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Managing Mares In Heat

Influence your mare"s cycle for performance-related reasons or for horse breeding.

If You Want to Breed
Start by scheduling a breeding soundness exam before the season begins, Dr. Wolfs­dorf says, so you'll have time to clear up infections or other problems that might prevent a successful pregnancy. Then monitor—or manipulate—your mare's cycle to time her breeding.

Check and confirm. The best and easiest way to recognize heat is with a teaser stallion or colt.

  • Walk the mare past the teaser daily, with a teaser rail or similar barrier ­between them, or bring him to a stall next to her. Be careful—if she's not ready, she may become aggressive and kick or strike at him.
  • If she's in heat she'll show interest and perhaps squat, wink her vulva or urinate. Signs vary widely, though, and some mares are quiet and shy. Tease throughout the cycle and record the changes in your mare's behavior, and you'll learn over time to recognize her signs.
  • Without a teaser, you'll need your veterinarian to monitor your mare and figure out where she is in her estrous cycle.

Either way, once you know your mare is in heat, have your veterinarian confirm it with transrectal palpation and ultrasound examinations. Your mare should be bred as close as possible to ovulation—no more than a day or two before and no more than six hours after. Since heat lengths vary depending on the time of year and the individual mare, the timing may be hard to judge. Rectal palpation and ultrasound allow the veterinarian to assess the size and softness of the ­developing follicle, the amount of edema (swelling) present within her uterus and how open her cervix is. The findings help determine when your mare is likely to ovulate. "If she has a 40-millimeter follicle and good edema, for example, she'll ovulate sooner than she will with a 20 mm follicle and developing edema," Dr. Wolfsdorf says.


Short cycling. Using hormones to bring your mare into heat—short cycling—lets you plan breeding more precisely. "This can be especially helpful when you need to coordinate delivery of shipped semen and a visit from your vet for artificial insemination," Dr. Wolfsdorf notes.

The hormones used are prostaglandins—a natural form ­(Lutalyse) or a synthetic (Estrumate). Like the prostaglandin produced by the lining of the mare's uterus at the end of diestrus, they break down the corpus luteum and stop the production of progesterone. That allows the mare to come into heat.

  • Your vet will examine the mare to be sure she's in diestrus and has a mature corpus luteum before administering the prostaglandin. Generally this will be six days after ovulation.
  • "Palpation and ultrasound are essential at the time of prostaglandin treatment, to assess follicle size," Dr. Wolfsdorf says. This allows the veterinarian to estimate the onset of estrus and the time of ovulation. "If you give prostaglandins and schedule artificial insemination for two days later without knowing this, you will most likely miss your opportunity."
  • Some mares respond to prostaglandins with sweating, and a few become colicky. Dr. Wolfsdorf says that the synthetic form seems to have fewer side effects in these mares.
  • Prostaglandins are given in an intramuscular injection; if given IV, the mare will collapse and convulse, which can be life threatening.

Getting a Head Start
Thoroughbred mares are often bred in February or March to ensure that their foals are born as close as possible to January 1—the official birthday of all racing Thoroughbreds. "But you can't just pull a mare out of the field in early spring and expect her to come into heat," Dr. Wolfsdorf says—the days are too short to trigger the hormones that govern her cycle. She needs at least 15 hours of daylight each day.

To breed in February, plan ahead and put her under lights. Increase the hours of light at the end of the day, say by keeping lights on in her stall until 11 p.m. "The light must be strong enough for you to easily read a newspaper everywhere in her stall," says Dr. Wolfsdorf, "because she'll tend to hang out in the darkest corner." Most mares will begin to cycle after about 60 days of extra light.

You can also try to induce the cycle with hormones. This works best in the transition stage, in early spring. Prostaglandins won't help at this stage if no corpus luteum is present, but a combination of progesterone and estradiol may. Follicle stimulating hormone, gonadotropin-­releasing hormone and other hormones are also used. "There are lots of protocols," Dr. Wolfsdorf says, "which tells you that none works perfectly.

This article originally appeared in the January 2012 issue of Practical Horseman magazine.

Posted in Anatomy, Behavior, Breeding, Dressage, English, Eventing, Health, Horse Care, Hunter/Jumper, Illnesses & Injuries, Other Horse Sports, Riding & Training | Leave a comment

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