• The average date of the first ovulation of the year in mares maintained under ambient light conditions is MID-APRIL TO EARLY MAY in North America
  • Provision of a stimulatory artificial photoperiod is used to advance the date of the FIRST OVULATION OF THE YEAR
  • The artificial photoperiod should begin on or about DECEMBER 1
  • Approximately 60 TO 70 DAYS of an artificial photoperiod is necessary to stimulate ovulation; if the artificial photoperiod is applied appropriately, mares should ovulate by early to mid-February
  • A minimum of 10 FOOT-CANDLES OF LIGHT are recommended; this may be achieved by use of a 100 or 200 watt light bulb in a 12 x 12 foot box stall
  • Automatic timers may be used to turn the lights ON AT DUSK AND OFF AT 11:00 PM
  • Mares should be allowed an 8 HOUR PERIOD OF DARKNESS; providing 24 hours of light is less effective
  • Maintain the artificial photoperiod until at least APRIL 1 or mares may revert back into anestrus
  • Late-term pregnant mares due to foal in January, February or March may be housed ‘UNDER LIGHTS’ to help ensure that they cycle after foaling and not experience post-partum anestrus


  • Mares in DEEP SEASONAL ANESTRUS have ovarian follicles less than 20 mm in diameter
  • TRANSITIONAL MARES have one or more ovarian follicles greater than 20 mm in diameter
  • Mares may remain in transition for SEVERAL WEEKS
  • Administration of LOWLOSE DESLORELIN can stimulate follicular development in transitional mares
    • A dose of 50 μg of deslorelin is administered intramuscularly twice daily
    • Treatment is discontinued when one or more follicles is at least 35 mm in diameter
    • The follicle is allowed to mature without any hormone therapy for 24 hours (a ‘coast’ period)
    • The mare is subsequently administered hCG to induce ovulation; note that administration of a ‘high-dose’ of deslorelin (i.e. 1.8 mg) is less effective at inducing ovulation in low-dose deslorelin treated mares
  • Low-dose deslorelin therapy is MORE EFFECTIVE IN TRANSITIONAL MARES than mares in deep anestrus
  • The AVERAGE DURATION OF LOWDOSE DESLORELIN THERAPY is 5 days; treatment should be discontinued if no significant follicular development is evident after 10 days


  • It is common practice to administer either HCG (Chorulon®) or DESLORELIN
  • Hormone therapy is generally effective if the mare is in behavioral ESTRUS and/or has endometrial EDEMA visible on ultrasonography and has a dominant follicle at least 35 MM
    in diameter (i.e. in Quarter Horse, Arabian and Thoroughbred mares)
  • Administration of HCG (2,500 IU, IV) will usually induce ovulation in approximately 36 HOURS, whereas DESLORELIN (1.8 mg, IM) treatment typically induces ovulation in approximately 40 HOURS
  • Mares to be bred with frozen semen may be administered hCG or deslorelin to induce a TIMED OVULATION and make breeding management more efficient
    • If two (2) doses of frozen semen are available, one can administor deslorelin at 8:00 am or administer hCG at 12:00 pm (noon) and anticipate ovulation at 12:00 am (midnight), in 40 or 36 hours, respectively. The mare is inseminated the evening before the anticipated ovulation and the morning after ovulation is confirmed
    • If one (1) dose of frozen semen is available, one can administer deslorelin at 8:00 pm and anticipate ovulation at 12:00 pm (noon) 40 hours later and inseminate post-ovulation
    • In both scenarios, ultrasound examinations are performed at specified time intervals after hormone therapy in case of an early ovulation


  • All mares experience UTERINE INFLAMMATION after either live cover or artificial insemination
  • Inflammation PEAKS AT 8 TO 12 HOURS and is usually resolved by 24 hours in normal mares
  • Endometritis may PERSIST for more than 24 hours in some mares
  • THERAPY for persistent mating-induced endometritis (PMIE) includes:
    • Uterine lavage to remove excessive fluid, inflammatory cells, dead sperm and debris:
      • The uterus is lavaged with sequential one-liter volumes of either sterile saline or lactated Ringer’s solution until the effluent fluid is clear
    • Administration of oxytocin or prostaglandins
      • Oxytocin will stimulate uterine contractions for 30 to 45 minutes
      • Cloprostenol (Estrumate®) will stimulate uterine contractions for 2 to 4 hours
  • PREVENTIVE MANAGEMENT procedures on each subsequent cycle may include:
    • Administration of dexamethasone (30 to 50 mg, IV) once at the time of breeding
    • Only breeding the mare one time to limit re-inflammation
    • Performing a uterine lavage 4 to 6 hours after breeding
    • Prophylactic administration of ecbolic agents beginning 4 to 6 hours after breeding


  • DIAGNOSIS of bacterial endometritis is based on:
    • Culture of a pathogenic organism from a uterine swab or low-volume lavage:
      • The most common bacterial organisms are Streptococcus equi subsp. zooepidemicus, Escherichia coli, Pseudomonas aeruginosa and Klebsiella pneumoniae
    • Presence of inflammatory cells (neutrophils) and possibly bacteria on uterine cytology
    • Detection of bacterial DNA using PCR analysis
  • TREATMENT includes:
    • Uterine lavage to remove bacterial organisms and inflammatory debris
    • Infusion of an antimicrobial agent into the uterus for 3 to 5 consecutive days in estrus
      • Selection should be based on results of antimicrobial susceptibility testing
    • Empirical choice may include:
      • Naxcel® or Timentin® as the only agent
      • A combination of a beta-lactam antibiotic (i.e. penicillin)
        plus an aminoglycoside
  • Systemic antibiotics may also be considered; common options include:
    • Ceftiofur crystalline free acid (Excede®); enrofl oxacin (Baytril®); trimethoprim-sulfamethoxasole


  • Diagnosis of fungal endometritis is based on:
    • Culture of a pathogenic organism from a uterine swab
      or low-volume lavage
      • The most common fungal organisms are the yeast Candida albicans, and hyphate fungal organisms Aspergillus fumigatus and Mucor sp.
    • Presence of inflammatory cells (neutrophils) and possibly fungal organisms on cytology
    • Detection of fungal DNA using PCR analysis
  • Treatment includes:
    • Uterine lavage to remove fungal organisms and inflammatory debris
    • Infusion of an antifungal agent into the uterus for 3 to 5 consecutive days in estrus
      • Our current choice for intrauterine infusion is nystatin
    • Systemic administration of an antifungal agent for 2 to 3 weeks
      • Fluconazole
  • A COMBINATION of a systemic antifungal agent and an intrauterine antifungal agent may be more effective than either treatment alone in challenging cases
  • It is common to subsequently culture the bacterium S. equi subsp. zooepidemicus after successful elimination of the fungal infection


  • Progesterone is a steroid hormone required for MAINTENANCE OF PREGNANCY
  • The vast majority of pregnant mares do not need progesterone supplementation
  • In some clinical cases, progesterone supplementation should be considered in a pregnant mare:
    • Small corpus luteum and/or evidence of uterine edema on ultrasound
    • Progesterone level less than 4.0 ng/ml
    • History of repeated pregnancy loss
  • Treatment protocols include:
    • Administration of altrenogest (0.044 mg/kg, PO, q 24 hr)
    • Administration of long-acting progesterone once per week
  • Options for discontinuing supplementation include:
    • Endogenous progesterone greater than 4.0 ng/ml in a mare treated with altrenogest
    • Ultrasound evidence of secondary corpora lutea formation (by approximately day 50 to 60)
    • Treatment until at least 90 to 120 days, at which time the placenta is producing sufficient progestagens to maintain pregnancy
  • ​​​​​​​NOTE: altrenogest is not detected in most traditional progesterone assays


  • In the event that a young foal is orphaned by the unexpected death of its dam, LACTATION CAN BE INDUCED in another mare that may serve as a nurse mare
  • The potential nurse mare must have GIVEN BIRTH AND LACTATED PREVIOUSLY, have good maternal instincts and have a gentle disposition
  • Lactation can be stimulated by a combination of ESTRADIOL 17ß and a DOPAMINE ANTAGONIST such as domperidone or sulpiride; lactation can also be stimulated by administration of a dopamine antagonist alone
  • Therapy for induction of lactation:
    • Pre-treatment with estradiol-17ß(3 to 5 mg) for 2 to 3 days
      • The goal is to enhance the prolactin response to domperidone therapy
    • Administration of domperidone (Equidone®); 1.1 mg/kg
      (approximately 5 mls of Equidone® gel) once daily for
      5 to 10 days
    • Begin hand-milking once lactation begins
    • Carefully introduce the foal to the nurse mare
  • ​​​​​​​NOTE: mares induced to lactate do not produce colostrum; an orphaned neonatal foal will need to be provided with an alternative supply of colostrum, a colostrum substitute or hyperimmune plasma

Diagnosis and treatment of placentitis

  • Ascending bacterial PLACENTITIS is one of the most common infectious causes of abortion
  • External CLINICAL SIGNS may include premature lactation, vaginal discharge or abortion
  • Diagnostic techniques include:
    • Ultrasonographic evaluation of the placenta per rectum to evaluate placental thickness (i.e. combined thickness of the uterus and placenta, CTUP)
    • Detection of placental separation from the uterus
  • Therapy for placentitis includes:
    • Systemic antibiotics
      • Trimethoprim-sulfamethoxasole (30 mg/kg, PO q 12h), twice daily for the duration of pregnancy
    • Progestin supplementation
      • Altrenogest (i.e. Regu-Mate®), 0.088 mg/kg (double-dose), for the duration of pregnancy
    • Non-steroidal anti-inflammatory medications (options):
      • Pentoxifylline (8.4 mg/kg, PO, q 6h to q 8h, for the duration of pregnancy
      • Flunixin meglumine (1.1 mg/kg, IV) - as needed
      • Firocoxib (Equioxx® Paste); loading dose: 0.3 mg/kg; daily dose: 0.1 mg/kg, PO, q 24 hrs, for the duration of pregnancy

Prediction of foaling

  • The gestation length of mares is approximately 340 days in duration
    • Mammary gland development
      • Begins about 7 to 14 days prior to foaling
    • Waxing of teat ends
      • Approximately 70 % of mares ‘wax-up’; mares that wax usually foal in 24 to 72 hrs
    • Increase in calcium levels in mammary secretions
      • A majority of mares have milk calcium levels greater than 200 ppm prior to foaling
        • However, not all mares will reach 200 ppm; some mares will foal with a milk calcium level less than 100 ppm
      • Most mares will foal within 24 to 48 hours after milk calcium reaches 200 ppm
    • Decrease in pH in mammary secretions
      • Mammary secretion pH is 7.0 to 7.4 in the days prior to foaling
      • pH of equine mammary fluid decrease to approximately 6.4 within 12 to 24 hrs prior to foaling
  • NOTE: changes in mammary pH are less diagnostic than changes in milk calcium

Induction of labor

  • It may be indicated to induce labor in a mare for medical or management reasons
  • Situations may include a previous history of giving birth to a stillborn foal, dystocia or injury during foaling, potential for neonatal isoerythrolysis or diagnosis of a potentially life-threating condition such as hydrops
    • Gestation length of at least 330 days
    • Mammary gland development
    • Waxing of the teat ends
    • Milk calcium levels of at least 200 ppm
    • Some clinicians also include relaxation of the cervix as a qualification for induction
  • The TECHNIQUE used at CSU for induction of labor is:
    • Administration of 5 units (0.25 mls) of oxytocin
    • A second dose of oxytocin (10 units) is administered 15 minutes later
    • Most mares ‘break their water’ 8 to 10 minutes after the second dose of oxytocin
  • Mares induced to foal have a higher incidence of RETAINED PLACENTA than mares with a spontaneous delivery

Treatment of retained placenta

  • RETAINED PLACENTA is common after dystocia, obstetrical intervention, induction of labor and abortion
  • Mares that experience one retained placenta are likely to retain their placenta in SUBSEQUENT births
  • A placenta is considered to be retained in the mare after 3 HOURS; considered a medical emergency
  • TREATMENT for retained placenta may include some or all of the following, depending on clinical circumstances, duration placenta has been retained, medical condition of the mare, etc.:
    • Administration of oxytocin (10 to 20 units, IV or IM)
    • Infusion of ~ 3 gallons warm water containing povidone-iodine into the allantoic cavity (‘Burns Technique’); fluid is held within the allantoic cavity for 3 to 5 minutes before being evacuated
    • Uterine lavage (i.e. fluid is infused outside of the retained membranes)
    • Careful manual manipulation to disconnect retained chorioallantoic membrane from endometrium
    • Systemic antibiotics
    • Local infusion of antibiotics into the uterine lumen
    • Administration of a non-steroidal anti-inflammatory drug (i.e. flunixin meglumine)
    • Tetanus toxoid
    • Frog support pads or deep bedding
    • Administration of 100 to 150 mls of 23 % calcium gluconate added to 1 liter of lactated Ringer’s solution (if mare is hypocalcemic)
  • NOTE: cannulation of a placental blood vessel followed by infusion of water may also be effective in resolution of a retained placenta

hCG response test for detection of cryptorchid testes

  • Aggressive or stallion-like behavior in a gelding may indicate the presence of CRYPTORCHID TESTES
  • Endocrine options for detection of the presence of cryptorchid testes include:
    Category Testosterone Level (pg/ml)
    Gelding < 100 pg/ml
    Cryptorchid 200 to 1,000 pg/ml
    Intact Stallion ?1,000 pg/ml
  • Initial blood sample is collected
  • HCG is administered (10,000 international units, IV)
  • Second blood sample is collected 1 to 2 hours later
  • Both samples are analyzed for TESTOSTERONE concentration and levels compared
  • An increase in testosterone level in the second sample suggests the presence of testicular tissue
    Category Testosterone Level (pg/ml)
    Gelding No change
    Cryptorchid Increased
    Intact Stallion Increased
  • Other diagnostic tests for the presence of cryptorchid testes include
    • Anti-Müllerian Hormone (AMH) concentration
    • Ultrasound examination – inguinal, flank and transrectal

Chemical ejaculation of a stallion

  • Pharmacologically induced ex copula ejaculation, or chemical ejaculation, may be used to COLLECT SEMEN from stallions with physical disabilities, penile abnormalities, behavioral issues or ejaculatory dysfunction
  • Semen collected by chemical ejaculation is typically LOW IN VOLUME and HIGH IN SPERM CONCENTRATION
  • The procedure should be performed in a quiet place with minimal distractions (i.e. the stallion’s regular stall)
  • Clinical procedure:
    • Administration of IMIPRAMINE HYDROCHLORIDE (2.2 mg/kg, PO)
    • Administration of XYLAZINE HYDROCHLORIDE 0.3 to 0.4 mg/kg intravenously one to two hours later
  • Passive emission of semen usually occurs within 3 TO 5 MINUTES after administration of xylazine as the horse is becoming sedated
  • Ejaculation may also occur 10 TO 15 MINUTES after administration as the sedation is wearing off
  • Semen can be collected using a hand-held cup attached to a long pole or into a collection device placed over the prepuce and tied over the back of the horse
  • Chemical ejaculation is effective in 30 TO 75% of horses treated