It is the period of sexual receptivity during which mating and ovulation occurs in most species and the corpus luteum begins to form
The graafian follicle is large and mature
Ovum undergoes certain maturation changes
There is increasing level of estrogen produced by the synergistic effects of the theca and granulosa cells.
Inhibin and increasing level of estrogen secreted from the graafian follicle inhibit hypothalamic and pituitary gonadotrophin secretion, thus inhibit the FSH release by negative feed back effect
Increased level of estrogen stimulates hypothalamic LHRH release by positive feed back mechanism, thus the ovulatory LH surge before ovulation.
Oviducts – The oviducts are tonic, the epithelium mature and cilia are active, contraction of the oviduct is occurring a close affinity to the Graffian follicle. Increase amount of oviductal fluid is being secreted.
The uterus is erect, turgid edematous. The blood supply to the uterus is increased. The mucosa is rapidly growing and mucus is secreted.
The vagina, vulval and cervical mucosa is pink, congested, due to increased vascularity; the mucus secretion is greatly increased strings of mucus hang from the vulva of the cow form pattern of discharge.
Towards the end of this period, there may be increased leucocytes migrating into the uterine lumen.
In most species the rupture of the mature ovisac or ovulation occurs towards the end of this period of estrum.
Duration of estrus varies from 14 to 18 hours in the cow, 7 to 10 days in mare and bitch
Other external signs – mounting, bellowing, frequent urination, discharge, reduced feed intake and milk yield.