Breeding management
Traditional number of mares for one stallion is 40 per year. By Society for Theriogenology guidelines, fertile stallions breeding by natural service can breed around 40 mares / season and those in an artificial insemination program should be able to service 120 mares / season. In reality, numbers are much higher than this. Artificial insemination is not allowed in Thoroughbreds. Stallion semen contains a large gel fraction. Normal sperm number is 9 billion per ejaculate.
Mares are long-day breeders with two transition periods, spring and fall, during which they go through without consistently ovulating. Owners can "speed up" the season by placing mares and stallions under artificial lights to lengthen the day, beginning about December 1. You want to breed the mares by Valentine's Day, aiming for a birthday as soon after New Year's as possible.
Reproductive physiology
Puberty onset is at 15-24 months of age and is dependent on body weight, age and season. Estrous cycle length is 18-21 days. Standing heat lasts about 4-7 days and ovulation occurs 24-48 hours before the end of standing heat. Determine the stage of the estrous cycle by "teasing" the mare with a stallion; this may enhance subtle signs of estrus. Length of diestrus is constant in normal mares at 14-15 days while estrus varies as a function of season, with shorter estrus periods correlated with increasing daylength.
Pregnancy diagnosis
Always double-check those pregnancies detected early as embryonic death (days 45-90 of gestation) is common in mares.
Rectal palpation - Palpation of the ovaries is not helpful since equine ovaries usually have no overt structures palpable. Follicles will be present on the ovaries throughout pregnancy. Rectal palpation can be used to assess uterine tone and to try to detect amniotic vesicles.
Ultrasound - This is the preferred technique. It can be used as early as 9 days post-breeding, with the embryonic vesicle consistently visible by days 12-13. Amniotic vesicles are visible days 15-20, heartbeats days 24-30 and an obvious embryo after day 30 of pregnancy. Ultrasound is useful for assessing twin pregnancies, which must be terminated or reduced to a singleton pregnancy very early in gestation. Examining a mare for pregnancy at day 13-15 is critical for early detection and management of twin pregnancies.
Pregnancy and parturition
Gestation length = 340 days. Labor is explosive. Dystocia is less common than in cattle but is more likely to result in fetal death if it occurs. Dystocia usually is due to fetal malposture or malpresentation due to length of the fetal limbs.
Rhinopneumonitis
Equine herpesvirus I causes abortion in the last 4 months of pregnancy. The placenta is normal grossly. The fetus is aborted "fresh" and has necrotic foci in the liver, and intranuclear inclusions in the liver, lung, thymus and adrenal glands. Diagnosis is by virus isolation from aborted tissues. There is no treatment. Prevention is by vaccination with a killed product at 5, 7 and 9 months of gestation.
Endometritis
Endometritis is common in "susceptible" mares, which probably have altered uterine motility decreasing their ability to clear bacteria and fluid from the uterus. Endometritis may be exacerbated by abnormal anatomy (urine pooling). Diagnose by ultasound examination, uterine cytology and culture, and endometrial biopsy. Treat with uterine lavage and treatment with uterine-contracting agents, with or without intrauterine infusion with antibiotics in those mares with positive uterine culture results.
Abortion
Infectious causes = Strep zooepidemicus (farms with poor hygiene), equine herpesvirus I, equine viral arteritis, equine infectious anemia
Non-infectious causes = Twinning, toxins, trauma, developmental abnormalities of the fetus
Endometrial biopsy
A strong relationship exists between foaling rate and the histology of the
endometrium:
Category I = no inflammation or fibrotic changes, > 80% chance of carrying
a pregnancy to term
Category II A = some focal inflammation and fibrosis, 50-80% chance of carrying
a pregnancy to term
Category II B = some focal inflammation and fibrosis, 10-50% chance of carrying
a pregnancy to term
Category III = widespread inflammation, uniform fibrosis, < 10% chance of
carrying a pregnancy to term
Neonatal isoerythrolysis
Neonatal isoerythrolysis = hemolytic disease of the foal due to an incompatible blood reaction between serum antibodies of the dam (concentrated in colostrum) and erythrocytes of the foal. The foal is clinically normal at birth but after nursing (within 24-36 hours after birth) becomes dull, weak, polypnic, and icteric. Collapse and death may follow. Diagnose by clinical signs and decreased PCV in foal. Treat by maintenance of foal's circulatory system (transfusion - good donor is the dam, since she will not lyse her own RBCs - fluids, antibiotics, glucocorticoids). Can allow foal to nurse again after 36 hours of life when can no longer absorb large proteins. Prevent by screening mares. Horses have 8 blood types but types A and Q are the worst offenders. If the mare has anti-A or anti-Q titers of 1:8 or greater 2-3 weeks prior to foaling, withhold colostrum from the foal.
Retained fetal membranes
Fetal membranes are considered retained if still present greater than 3 hours after delivery of foal. Mares are predisposed by abortion, dystocia, uterine infection, and by breed (draft mares more susceptible). Sequelae are severe and include metritis and laminitis. Treat with oxytocin (IM or IV, if give as slow drip decrease incidence of colic). Manual removal may cause hemorrhage. Can try to treat with uterine infusion with saline or dilute antiseptic (causes release of endogenous oxytocin). May help to treat with Banamine as well.
Thanks to Dr. Margo Macpherson, University of Florida, for looking over
this material.