Phases of the Estrous Cycle
The basic pattern of the estrous cycle is the same in all domestic animals, but some species differences are found in specific parts of the cycle. Some specifics about relevant farm species are summarized in Table 27-1.
The estrous cycle may be divided into several phases based on behavioral changes or structural changes in internal and external genitalia. These phases are called proestrus, estrus, metestrus, and diestrus.Proestrus
The first phase (proestrus) of the estrous cycle is the building-up phase. During this phase the ovarian follicle (under the influence of FSH and LH) enlarges and begins to secrete estrogens. In polyestrous species, proestrus usually begins within a day or two of regression of the corpus luteum from the previous cycle.
Table 27-1. Average Ages or Times of Reproductive Parameters for Selected Species
| Animal | Onset of Puberty | Age of First Service | Estrous Cycle | Estrus | Gestation |
| Mare | 18 mo | 2-3 yr | 21 d | 6 d | 336 d |
| Cow | 1-2 yr | 1-2 yr | 21 d | 18 hr | 282 d |
| Ewe | 8 mo | 1-1.5 yr | 17 d | 1-2 d | 150 d |
| Sow | 7 mo | 8-10 mo | 21 d | 2 d | 114 d |
Estrogens absorbed from the follicles into the blood stimulate increases in vascularity and cell growth of the tubular genitalia in preparation for estrus and pregnancy.
Late in proestrus the vaginal wall thickens, and the external genitalia may increase in vascularity (e.g., swelling and redness) in preparation for copulation. in some species, the vulva may discharge mucus late in proestrus.Estrus
Estrus, the period of sexual receptivity, is primarily initiated by the elevation in estrogens from mature follicles just prior to ovulation. in most domestic species, ovulation occurs within a day or two after the onset of behavioral estrus, which is about the end of behavioral estrus. Progesterone from preovulatory follicles, developing corpora lutea, or corpora lutea from previous cycles also promotes behavioral estrus in some species.
Metestrus
The end of sexual receptivity marks the beginning of metestrus, the postovulatory phase dominated by corpus luteum function. During this period, serum estrogens decrease and progesterone increases. A fully developed corpus luteum has a notable influence on the uterus. The endometrial lining of the uterus thickens; uterine glands enlarge; and uterine muscles show increased development. The external genitalia return to their state before estrus as plasma estrogens decrease.
Diestrus and Anestrus
Polyestrous animals have a short period of inactivity before the proestrus phase of the next cycle. This is diestrus. Animals with long periods between cycles or polyestrous animals that stop cycling (e.g., due to change in season) enter a long period of inactivity termed anestrus. For example, sheep have a short diestrus while cycling during a breeding season but enter anestrus if pregnancy is not established during the breeding season. During anestrus the uterine tubes, uterus, and vagina shrink and remain small until the next breeding season.
Puberty
Puberty in female animals can be defined as the first estrus accompanied by ovulation. The endocrine basis for puberty in female animals is the development of the hypothalamic mechanisms responsible for GnRH release. The adenohypophysis is capable of releasing FSH and LH before GnRH becomes available to stimulate their release. Great variations in the timing of puberty can be found within a single species, depending on climate, level of nutrition, and heredity.