Oestrous Cycle

The oestrous cycle of a cow is generally about 21 days long but can span 17-24 days.

The cycle begins with a cow in oestrus.

At this moment the cow reaches its peak oestrogen secretion, which can last from 6 to 24 hours.

Ovulation generally occurs 24 to 32 hours after the beginning of cattle oestrus.

Ovulation marks the beginning of the luteal phase.

After ovulation, germ cells mature in a process called oogenesis.

They are incorporated into follicles that contain receptors for follicle stimulating hormone (FSH). Follicles grow and mature in follicular waves.

Follicular development and size during the bovine oestrus cycle

Oestrus (day 0 of the cycle) lasts on average 4-24 hours.

  • During oestrus, the cow or heifer is receptive to a bull and stands for mating (standing heat).
  • Ovulation takes place 30 hours after the onset of oestrus and after behavioural signs have ceased.
  • Restlessness
  • Decreased milk production
  • Standing to be mounted
  • Presence of clear mucus
  • Swelling and reddening of vulva

Metoestrus (day 2-4) starts when oestrus ends.

  • A corpus luteum (CL) from the ovulated follicle is formed.
  • Progesterone (P4) levels in circulation begin to rise.
  • P4 exerts a negative feedback on the gonadotropin releasing hormone (GnRH) neurons in the hypothalamus limiting the production of cattle oestrogen.

Dioestrus (day 5-17) is the period of maximum CL size and function.

  • High levels of progesterone in circulation.
  • At the end of dioestrus, luteolysis of the CL begins because of PGF2α produced by the endometrium.

Pro-oestrus (day 17-20) consists of ovarian follicles growing rapidly.

  • The CL of the previous cycle regresses.
  • Progesterone concentrations in circulation drop.
  • The dominant follicle is selected and grows producing increasing amounts of oestradiol.
  • Mucosal layers of vagina and uterus multiply.