Equis ISSN 2398-2977

Female: prolonged diestrus

Contributor(s): Terry Blanchard, David Dugdale, Graham Munroe, Sarah Stoneham, Madeleine L H Campbell


  • Persistence of the corpus luteum → prolonged diestrus of up to several months.
  • Cause: failure of normal luteolysis, mechanism unclear, possibly presumably due to a deficiency of prostaglandin release from uterus.
  • Signs: delayed/non-return to estrus.
  • Diagnosis: examination of genital tract; plasma progesterone concentration.
  • Treatment: exogenous prostaglandin administration.
  • Prognosis: most mares return to estrus within a few days; the response is variable, however, and some mares fail to respond.



  • Failure of the endometrial release of prostaglandin at the proper time.
  • This may be for unexplained reasons, or may (less commonly) be due to insufficient release of prostaglandin from the endometrium in mares with degenerative endometritis/pyometra.


  • See also Reproduction: management - female.
  • Normal diestrus has an average duration of 16 days, and extends from ovulation to luteolysis.
  • Following ovulation, plasma progesterone concentration increases from 0 ng/ml to c8 ng/ml by day 5. Peak concentration of 5-20 ng/ml is achieved between days 5 and 8 and maintained until days 14-15.
  • Progesterone exerts a negative feedback on gonadotropin releasing hormone (GnRH) which in turns decreases the concentration of luteinizing hormone (LH).
  • Follicle stimulating hormone (FSH) is unaffected and often peaks mid-diestrus.
  • Diestrus is normally terminated when prostaglandin F2 alpha is released from the endometrium in the absence of pregnancy and → lysis of the corpus luteum.
  • A rapid decline in plasma progesterone concentration follows which:
    • Initiates behavioral signs of estrus by 'unmasking' estrogen.
    • Allows an increase in LH (which stimulates ovulation) by reducing the negative feedback from progesterone to GnRH production.
  • A failure of this normal luteolytic mechanism → prolonged diestrus.
In the mare, unlike some other species, follicular activity may continue during diestrus with occasional diestrus ovulation Female: diestral ovulation (though most follicles formed during luteal function do not ovulate).


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Santos V G, Bettencourt E M & Ginther O J (2015) Hormonal, luteal, and follicular changes during initiation of persistent corpus luteum in mares. Theriogenology 83 (4), 757-765 PubMed.
  • Newcombe J R (1997) The incidence of ovulation during the luteal phase from day 4 to day 20 in pregnant and non-pregnant mares. J Equine Vet Sci 17 (3), 120-122 ScienceDirect.
  • Lofstedt R M (1988) Control of the estrous cycle in the mare. Vet Clin North Am Equine Pract 4 (2), 177-196 PubMed.

Other sources of information

  • Lofstedt R M (2011) Diestrus. In: Equine Reproduction. Eds: McKinnon A O, Squires E L, Vaala W E & Varner D D. Wiley Blackwell. pp 1728-1731.