The Bovine Estrous Cycle
259-264

• The cow is a non seasonal polyestrus species. In other words she continually has estrous cycles all year around.  

• The entire estrous cycle averages 21 days long, but can be as short as 18 days and as long as 24 days.

•   The stages of the bovine cycle are

  • proestrus, 

  • estrus, 

  • metestrus, 

  • diestrus, 

  • Day 0 is considered to be estrus. 

  • Days 1-5 are metestrus.  

  • Days 6-17 are diestrus.  

  • Days 18-20 are proestrus.

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During the late luteal phase, if a pregnancy recognition signal is not present by day 17-18, prostaglandin is released. This casuses luteal regression, which means progesterone declines. With the loss of progesterone inhibition, GnRH rises and causes stimulation of LH and FSH. FSH causes maturation of follicles past the antral stage. The dominant follicle establishes itself and is the main follicle that grows. The growing follicle produces estrogen in the granulosa cells. It is the estrogen that causes the signs of estrus in the cow. Estrogen actually peaks before heat (estrus) and it is the estrogen that causes the final LH release. Inhibin is also produced by the growing follicle and prevents other smaller follicles from growing. This inhibition of smaller follicles assures selection of a single follicle to ovulate. The estrogen causes progesterone receptor synthesis, which allows LH binding to luteal cells. 

Estrus  

During estrus the estrogen and FSH are declining. The LH peaks during standing estrus. This is the time the cow is in 'standing heat', and will stand with all four legs firmly braced to be mounted to be mounted by another cow. Estrus lasts 18-20 hours, but may be shorter in the hot Southeastern summer due to heat stress. The thecal cells start producing progesterone, which inhibits LH and FSH release. Ovulation occurs 12-18 hours after the end estrus.

Metestrus  

Metestrus is 3-5 days long and is the time of luteal development. The corpus hemorrhagicum (CH), which is the 'bloody body' that formed from the follicle after ovulation, is developing into a corpus luteum. However, during metestrus the corpus luteum is not yet mature, but still growing so progesterone is rising. Since the corpus luteum is not mature, there are no prostaglandin receptors on it, thus making luteolysis via prostaglandin impossible. The FSH surge that occurs at this time may choose the first folliclular wave for the next cycle.

 Diestrus     

Diestrus lasts from days 5-17 of the estrous cycle and is the time that the mature corpus luteum ('yellow body') is producing progesterone. Progesterone is produced by large and small luteal cells. The large luteal cells are derived from granulosa cells and the small luteal cells are derived from the thecal cells. The large luteal cells produce most of the progesterone and have the prostaglandin receptors, while the small luteal cells have LH receptors (this makes them capable of progesterone production, but they only produce about 20% of the progesterone in normal circumstances). Small luteal cells can eventually transform into large luteal cells. 

During diestrus there are 2, 3 or 4 waves of follicular growth, depending on the individual cow. The follicles grow, become static for about 2 days and then become decline (become atretic ). In 2 wave cows; wave 1 starts on day 2, wave 2 starts day 11 and it is the second wave that ovulates. In a 3 wave cow; wave 1 starts on day 2, becomes static on days 8-12, and declines on days 12-16; wave 2 starts day 9 and ends day 17; and then wave 3 starts day 16 and eventually becomes the follicle that ovulates. These follicular waves are very important in understanding a cow's response to estrous cycle synchronization. At the end of diestrus, if no pregnancy signal is received by the corpus luteum, the luteolytic cascade starts.

The luteolytic cascade  

If the pregnancy signal [interferon tau (IFN- tau), or also called bovine trophoblastic protein-1 bTP-1 is not received by the corpus luteum, then the prostaglandin that has been synthesized by the uterus is transferred to the ipsilateral corpus luteum by a local utero-ovarian countercurrent exchange mechanism. The prostaglandin then binds to the large luteal cells and causes luteal death via either direct action or a vascular constriction. The prostaglandin also causes release of oxytocin from the large luteal cells, which causes the uterus to release more prostaglandin. This gives a 'fail-safe' mechanism to ensure luteal death and return to estrus for a cow that is not pregnant. Estrogen from the dominant follicle is important in that it induces the uterine prostaglandin synthesis and the uterine oxytocin receptors. Truly an amazing cascade of events!