The interval from the first signs of sexual receptivity at Oestrus (heat) to the next estrus is called estrus cycle.
The chain of physiological events that begins at one Oestrus period and ends at next is called as Oestrus cycle.
The cycle is of 20 + 2 days in cows for normal female in quite regular cycles. This cycle may be studied in four distinct phases as designated by Marshall i.e. Proestrum, Oestrum, Metestrum and Dioestrum.
This phase is indiction of animal coming in heat. The ovary is surrounded by follicular fluid containing high level of estradiol. The Graafian follicle within ovary grows. The increased level of estradiol is absorbed into blood making effect to oviduct causing growth of cells lining the tube & increasing in the number of cilia which are shortly helpful to transport ova to uterus. Also, epithelial wall of vagina increases in thickness to accommodate smooth coitus this period is of S hrs to 2 days.
This is period of sexual desire. The Graffian follicles are mature or ripe in this stage. This phase period comes to close by rupture of follicle of ovulation i.e. release of Ovum. This period lasts for 12-24 hours in cow while 1-2 days in ewe. The symptoms exhibited during this period by cow are it bellows frequently, mounts other animals, excited, licking to other animals and stands to be ridden by other animals. This period is called period of standing heat. The proper period to breed is 8 to IS hrs, for getting high fertility rate.
Period during which reproductive organs return to normal condition. The phase is of 1-5 days in which the cavity of the Graafian follicle from which ovum had been expelled becomes reorganized and forms new structure known as corpus leteum (C.L.) which secretes progesterone hormone having following functions:
Prevents maturation of further Graffian follicles which prevent occurrence of further estrus period for a time.
It is essential for implementation of fertilized eggs.
It initiates the development of mammary gland.
This is the longest phase of cycle. The corpus leteum is fully grown, making its effect on uterine wall to accommodate the embryo. The muscles of uterus develop. The uterine milk is produced to nourish embryo. If pregnancy succeeds, this stage is prolonged throughout gestation remaining C.L. intact for the period.
In absence of fertilized eggs, the C.L. undergoes retrogressive changes the cell becomes vaculated in the lipid droplets. Since the C.L. got reabsorbed, the level of progesterone is .declined and the level of estradiol increases, bring the animal in heat and the cycle is repeated in case of failure of fertilization.