Sheep - AgRIS

symptoms, it is difficult to detect estrous in ewe. The duration of estrous in ewes varies from
12-72 h (Taparia 1972; Castilo Roias et al., 1977; Gaillard 1979; Yenikoye ta 1. 1981) with an
average of abot 45 h. Breed of ewe does not affect the duration of estrous. Ewe lambs, usually
have the shortest estrous, older ewes have the longest and the yearlings are intermacdiate. The
duration of the estrous cycle is shorter at the beginning and at the end of the breeding season.
The first ovulation at the onset of puberty is frequenctly not associated with psychic
estrous. It appears that a small amount of progesterione is necessary to trigger psychological
estrus. In the cycling ewes this comes from the corpus luteum of previous cycle. Therefore, the
first ovulation after the ovary is awakened from the quiescent state finds the ovary without an
old corpus luteum to produce the small amount of progesterone necessary to bring about
psychological estrous.
The ovulation takes places about 12 h before the end of estrous Right ovary ovulates more
frequently than the left (60 and 40% respectively).
During estrous swelling and reddening of vulva occurs. Ewes become restless. Pheromones
begin to appear during proestrous. Ewes normally come in heat after the weaning of lamb.
Some nonseasonal breeding ewes may show a post partum estrous a few days after delivery,
but others wait for 4 to 6 weeks.
Two to 3 days before the onset of estrous grafian follicles destined for ovulation begin to
enlarge rapidly and become turgid. The theca interna hypertrophies and the ovum, with
attached cumulus oophorus, moves from the embedded position in the granulosa layer toward
the enlarged fluid filled antrum of grafian follicle. A transparent area appears in the follicle
membrane near the apex when ovulation is impending.
2. Merestrous
The ewe enters metestrous after estrous. This phase lasts for 2 days. The corpus luteum
organises during this period and becomes functional. The progesterone level rises rapidly.
3. Diestrous
Diestrous is the period during which the influence of luteal progesterone on accessory sex
structures predominates. This phase is referred to as the phase of the corpus luteum. Metestrous
and diestrous are referred to as the luteal phase. Large amounts of progesterone enter general
circulation resulting in the development of endometrial growth and also of mammary glands.
The myometrium develops under the influence of progesterone and the uterine glands secrete a
thick viscous material which will nourish the zygote. If a zygote reaches the uterus, the corpus
luteum continues through out pregnancy, but if the ovum is not fertilized, the corpus leuteum
remains functional only until day 12 or 13. The decline of progesterone perrnits a surge of FSH
and growth of another follicle, resulting in initiation of new estrous cycle. In short the estrogen
dominates for 34 days of cycle and progesterone dominates for about 13 days.
4. Proestrous
It is the period after which the corpus luteum fails, when the progesterone level drops, FSH
release stimulates follicle growth and rising estrogen levels lead to estrous. Proestrus and
estrous are of- ten referred to as the follicular phase. Estradiol increases the blood supply to the
tubular genital tract from vulva to the uterine tube and causes oedema. The vulva swells, the
vestibule becomes hyperemic and the glands of the cervix and vagina secrete a serous secretion
which appears as a vagina discharge.
Anterior pituitary and the gonads
The primary target organs of the pituitary gonadotropins are the ovary and testes. The
growth and development of ovarian follicles in mamnis are dependent upon FSH but LH is
essential for follicle maturation. Both FSH and LH are essential for the synthesis of estrogen.
Rising estrogen blood levels suppress release of FSH and facilitate release of LH.
Apparently FSH and LH are secreted continously by the anterior pituitary throughout
estrous cycle, but the proportions and levels of each, change during the different stage of the
cycle.
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