Estrous Suppression

When mares misbehave or behave in ways contrary to the owner’s wishes the reason is often felt to be estrous cycle related.

Common names used for this behavior include:

  • Marish behavior
  • Moody
  • Feisty
  • Pissy mare behavior
  • Mare madness

However, these behaviors often are not the result of cyclic activity.
Most problematic mare type behaviors are the result of:

  • Submissive behavior
  • Urogenital discomfort
  • Stallion like behavior

It is the job of the veterinarian to rule out these other causes of problematic behavior. Some common causes of these behaviors include:

  • Ovarian tumors
  • Pneumovagina
  • Uterine infection
  • Vaginitis
  • Bladder infection

Common Complaints of Estrous Related Behavior Problems:

  • Intense behavioral signs of heat that prevent training and performance
  • Less cooperative
  • Inattentive

Most commonly request for estrous suppression come from trainers and owners of mares engaged in some sort of training or performance.


-Synthetic Progesterone
-Given Orally Daily
-Highly Effective For Suppressing Estrous For The Duration Of Treatment
-Mares Come Back Into Heat When Drug Is Discontinued
-Safe For Long Term Use If Mare Is Reproductively Healthy
-Contraindicated For Use In Mares With Uterine Inflammation


-Inconvenient daily dosing
-Human health risk
-Progesterone in Oil
-Requires daily injections intra muscularly
-Muscle soreness is a complication
-Medroxyprogesterone Acetate

  • Depo-Provera, a human contraceptive
  • Has not been evaluated for use in horses
  • Wide variety of recommended doses
  • Wide variety of therapeutic responses

Hydroxyprogesterone Caproate:

  • Synthetic progesterone
  • Has a very low binding for equine progesterone receptors
  • Low efficacy in the horse
  • Wide range of reported doses and a wide range of reported duration of activity
  • It is reported to have a very poor ability to suppress estrous behavior in the mare.

Cattle Implants

  • Progestin containing implants that are designed for use in food animal to increase weight gain
  • Synovex is the most widely used
  • Once implanted under the skin the dose is slowly absorbed over several months
  • Scientific controlled experiments showed no efficacy for suppressing estrous in mares at 10x the recommended dose.
  • In spite of this evidence many veterinarians and horse owners/trainers continue to recommend its use.


  • Deslorelin
  • Developed as a ovulation induction agent for mares
  • GnRH analogue
  • Implanting a double dose of Ovuplant will suspend estrous in most mares for 30 days or more
  • Work best in spring and summer
  • May be repeated as needed.


IM injection of 60 units Oxytocin twice daily on days 7 & 14 post ovulation blocked luteolysis and prevented mares from coming back into heat in 100% of mares tested for at least 30 days.


  • Placing a glass marble in the mare’s uterus will suppress estrous in some mares
  • Theoretically suppresses estrous by extending the luteal phase of the cycle
  • Developed by a Dutch veterinarian that claimed success rates of 75%
  • Recent studies proved only a 40% success rate
  • When effective the marble will suppress estrous for up to 90days.
  • Protocol:
    • Evaluate the mare’s reproductive health
    • A sterile 35mm diameter marble is placed in the uterus at the end of estrous within 24 hr post ovulation through the open cervix.
    • Marble must be removed from the uterus, some times difficult in mares with pendulous uteri.

GnRH Immunization
Immunization against GnRH will suppress the hormones LH and FSH causing immune mediated estrous suppression
Approved for use in Australia but not US
2 injections required one month apart
98% of vaccinated mares ceased estrous within 4 wk of the second vaccination and ceased cycling for the remainder of the breeding season
90% of the treated mares returned to estrous behavior the following season and had normal pregnancy rates when bred.

What Tucson Horse Owners Need to Know About Equine...
Influencing The Sex Of Offspring

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