mares have variable durations of estrus, and are influenced by season (photoperiod), an understanding of normal duration (5 – 7 days) and time of ovulation within estrus (last 1/3) is helpful. Thus, knowing when the mares first displayed estrous behavior is an important piece of information when managing mares.

When breeding mares through teasing alone, breeding begins on day 2 or 3 of estrus depending on the time of the breeding season: early season start on day 3 and breed every other day until mare no longer teases or stands for stallion. If late season, start on day 2. Typically, only two covers are needed in cyclic mares with this approach.

Development and subsequent diminishing of uterine edema is our second piece of information. Breeding begins when deep endometrial folds exist, or at the time they begin to diminish — depending on the type of semen being used (is there a need to limit inseminations to one insemination only?), early or late season, and scheduling issues associated with the stallion.


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