Controlling the Equine Estrous Cycle



  • Group mares for breeding with shipped semen
  • Preventing estrus during performance races, shows, and events
  • Preventing groups of mares from coming into estrus when breeding with a single stallion.
  • Stop 'horsey' behavior



  • Prostaglandin (PGF) only works on the mature corpus  luteum, which is about 5 days after ovulation.
  • The prostaglandin binds receptors on the mature corpus  luteum and causes luteolysis.
  • After the administration of prostaglandin to a mare with a mature corpus luteum the mare will come into heat 2-5 days later. 
    • The time to estrus varies depending on follicular development on the ovary. 
    • If a large follicle is present, the response is more variable -the mare may ovulate the large follicle without coming into heat if the follicle is very large and growing at the time of PGF administration, or if it is large but beginning atresia when the prostaglandin is given she may regress that follicle and develop a new wave from which a preovulatory follicle will arise, in which case it will take her longer to come into estrus and longer to ovulate. 
    • Mares with small follicles are somewhat more predictable in their response to PGF.
  • The mare will then ovulate in 7-10 days (ave.).

Lutalyse - Dinoprost tromethamine
  • This is the natural prostaglandin.
  • The dose is 5-10 mg (1-2 cc/mare) IM.
    • As low as 1.25 to 2.5 mg has induced a normal estrus in 11 mares with 32 cycles (Barker. Theriogenology 66 (2006) 686.
  • Side effects include transient sweating and colic.
  • Alternatively, a dose of 0.1 to 0.5 mg (0.1 cc) given twice at a 24 hr interval can be used.  Side effects are avoided and efficacy is just as good

  • This is a synthetic prostaglandin analogue, cloprostenol.
  • The dose is 200 mcg/mare IM.
  • There are fewer side effects to this prostaglandin product.
  • The mare needs a mature corpus luteum, so it will work in the luteal phase only.

A Mature Corpus Luteum seen on the cut surface of the ovary.


  • It will not work to bring anestrus mares into heat!
  • It can be used to "short cycle" a mare, although the time to ovulation is not as predictable as in cows, nor is the amount of time "saved"
  • It can be use to terminate a prolonged diestrus
  • It will abort a mare in early pregnancy, so make sure they are not pregnant.
  • The induced estrus is normal, and fertility is no better or worse.
  • Administration to a group of cycling mares allows 'synchronization' of estrous for AI, embryo transfer or specific breeding arrangements. The true synchrony is not very good however, because of the variability of the time to estrus, estrus length and ovulation.
  • The drug also allows 'desynchronization', so if natural cover is used you can keep the mares cycles spread out so that breeding can be done over a longer time period.
  • Careful planning is needed to group mares or keep mares out of estrus at a particular time
  • Regumate is an oral progestagen, allyltrenbolone or altrenogest/
  • Administration suppresses estrus by forming an artificial luteal period.
  • The dose is 1 ml/50 kg (1 ml/110 lbs) orally or in feed for 14 days.
  • Heat occurs 4-5 days after withdraw and the mares have normal fertility.
  • The addition of prostaglandin at drug withdrawal helps the synchrony.
  • The drug is fairly expensive, but one of its benefits is that it has no cross reaction with progesterone assays, so the endogenous progesterone can be measured while the drug is being administered.
  • The drug can also used in transitional mares to ease the mare through the transitional period.
  • Can also be used at double dose (1 ml/25 kg, or 2 ml/110 lbs) for 2 days during late estrus in an attempt to delay ovulation

Injectable progesterone
Estrus suppression
  • Progesterone alone does not completely inhibit follicular development and ovulation
  • Injectable progesterone can be used to suppress estrus, but there is a dose dependent response for ovulation suppression. For example, 50 mg/day prevents estrus, but not ovulation; while 100 mg/day prevents estrus and ovulation.

  • Administration of 500 mg repositol (alcohol) every 10 days gives less consistent results for estrus suppression.

  • Commercial products
    • BioRelease P4 LA150
    • Progesterone 150mg/mL in 'biorelease technology vehicle'
    • $160 for 100 ml
    • 10 ml/day (1500 mg) every 7 days for "pregnancy maintenance"
    • ($8/wk)

  • Commercial products for estrus synchronization
  •  P&E (BET Labs)
  • 150 mg progesterone & 10 mg estradiol
  • give daily for 10 d


Estrous Synchronization
  • Estrogen more profoundly suppresses follicular development and ovulation than does progesterone
  • Intramuscular administration of 150 mg/day progesterone and 10 mg/day estradiol 17beta for 10 days, combined with a dose of prostaglandin on the last day of treatment improves estrous synchronization.
  • Addition of 2500 IU hCG when a 35 mm is present further improves synchronization of ovulation.
  • Estradiol 17beta, however is not commercially at BET labs. 

Future Progesterone Products

  • P+ Microspheres Progesterone in microspheres 
    • Inject every 2 weeks 
  • SABER Progesterone in liquid delivery system 
    • Inject every 2 weeks

Alternative Estrus Suppression Techniques


  • Glass Balls



    • 24 mares - 35 or 25 mm glass ball + cloprostenol + ticarcillin
    • 6/12 25 mm expelled (5/6 in 24 hrs)
    • 7/18 - mean 86 days luteal function
    • Biopsy not affected
    • Balls can break down

  • Cattle CIDRS
    • Do not use
    • Vaginitis
  • Cattle implants - have shown to not be effective


  • hCG to induce prolonged CLs

    •  8 days post ovulation with > 30 mm follicle

    • 3,000 IU hCG

    • 3/4 ovulated and had diestrus 50 to 80 days

    • **** 5/9 needed multiple cycles to get 30 mm follicle)***

    • other studies show deslorelin to be ineffecive


  • Oxytocin

    • 60 IU IM BID days 7-14 post ovulation

    • Prolonged CL function (N=6) to 30-40 days

    • SID seemed as effective (5/8 vs 5/7 BID vs 1/7 control)


  • Pregnancy

    • Breed and abort

    • Endometrial cups - prolonged time to estrus

    • Time and expense to breed

    • Stigma of pregnancy and abortion


  • GnRH vaccine

    • Equity- Australia

    • Effective 25-30 weeks

    • Anovulatory, but some still show estrus behavior

    • Some never recover


  •  Ovaricetomy

    • Flank/Midline ovariectomey

    • Colpotomy
      • Vaginal technique using an ecraseur
      • Inexpensive
      • Potential complications
        • Bleed out
        • Remove loop of bowel
        • Eviseration


  • Laparoscopy - equipment and expertise



Inducing Ovulation
  • hCG is a glycoprotein produced by the human placenta (human Chorionic Gonadotropin) and found in the urine of pregnant women.
  • The normal dose to induce ovulation is 1,500-3,000 IU IV once during estrus once a 35 mm follicle is present on the ovary.
  • Ovulation normally occurs within about 36-48 hours. ("80% of mares with a follicle > 35 mm will ovulate within 48 hr")
  • Work has shown that antibodies will develop during a breeding season with repeated doses, but the response to the drug apparently does not diminish.
  • Antibodies are carried into the following year but do not affect the response to hCG
  • Some mares fail to respond after use over several cycles but failure to respond does not correlate with antibody presence or levels


Click to enlarge.
  • Ovuplant is a GnRH analog deslorelin
  • It was offered as a subcutaneous implant that was given when a follicle greater than 30 mm was present on the ovary.
  • Deslorelin is now available in injectable form
  • Ovulation will occur within 38-60 hours. Most people that have used it feel that ovulation occurs about 48 hours and that it is more consistent than hCG.  Published studies show little difference between hCG and deslorelin in percent of mares responding, time to ovulation or conception rate.
  • Since it is only a decapeptide, antibodies are not produced.
  • It may cause persistent anestrus if prostaglandin is administered 7 days later, however this in not a consistent occurrence in all mares.
  • With the deslorelin implants, a prolonged interestrus interval in a fairly high percentage of mares was well documented.  Effects on the interestrus interval with the injectable deslorelin are not yet clear.

contributed by Bruce E Eilts and modified on 11 October 2004 
assisted by Emma Jones 

Equine Index

Next Page


contributed by Bruce E Eilts on 25 September 2012


To  Main

Send an email to ask a question that has not been adequately covered.