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Estrous Cycle

Estrous cycle is recurring period of sexual activity which is accompanied by maturation of one or more follicles on ovary and terminates with shedding of one or more ovum. The estrus cycle on an average is 21 days with the range of 18-24 days.

Cycle is stopped:

  • When female is conceived/ pregnant

  • Any pathological condition

  • Due to unfavourable condition

Estrous Period:

It is the period during which female shows excitement.

Lack of cyclicity or estrus. In some spices it is normal due to physiological changes.

Phases of Estrous Cycle

There are four phases of Estrous Cycle:

  • Proestrus

  • Metestrus

  • Estrus

  • Diestrus

Stage of cycle



Estrus cycle (d)



Proestrus (d)



Estrus (hours)



Metestrus (d)



Diestrus (d)




  • In this period uterus prepares for estrus

  • There is regression of CL due to PGF2α  and follicle starts growing

  • Tonicity of uterus increases

  • Secretary activity increases and reaches its peak on day first of cycle


Estrus is characterized as the time period when the female exhibits sexual desire and acceptance of the male by standing to be mounted.

Internal Changes:

  • Dilation of cervix (diameter is relatively large, closed cervix means diameter is relatively small)

  • Uterine tonicity (mild, moderate, or severe)

  • Grafian follicle on ovary (usually one in bovine).

  • Mucous production which is directly related to hormone (estrogen). It is secreted from secretary cells. So increased estrogen, increased mucous.

  • Change in color of vagina (hyperemia). 

External changes:

  • Vulva is swollen (wrinkles are disappeared)

  • Tonicity is due to increase blood flow to uterus. It is beneficial. This blood flow to the uterus prevent from establishing of infection because blood contains macrophages.

  • Uterus is more prone to infection under the influence of progesterone.

  • If we palpate the ovary, follicle becomes thin and thin as it moves toward the end of estrus.

  • Estrus is only visible phase during which we see the changes in the behavior of animal. Estrus period and signs of estrus are quite variable in bovines. In buffalos it is difficult to detect estrus as they show silent estrus. Buffalos have nocturnal behavior. They are active in night specially late night due to low temperature.

Primary Signs of Estrus: Stand to be mounted, bellowing, restlessness, swollen and reddish vulva, mucous discharge.

Secondary Signs of Estrus: Increased urination, low feed intake, rough hair on back and tail, isolation from other animals, low milk production


  • Period following the estrus is called metestrus.

  • During this period ovulation occurs and ovulation fossa or depression is formed on grafian follicle

  • This depression is gradually filled up by granulosa cells which transfer into luteial cells and later grafian follicle is transformed into carpus luteum (CL). CL is palpatable at 5th day which is soft, conical and reddish in colour.

  • Aggressiveness decreases

  • Tonicity in uterus gradually decreases

  • Mucous secretion is also decreased due to decrease in estrogen because it is directly proportional to the estrogen.

  • Externally wrinkles are appeared on vulva.

  • Color of vagina changes from hyperemic to pale.

  • Animal is reluctant to accept male.


  • It is longest period of cycle.

  • Animal is silent or quiescent.

  • During diestrus CL further grows and become mature on day 9th. A mature CL has depression in centre. Mature CL remains upto 16 day.

  • During this phase there is dominance of progesterone.

  • Organs are flaccid. No tone at all.

  • One follicle matures at 11th day which is called midcycle follicle. At 10th and 11th day we find both structures; CL and mature follicle (mid cycle follicle). Mid cycle follicle subsides and becomes atretic follicle within next one or two days called follicle atrasia.

  • On 17th day regression of CL starts.

Gestational estrus:
During pregnancy signs of estrus are shown by buffalo. It is about 5 % and generally occurs at 5th month of pregnancy due to change in hormone. It is known as gestational estrus. This function is performed by placenta.

Signs of Estrus in Rectal Palpation


o    Slight tonicity of uterus

o    Vulva is slightly swollen

o    Vulva is pink to red

o    Follicle on ovary


o    Graphian follicle

o    Increase in tonicity of uterus

o    Relaxation of cervix

o    Follicle is like blister


o    Ovulation fossa on ovary

o    Decrease in tonicity

o    Post estrus bleeding in cows


o    Corpus luteum (liver like)

o    Flaccid horns, uterus

o    Animal shows resistance for palpation



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