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Mare Care For Healthy and Sound Breeding


The University of Tennessee Extension Service’s Master Horse Owner Seminar offered detailed information about breeding, including “Stallion Reproductive Anatomy and Management” and “Mare Reproductive Anatomy and Physiology.” Following are excerpts from the latter, written by Christine R. Brown, MS Candidate, Department of Agriculture Education and Extension, and by Bridgett J. McIntosh, PhD, Department of Animal Science, University of Tennessee. Excerpts include Breeding Patterns, Mare Management, Pregnancy, and Routine Care of the Pregnant Mare.

Breeding Patterns. Mares are seasonally estrous, and the natural breeding season occurs from late spring through summer. Most mares exhibit a period of seasonal anestrous during the winter months. This is followed by a spring transition period with variable ovulation. Mares they cycle regularly and ovulate predictably from April until October – referred to as the physiologic breeding season. A fall transition with variable ovulation generally precedes the next winter anestrous period. Most breeding facilities begin breeding in February, because m any breed associations consider a horse one year of age on January 1 of the following year. The result is a larger foal at weaning compared to foals born later in the same year.

A mare’s estrous cycle occurs about every 21 days, and occurs into two physiological stages. Estrus, referred to as heat, is the shortest stage of the cycle, lasting three to seven days. During estrus the mare is receptive to the stallion and will ovulate near the end of estrus. At the end of estrous the follicle that ovulated will develop into a corpus luteum (CL). Diestrus will follow and the mare is no longer receptive to the stallion. This lasts approximately fifteen to nineteen days.

Human Intervention in the Estrous Cycle. Due to human desires, requirements of foals to be large for competitive events, humans often manipulate the onset of estrus in mares by using lights and synthetic hormones. If mare owners elect to breed mares in February, the mare should be put under artificial lights beginning in December of the previous year. The mare’s cyclic activity is a function of daily-perceived length of darkness, on the release of melatonin from the pineal gland. The lights stimulate the mare’s pineal gland and induce early onset of cycling. Photo-manipulation requires a 200-watt incandescent light, placed in the mare’s stall five feet above her head. Lights are turned on in the evening, beginning December 15, so the mare is exposed to a total of 16 hours of light per day.

Control of ovulation can be accomplished by feeding an approved oral progesterone to mares daily for 15 days starting on any day in a cycling mare. Then stop feeding, and estrus will occur an average of five days later.

 Another option is to feed oral progesterone for eight days in conjunction with prostaglandins. Prostaglandin is given on the last day that oral progesterone is fed. All options should be discussed with a veterinarian (Blanchard et al., 2003).
Mare Management.  Nutrition and management of the photoperiod are the two most important factors for successful breeding outcomes. Also, the mare requires vaccinations, a parasite control program, and daily exercise to optimize fertility rates. Gestating mares should be kept off fescue grass and hay for a minimum of three months before foaling. Fescue toxicity is caused by an endophyte present on tall fescue pasture and hay. Ingestion of endophyte infested fescue hay is prolonged gestation, difficult birth, thickened and retained placenta, and little or no milk production.
It is important to consider the mare’s breeding history before making the financial and long-term commitment to breed her.

Pregnancy. The average gestation of the horse is 340 days. It is not uncommon for a mare to carry a foal as long as twelve months. Several factors can shorten or lengthen gestation: sex of the foal can increase gestation. Colts tend to be carried on average two days longer. Shorter gestation can be caused by winter foaling, twins, and mares kept under lights (Salazar et al., 2006).

Routine Care of the Pregnant Mare. Pregnant mares should receive the minimum core vaccinations recommended by the American Association of Equine Practitioners (AAEP). In addition, it is recommended that pregnant mares be vaccinated for Rhinopneumonitis at the 5th, 7th, and 9th months of gestation. Most veterinarians recommend a 4th dose, administered 4 to 6 weeks prior to foaling. Infection of Rhinopneumonitis is one of the most common causes of pregnancy losses in mares (AAEP, 2008). The timing of vaccinations is essential to the foal’s health. Vaccinations can be transferred to the foal through the absorption of antibodies in the mare’s colostrum. Most veterinarians recommend that mares be treated for internal parasites every two to three months, and the day after foaling so there is reduced chance of the foal contracting internal parasites.

Rectal palpation and ultrasound examinations throughout gestation are other important steps in routine mare care. During these examinations, the mare should be evaluated for cervical tone, appearance and tone of the uterus, structures present on the ovaries, and normal embryo size and shape for gestational age. Routine examinations should be performed at day 14, days 18-20, days 25-30, days 35-45, days 60-80, and days 100-120 (Schweizer, 2003).
Consistent monitoring of the mare during the last few weeks of gestation is crucial. A mare may show several signs of impending birth. The mare should “bag up” (enlaragement of the udder) two to four weeks before foaling.

Conformational changes (dropping of abdomen and tail head relaxation) should occur one week to one month prior to foaling. Teats will fill with milk one day to one week before foaling. Dripping milk should occur one day to one week prior to foaling. Wax-like beads or waxy substance typically drips from teats one to four days prior to foaling. Changes in calcium milk concentration occur one day to 1½ days prior to foaling. Relaxation of external genitalia occurs ½ to one day prior to foaling.


American Association of Equine Practitioners (January 2008). Recommended Vaccinations.
Blanchard, Terry L., Steven P. Brinsko, Charles C. Love, Sherri L. Rigby, James Schumacher, and Dickson D. Varner (2003) Manual of Equine Reproeduction, 2nd edition. St. Louis, MO: Mosby, Inc.
Salazar, Tricia, Mary Beth Stanton, and Bryan M. Waldridge. (January 2006). Is  prolonged pregnancy dangerous? Many factors might contribute to a broodmare safely exceeding her foaling date.
Schweizer, Christine M. (2003) The Broodmare: Your Guide to Horse Health Care and Managament. Lexington, KY: The Blood-Horse, Inc.

For the complete article, please contact Bridgett McIntosh, PhD, UT Extension, Department of Animal Science, Nashville, TN (615) 832-6550. Website: Click “Extension and Outreach,” then “Horse” from the left-hand menu items.

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