The Normal Equine Estrous Cycle

  • The normal estrous cycle in the mare is 21 to 22 days long, as defined by the intervals between ovulation. The duration of estrus, however can vary, so the most consistent period in the estrous cycle is the length of diestrus. 
  • Diestrus has a fairly consistent 14 day (hormonal) or 15 day (behavioral) duration.
  • Estrus is the time the mare is "hot" , "in" ,or "standing" It is the "follicular phase", as the overt signs of estrus are attributable to the estrogen production by the follicle on the ovary. 
  • The duration is normally considered to be 7 days, but the duration actually varies with time of year. 
  • The duration of estrus is inversely proportional to day length, which means it becomes shorter at the peak of the cycling season.
  • Signs  
    • The mare will show interest in the stallion, and may actually seek out a stallion when she is in estrus.  

    Stallion is in the window.


    • The mare will move her ears forward, elevate her tail, squat to urinate (termed "breaking down"), evert the clitoris (termed "winking") and accept the stallion for breeding.  
    • Some mares may not show sexual receptivity (See "aberrations of the cycle").
  • Ovulation  
    • Ovulation is often said to occur on about day 5 of cycle, or 1 to 2 days before end the of estrus. 
    • This can be confusing as the duration of estrus changes during the season, and the mare does not base when to ovulate on when she anticipates going out of estrus. 
    • A better way to state when ovulation occurs is that the mare goes out of heat 1 to 2 days after ovulation. This makes much more sense in that after ovulation the follicle is a CH and is not producing estrogen, therefore the mare will not show signs of heat after ovulation.







  • Diestrus is called "cold", "out", or the luteal phase, since it is dominated by progesterone produced by the corpus luteum. 
  • The duration of diestrus is a consistent 14 -15 days. 
  • When counting days in a mare's cycle, it is best to start counting the day the mare goes out of heat and count forward 14 -15 days to predict when she will next come into heat. In this way you can anticipate when the next heat will be better than counting the traditional 21 days.

    • Signs  
      • During diestrus the mare rejects the stallion's advances. She does this by pinning her ears back and kicking at the stallion.
        There is really no metestrus or proestrus during the equine estrous cycle.


        Click here to see a video of mare teasing. 

        Hormonal Events of the Estrous Cycle

    • FSH  
      • FSH is responsible for the growth of new follicles, granulosa cell multiplication and LH receptor induction.
      • The timing of FSH is somewhat odd in that it actually peaks during diestrus. 
      • This causes 1-2 waves of follicular growth. 
      • The first wave usually starts to grow about 9 days after ovulation. This first wave may be the only dominant follicle or another dominant follicle wave may arise.

    • Inhibin  
      • Inhibin is a protein produced by the granulosa cells of the dominant follicle. 
      • Inhibin inhibits further FSH production and therefore inhibits the growth of new follicles.  
      • In a sense, the dominant follicle ensures selection of itself over all other follicles by preventing their growth.
    • LH  
      • LH rises at the beginning of estrus and, unlike in other species, peaks 2 days post ovulation.  
      • In the mare, LH has a long half-life due to the sialic acid residues present.  
      • The final follicular rescue is LH dependent and the secretion of LH is modulated by endogenous oxytocin in estrus. 
      • Repeated oxytocin administration or sexual stimulation may induce an LH peak and thereby enhance the ovulatory surge.

    • Progesterone  
      • Progesterone is from the transformed granulosa cells (or thecal cells in the mare) and rises shortly after ovulation. It is the dominant hormone of diestrus.

    • Prostaglandin  
      • Prostaglandin is produced by the endometrium when there is no pregancy signal by the early embryo. 
      • Prostaglandin enters the systemic circulation via A-V or lymphatic transfer (the mare lacks the local countercurrent exchange mechanism seen in the cow) and causes lysis of the mature corpus luteum. 

      • The corpus luteum is responsive to prostaglandin about 5 days after ovulation  (but may be sooner or later).  
      • If the endometrium is damaged, there may be no prostaglandin production, so the mare may not return to estrus regularly.  
      • Prostaglandin can be released by cervical stimulation, intrauterine infusions, biopsy, culture, or endometritis. This will result in a premature return to estrus.

    contributed by Bruce E Eilts and modified 4 September 2009
    assisted by Emma Jones and Eric Huey 

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    contributed by Bruce E Eilts on 25 September 2012


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