ISSN 2398-2977      

Female: estrus suppression


Synonym(s): Induction of diestrus, Prolonging of luteal phase, Suppression of ovarian follicular activity


  • Cause: certain mares display unwanted behaviors during estrus, which may detract from the mares’ competitive performance, or make them difficult to manage/ride.
  • Signs: behaviors consistent with a mare being in estrus include; ‘winking’ the vulva, frequent urination, refusal to move away from or posturing towards other horses, signs of abdominal discomfort and back pain (which may be associated with ovulation), and an apparent object to being ridden. Successful suppression of estrus will abolish these signs.
  • Diagnosis: owners frequently blame behavioral problems on ‘being in season (estrus)’, but this is not always accurate. It is therefore important to determine, initially by a combination of ultrasonography and serum progesterone analysis that episodes of behavioral problems do indeed correlate with the mares’ estrous cycle. Clients are advised to keep a diary of their mare’s behavior to see if there is a correlation with cyclic activity. It is important to rule out other issues that could result in poor performance such as musculoskeletal, gastrointestinal, urinary tract and behavioral issues.
  • Treatment:
    • Administration of exogenous progesterone/progestins.
    • Prolonging luteal function (repeated oxytocin injections; injection of human chorionic gonadotrophin during diestrus; insertion of an intrauterine device (‘marble’); intrauterine infusion of plant oils; establishing pregnancies and manual reduction of embryos after endometrial cup formation to induce pseudopregnancy (this treatment will not be further discussed, as the author considers it to be unethical).
    • Suppressing ovarian follicular activity (Gonadotrophin releasing hormone (GnRH) vaccines, GnRH analogues).
    • Ovariectomy.
The only medicine currently licensed for oestrus suppression in mares in the UK is the synthetic progestin altrenogest, administered orally.
  • Prognosis: 
    • Treatments have variable success in terms of estrus suppression. Many treatment methods have adverse side effects (see below). Ovariectomy not only results in permanent loss of reproductive function but also may not eliminate unwanted behavior in certain mares. It is important to understand that seasonal anestrus and ovariectomized mares may display paradoxical estrus behavior due to hormone secretion from the adrenal cortex.
    • Suppression of estrus, particularly using medicinal methods, but also using covert ovariectomy and possibly some non-medicinal methods may not be allowed by some competition regulatory authorities – always check this before initiating treatment, and make sure that owners are advised of this fact, and that a written record of owners having been advised accordingly is kept.


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Further Reading


Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Campbell M L H, Hampshire D, Hamstead L E et al (2017) The effects of intrauterine infusion of peanut oil on endometrial health, salivary cortisol and interovulatory period in mare. Theriogenology 102, 116-125 PubMed.
  • Hornberger K, Lyman C C, Coffman E & Holyoak R G (2017) Mares behaving badly: a review of methods for estrus suppression in the mare. Clin Theriogenology 9 (4), 583-594.
  • Diel de Amorim M, Nielsen K, Cruz R K S, Card C (2016) Progesterone levels and days to luteolysis in mares treated with intrauterine fractionated coconut oil. Theriogenology 86 (2), 545-50 PubMed.
  • Vanderwall D K, Parkinson K C & Rigas J (2016) How to Use Oxytocin Treatment to Prolong Corpus Luteum Function for Suppressing Estrus in Mares. J Eq Vet Sci 36, 1-4 VetMedResource.
  • McConaghy F, Green L, Colgan S & Morris L (2016) Studies of the pharmacokinetic profile, in vivo efficacy and safety of injectable altrenogest for the suppression of oestrus in mares. Aust Vet J 94 (7), 248-255 PubMed.
  • Katila T (2015) Techniques to suppress oestrus in mares. Eq Vet Ed 27 (7), 344-345 VetMedResource.
  • Schulman M L, Botha A E, Muenscher S B, Annandale C H et al (2013) Reversibility of the effects of GnRH‐vaccination used to suppress reproductive function in mares. Equine Vet J 45 (1), 111-113 PubMed.
  • Gee E K, Gillespie L & Bolwell C F (2012) Effect of oxytocin on suppression of oestrus in mares exhibiting normal oestrous cycles. N Z Vet J 60 (3), 189-193 PubMed.
  • Wilsher S & Allen W R (2011) Intrauterine administration of plant oils inhibits luteolysis in the mare. Equine Vet J 43 (1), 99-105 PubMed.
  • Argo C M & Turnbull E B (2010) The effect of intra-uterine devices on the reproductive physiology and behaviour of pony mares. Vet J 186 (1), 39-46 PubMed.
  • Elhay M, Newbold A, Britton A et al (2007) Suppression of behavioural and physiological oestrus in the mare by vaccination against GnRH. Aust Vet J 85 (1-2), 39-45 PubMed.
  • Kamm J L & Hendrickson D A (2007) Clients' Perspectives on the Effects of Laparoscopic Ovariectomy on Equine Behavior and Medical Problems. J Equine Vet Sci 27 (10), 435-438 VetMedResource.
  • Hedberg Y, Dalin A M, Santesson M & Kindahl H (2006) A preliminary study on the induction of dioestrous ovulation in the mare – a possible method for inducing prolonged luteal phase. Acta Vet Scand 26, 48:12 PubMed.
  • Stout T A E & Colenbrander B (2004) Suppressing reproductive activity in horses using GnRH vaccines, antagonists or agonists. Anim Reprod Sci 82-83, 633 PubMed.
  • Nie G J, Johnson K E, Braden T D & Wenzel J G W (2003) Use of an intra-uterine glass ball protocol to extend luteal function in mares. J Eq Vet Sci 23 (6), 266-273 VetMedResource.
  • Fitzgerald B P, Peterson K D, Silvia P J (1993) Effect of constant administration of a gonadotropin-releasing hormone agonist on reproductive activity in mares: preliminary evidence on suppression of ovulation during the breeding season. Am J Vet Res 54 (10), 1746-51 PubMed.
  • Hooper R N, Taylor T S, Varner D D, Blanchard T L (1993) Effects of bilateral ovariectomy via colpotomy in mares: 23 cases (1984-1990). JAVMA 203 (7),1043-6 PubMed.

Other sources of information

  • Vanderwall D K & Nie G J (2011) Estrus Suppression. In: Equine Reproduction. 2nd edn. Eds: McKinnon A O, Squires E L, Vaala W E & Dickson D V. Wiley Blackwell, USA, pp 1845-1853.

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