Page 499 - Anatomy and Physiology of Farm Animals, 8th Edition

484 / Anatomy and Physiology of Farm Animals

Table 27-1. Average Ages or Times of Reproductive Parameters for Selected Species
VetBooks.ir Animal Onset of Puberty Age of First Service Estrous Cycle Estrus Gestation
336 days
21 days
18 months
6 days
Mare
2–3 years
Cow 1–2 years 1–2 years 21 days 18 hours 282 days
Ewe 8 months 1–1.5 years 17 days 1–2 days 150 days
Sow 7 months 8–10 months 21 days 2 days 114 days

Proestrus During this period, serum estrogens
decrease and progesterone increases. A fully
The first phase (proestrus) of the estrous developed corpus luteum has a notable
cycle is the building‐up phase. During influence on the uterus. The endometrial
this phase the ovarian follicle (under the lining of the uterus thickens; uterine glands
influence of FSH and LH) enlarges and enlarge; and uterine muscles show increased
begins to secrete estrogens. In polyestrous development. The external genitalia return
species, proestrus usually begins within a to their state before estrus as plasma estro-
day or two of regression of the corpus gens decrease.
luteum from the previous cycle.
Estrogens absorbed from the follicles Diestrus and Anestrus
into the blood stimulate increases in
vascularity and cell growth of the tubular Polyestrous animals have a short period of
genitalia in preparation for estrus and inactivity before the proestrus phase of the
pregnancy. Late in proestrus the vaginal next cycle. This is diestrus. Animals with
wall thickens, and the external genitalia long periods between cycles or polyestrous
may increase in vascularity (e.g., swelling animals that stop cycling (e.g., due to change
and redness) in preparation for copulation. in season) enter a long period of inactivity
In some species, the vulva may discharge termed anestrus. For example, sheep have a
mucus late in proestrus. short diestrus while cycling during a breed-

ing season but enter anestrus if pregnancy is
Estrus not established during the breeding season.
During anestrus the uterine tubes, uterus,
Estrus, the period of sexual receptivity, is and vagina shrink and remain small until
primarily initiated by the elevation in the next breeding season.
estrogens from mature follicles just prior
to ovulation. In most domestic species, Puberty
ovulation occurs within a day or two after
the onset of behavioral estrus, which is Puberty in female animals can be defined
about the end of behavioral estrus. Proges- as the first estrus accompanied by ovula-
terone from preovulatory follicles, devel- tion. The endocrine basis for puberty in
oping corpora lutea, or corpora lutea from female animals is the development of the
previous cycles also promotes behavioral hypothalamic mechanisms responsible for
estrus in some species. GnRH release. The adenohypophysis is
capable of releasing FSH and LH before
Metestrus GnRH becomes available to stimulate their
release. Great variations in the timing of
The end of sexual receptivity marks the puberty can be found within a single spe-
beginning of metestrus, the postovulatory cies, depending on climate, level of nutri-
phase dominated by corpus luteum function. tion, and heredity.