The Estrous Cycle and Induction of Ovulation in Mares

  • Yoon, Min-Jung (Division of Animal Science and Biotechnology, Kyungpook National University)
  • Received : 2012.06.07
  • Accepted : 2012.06.14
  • Published : 2012.06.30


For horse breeders or managers, it is critical to understand the estrous cycle of mares. Breeding of mares cannot be successfully achieved throughout the whole year as mares breed seasonally. Mares are only able to breed when day length is more than 16 h, and this period is known as the breeding season. Their estrous cycle is approximately 21 days with 5-7 days of estrus and 14 to 15 days of a diestrus period. The estrous cycle of the mare is mainly controlled by gonadotropins, which control follicular development and ovulation. Mares exhibit unique ovulatory events which are not observed in other species. A LH surge occurs for several days, with levels of LH reaching their peak after ovulation. The LH level at the time of LH peak is lower than most other species. The unique anatomical structure of the ovaries of mares is known to limit the number of eggs ovulated. Several attempts have been made to develop chemical/hormonal agents which might be used to manipulate the timed ovulation of mares. Agents that have been tested include hCG, native GnRH, Deslorelin (Ovuplant, GnRH-agonist), Buserelin (GnRH analogue), equine pituitary extracts and equine chorionic gonadotropin (eCG or PMSG). However, the function, purity or stability of these agents is not reliable. Recombinant equine LH, an alternative agent for the timed ovulation, has been developed and tested for its biological activities, through the use of both in vitro and in vivo experiments. The reLH was suggested to be a reliable agent in inducing ovulation within 48 h after being administered through injection, when the size of dominant follicle is 35 mm in diameter.


Mares;Estrous cycle;Follicular development;Ovulation;Timed ovulation


  1. Adashi, E. Y. 1996. The ovarian follicular apparatus. Lippincott- Raven, Philadelphia.
  2. Al-Inany, H. G., Aboulghar, M., Mansour, R. and Proctor, M. 2005. Recombinant versus urinary human chorionic gonadotrophin for ovulation induction in assisted conception. Cochrane Database Syst Rev: CD003719.
  3. Alexander, S. L. and Irvine, C. H. 1987. Secretion rates and short-term patterns of gonadotrophin-releasing hormone, FSH and LH throughout the periovulator yperiod in the mare. J Endocrinol 114:351-362.
  4. Allen, W. E. 1975. Ovarian changes during early pregnancy in pony mares in relation to PMSG production. J Reprod Fertil Suppl: 425-428.
  5. Allen, W. R. and Moor, R. M. 1972. The origin of the equine endometrial cups. I. Production of PMSG by fetal trophoblast cells. J Reprod Fertil 29:313-316.
  6. Andrews, F. and McKenzie, F. 1941. Estrus, Ovulation and Related Phenomena in the Mare. University of Missouri-Columbia.
  7. Angus O. McKinnon and Voss, J. L. 1993a. Equine reproduction. Williams & Wilkins, Media, PA.
  8. Angus O. McKinnon and Voss, J. L. 1993b. Equine reproduction. Williams & Wilkins, Media, PA.
  9. Baldwin, D. M., Roser, J. F., Muyan, M., Lasley, B. and Dybdal, N. 1991. Direct effects of free and conjugated steroids on GnRH stimulated LH release in cultured equine anterior pituitary cells. J Reprod Fertil Suppl 44:327-332.
  10. Barrier-Battut, I. et al. 2001. Use of buserelin to induce ovulation in the cyclic mare. Theriogenology 55:1679-1695.
  11. Becker, S. E. and Johnson, A. L. 1992. Effects of gonadotropinreleasing hormone infused in a pulsatile or continuous fashion on serum gonadotropin concentrations and ovulation in the mare. J Anim Sci 70:1208-1215.
  12. Bergfelt, D. R. and Ginther, O. J. 1985. Delayed follicular development and ovulation following inhibition of FSH with equine follicular fluid in the mare. Theriogenology 24:99-108.
  13. Bergfelt, D. R. and Ginther, O. J. 1992. Relationships between circulating concentrations of FSH and follicular waves during early pregnancy in mares. Equine Vet Sci. 12:274-279.
  14. Brannstrom, M. and Janson, P. O. 1989. Progesterone is a mediator in the ovulatory process of the in vitro-perfused rat ovary. Biol Reprod 40:1170-1178.
  15. Brook, C. G. D. and Marshall, N. J. 1983. Essential Endocrinology. Fourth edition ed. The Blackwell Science Ltd., Bodmin, Cornwall.
  16. Butt, W. R., Crooke, A. C. and Ryle, M. 1970. Gonadotrophins and ovarian development, Livingstone, Edinburgh.
  17. Camillo, F. et al. 2004. Clinical use of twice daily injections of buserelin acetate to induce ovulation in the mare. Vet Res Commun 28 Suppl 1:169-172.
  18. Cole, H. H. and Hart, G. H. 1930. The potency of blood serum of mares in progressive stages of pregnancy in effecting the sexual maturity of the immature rat. Am. J. Physiol. 80:57-68.
  19. Cole, L. A. and Kardana, A. 1992. Discordant results in human chorionic gonadotropin assays. Clin Chem 38:263-270.
  20. Day, F. T. 1939. Ovulation and the descent of the ovum in the fallopian tube of the mare after treatment with gonadotrophic hormones. J. agric. Sci. Camb. 29:459-469.
  21. Day, F. T. 1940. Clinical and experimental observations on reproduction in the mare. J. Agric. Sci. Camb. 30:244-261.
  22. Dennefors, B. et al. 1983. Aspects concerning the role of prostaglandins for ovarian function. Acta Obstet Gynecol Scand Suppl 113:31-41.
  23. Donadeu, F. X. and Ginther, O. J. 2001. Effect of number and diameter of follicles on plasma concentrations of inhibin and FSH in mares. Reproduction 121:897-903.
  24. Donadeu, F. X. and Ginther, O. J. 2002. Changes in concentrations of follicular fluid factors during follicle selection in mares. Biol Reprod 66:1111-1118.
  25. Duchamp, G., Bour, B., Combarnous, Y. and Palmer, E. 1987. Alternative solutions to hCG induction of ovulation in the mare. J Reprod Fertil Suppl 35:221-228.
  26. Evans, J. W. 2001. Horses. 3th ed. Henry Holt, New York.
  27. Evans, M. J. and Irvine, C. H. 1975. Serum concentrations of FSH, LH and progesterone during the oestrous cycle and early pregnancy in the mare. J Reprod Fertil Suppl: 193-200.
  28. Farquhar, V., PM, M., DK, V. and EL, S. 2000. Efficacy of the GnRH agonist deslorelin acetate for inducing ovulation in mares relative to age of mare and season. J Equine Vet Sci. 20:8-11.
  29. Farquhar, V. J., McCue, P. M., Nett, T. M. and Squires, E. L. 2001. Effect of deslorelin acetate on gonadotropin secretion and ovarian follicle development in cycling mares. J Am Vet Med Assoc 218:749-752.
  30. Filicori, M. and Cognigni, G. E. 2001. Clinical review 126: Roles and novel regimens of luteinizing hormone and folliclestimulating hormone in ovulation induction. J Clin Endocrinol Metab 86:1437-1441.
  31. Fleury, P. D. C., Alonso, M. A., Alvarenga, M. A. and Douglas, R. H. 2004. Intervals to ovulation after treatment with oestradiol cypionate (ECP) or biorelease deslorelin (BRT-DES). In: Proceedings of the 6th International Symposium on Equine embryo transfer, Rio de Janeiro, Brazil.
  32. Gastal, E. L., Gastal, M. O., Bergfelt, D. R. and Ginther, O. J. 1997. Role of diameter differences among follicles in selection of a future dominant follicle in mares. Biol Reprod 57: 1320-1327.
  33. Ginther, O. J. 1979a. Reproductive biology of the mare Madison, Wisconsin.
  34. Ginther, O. J. 1979b. Reproductive biology of the mare, Madison, Wisconsin.
  35. Ginther, O. J. 1993. Major and minor follicular waves during the equine estrous cycle. Journal of Equine Veterinary Science 18-25.
  36. Ginther, O. J., Beg, M. A., Bergfelt, D. R., Donadeu, F. X. and Kot, K. 2001. Follicle selection in monovular species. Biol Reprod 65:638-647.
  37. Ginther, O. J., Beg, M. A., Gastal, M. O. and Gastal, E. L. 2004. Follicle dynamics and selection in mares. Anim Reprod. 1: 45-63.
  38. Ginther, O. J., Whitmore, H. L. and Squires, E. L. 1972. Characteristics of estrus, diestrus, and ovulation in mares and effects of season and nursing. Am J Vet Res 33:1935-1939.
  39. Ginther, O. J., Woods, B. G., Meira, C., Beg, M. A. and Bergfelt, D. R. 2003. Hormonal mechanism of follicle deviation as indicated by major versus minor follicular waves during the transition into the anovulatory season in mares. Reproduction 126:653-660.
  40. Guraya, S. S. and Dhanju, C. K. 1992. Mechanism of ovulationan overview. Indian J Exp Biol 30:958-967.
  41. Hughes, J. P., Stabenfeldt, G. H. and Evans, J. W. 1972. Estrous cycle and ovulation in the mare. J Am Vet Med Assoc 161: 1367-1374.
  42. Irvine, C. H. and Alexander, S. L. 1994. The dynamics of gonadotrophin-releasing hormone, LH and FSH secretion during the spontaneous ovulatory surge of the mare as revealed by intensive sampling of pituitary venous blood. J Endocrinol 140:283-295.
  43. Irvine, D. S., Downey, B. R., Parker W. G. and Sullivan, J. J. 1975. Duration of oestrus and time of ovulation in mares treated with synthetic Gn-RH (AY-24,031). J Reprod Fertil Suppl: 279-283.
  44. Jablonka-Shariff, A. et al. 2007. Expression and bioactivity of a single chain recombinant equine luteinizing hormone (reLH). Theriogenology 67:311-320.
  45. Jochle, W. 1995. [Control of ovulation in the mare with Ovuplant (short-term release of the GnRH analog deslorelin acetate). Overview of investigations from 1990 to 1994]. Tierarztl Prax 23:381-393.
  46. Johnson, A. L. 1986. Induction of ovulation in anestrous mares with pulsatile administration of gonadotropin-releasing hormone. Am J Vet Res 47:983-986.
  47. Johnson, C. A., Thompson, D. L., K. M. K. Jr. and Guitreau, A. M. 2000. Prolonged interovulatory interval and hormonal changes in mares following the use of OvuplantTM to hasten ovulation. J. Equine Vet. Sci. 20:331336.
  48. Kilicarslan, M. R. et al. 1996. Effect of GnRH and hCG on ovulation and pregnancy in mares. Vet Rec 139:119-120.
  49. Lipner, H. and Greep, R. O. 1971. Inhibition of steroidogenesis at various sites in the biosynthetic pathway in relation to induced ovulation. Endocrinology 88:602-607.
  50. McCue, P. M., Farquhar, V. J., Carnevale, E. M. and Squires, E. L. 2002. Removal of deslorelin (Ovuplant) implant 48 h after administration results in normal interovulatory intervals in mares. Theriogenology 58:865-870.
  51. McKinnon, A. O. et al. 1993. Predictable ovulation in mares treated with an implant of the GnRH analogue deslorelin. Equine Vet J 25:321-323.
  52. McKinnon, A. O. and Voss, J. L. 1992. Equine reproduction. Williams & Wilkins, Media, PA.
  53. Meinert, C. et al. 1993. Advancing the time of ovulation in the mare with a short-term implant releasing the GnRH analogue deslorelin. Equine Vet J 25:65-68.
  54. Morehead, J. and Blanchard, T. 2000. Clinical experience with deslorelin ($Ovuplant^{TM}$) in a Kentucky Thoroughbred broodmare practice (1999). J Equine Vet Sci. 20:358402.
  55. Mori, T., Suzuki, A., Nishimura, T. and Kambegawa, A. 1977. Inhibition of ovulation in immature rats by anti-progesterone antiserum. J Endocrinol 73:185-186.
  56. Natraj, U. and Richards, J. S. 1993. Hormonal regulation, localization, and functional activity of the progesterone receptor in granulosa cells of rat preovulatory follicles. Endocrinology 133:761-769.
  57. Park, O. K. and Mayo, K. E. 1991. Transient expression of progesterone receptor messenger RNA in ovarian granulosa cells after the preovulatory luteinizing hormone surge. Mol Endocrinol 5:967-978.
  58. Pinaud, M. A., Roser, J. F. and Dybdal, N. 1991. Gonadotropin releasing hormone (GnRH) induced luteinizing hormone (LH) secretion from perifused equine pituitaries. Domest Anim Endocrinol 8:353-368.
  59. Roser, J. F., Kiefer, B. L., Evans, J. W., Neely, D. P. and Pacheco, D. A. 1979. The development of antibodies to human chorionic gonadotrophin following its repeated injection in the cyclic mare. J Reprod Fertil Suppl: 173-179.
  60. Rowson, L. E. 1971. Egg transfer in domestic animals. Nature 233:379-381.
  61. Squires, E. L., Garcia, R. H., Ginther, O. J., Voss, J. L. and Seidel, G. E. 1986. Comparison of equine pituitary extract and follicle stimulating homone for superovulating mares. Theriogenology 26:661-670.
  62. Stewart, F. and Allen, W. R. 1979. The binding of FSH, LH and PMSG to equine gonadal tissues. J Reprod Fertil Suppl: 431-440.
  63. Sullivan, J. J., Parker, W. G. and Larson, L. L. 1973. Duration of estrus and ovulation time in nonlactating mares given human chorionic gondotropin during three successive estrous periods. J Am Vet Med Assoc 162:895-898.
  64. Tjugum, J., Norstrom, A. and Dennefors, B. 1983. Influence of prostaglandin E2 on proteoglycan synthesis in the human ovarian follicle wall. Prostaglandins 25:71-77.
  65. Vanderwall, D. K., Juergens, T. D. and Woods, G. L. 2001. Reproductive performance of commercial broodmares after induction of ovulation with hCG or Ovuplant (deslorelin). J. Equine Vet. Sci. 21:539-542.
  66. Vidament, M., Arnaud, G., Trillaud-Geyl, C., Duchamp, G. and Palmer, E. 1992. Analogue of GnRH (buserelin) and of PGF2a do not induce ovulation in mares. Proceedings of the 12th International Congress Animal Reproduction and Artificial Insemination, The Hague 4:1927-1929.
  67. Voss, J. L., Pickett, B. W., Burwash, L. D. and Daniels, W. H. 1974. Effect of human chorionic gonadotropin on duration of estrous cycle and fertility of normally cycling, nonlactating mares. J Am Vet Med Assoc 165:704-706.
  68. Wallace, R. A., Squires, E. L., Voss, J. L. and Pickett, B. W. 1977. Effectiveness of GnRH or GnRh analogues in inducing ovulating and shortening estrus in mares. In: 69th Annu Meet Anim Sci. Abstr# 535, Madison WI.
  69. Weissman, A., Lurie, S., Zalel, Y., Goldchmit, R. and Shoham, Z. 1996. Human chorionic gonadotropin: pharmacokinetics of subcutaneous administration. Gynecol Endocrinol 10:273-276.
  70. Wesson, J. A. and Ginther, O. J. 1981. Influence of season and age on reproductive activity in pony mares on the basis of a slaughterhouse survey. J Anim Sci 52:119-129.
  71. Wilson, C., Downie, C. and Hughes, J. 1990. Effects of repeated hCG injections on reproductive efficiency in mares. J Equine Vet Sci. 10:301-308.
  72. Yen, S. S., Llerena, O., Little, B. and Pearson, O. H. 1968. Disappearance rates of endogenous luteinizing hormone and chorionic gonadotropin in man. J Clin Endocrinol Metab 28: 1763-1767.
  73. Yoon, M. J. et al. 2007. The efficacy of a single chain recombinant equine luteinizing hormone (reLH) in mares: induction of ovulation, hormone profiles, and inter-ovulatory intervals. Domest Anim Endocrinol 33:470-479.

Cited by

  1. Evaluation of an intravaginal triptorelin acetate gel for inducing ovulation in mares1 vol.95, pp.8, 2017,
  2. Effect of breed and other animal-related factors on conception rate to artificial insemination with frozen semen in mares in Ethiopia pp.1573-7438, 2019,