The Mare's Estrous Cycle

By Jos Mottershead

(These charts and much of the information appearing in this article are reproduced from the Equine-Reproduction.com seminar course notes, at which this topic is dicussed in detail. For more information on the educational seminars, please "click" here)

Terminology

"Estrous" (œstrous in many parts of the world outside North America) refers to the entire cycle;
"Estrus" (œstrus) refers to the "heat" stage of that cycle when the mare is receptive to the stallion's advances;
"Diestrus" (diœstrus) refers to the period in between the estrus phases when the mare is not receptive to the stallion;
"Anestrus" (anœstrus) refers to the compete absence of estrus;
The mare is a "seasonally polyestrus" (polyœstrus) animal, meaning that she undergoes regular estrus cycles during a portion of the year (late spring, summer and early fall) and none at others (winter). This is nature's way of preventing the arrival of a foal during bad weather.

These cycles are controlled by the mare's hormones, which in turn respond to an increase or decrease in daylight duration with the onset of spring or fall, which affects the pineal gland. This study of hormonal activity is known as endocrinology.

It is important to understand that there is a closely linked feedback system between many of the reproductive hormones present in the mare which will alter the level or presence of some hormones as levels of other different hormones increase or decrease. This means that artificially altering a single hormone will be likely to have an effect on one or more of the other hormones. The same can be said of natural hormonal changes - whether they are happening in a correct manner or not. Many of these hormonal changes do occur naturally, but when something becomes unbalanced either naturally or artificially, we can see estrous cycle problems develop in the mare.

Hormonal feedback in the mare


Hormones Active during the Estrous Cycle of the Mare:

A Brief Sequential Overview of the Regular Estrous Cycle

The Hormones of the Mares Estrous Cycle

© 2001 Jos Mottershead and Equine-Reproduction.com
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