4/20/2010 Spring Breeding - the Transitional Period

Vet tip of the Day: The Mare's Transitional Period

It's the time of year when I get lots of calls regarding breeding mares.  In the past week I've also had several  about performance mares exhibiting unusual behavior and brood mares showing irregular heat cycles.  Both of these problems are related to the seasonal nature of mare's reproductive cycle, and may be particularly evident this year because of our unusual weather patterns. Today I am going to briefly review the mare's estrous cycle with emphasis on the transitional period that affects many mare's between January and April.

Seasonal variation in the duration of daylight has a profound influence on mare reproductive performance.  The horse is a seasonal breeder - increasing daylight improves the mare's reproductive efficiency while shortened days results in poor reproductive regulation.  Daylight is believed to act by stimulating the production of melatonin by the pineal gland, located within the brain.  This melatonin in turn causes the hypothalamus to release GnRH (gonadotropin releasing hormone).  GnRH acts on the pituitary, causing production of FSH (follicle stimulating hormone) and LH (luteinizing hormone) which influence the ovaries to develop and release follicles. In order for successful conception to occur, a mature follicle, or egg, must be fertilized by healthy sperm and then arrive in a uterus which is ready to accept and nourish it as it develops into a budding embryo. 

The transition from the short days of winter when most mares stop cycling all together, to the long days of June, when fertility is at its highest, is a gradual, progressive process.  During the spring and fall, mares enter a period of anovulatory receptivity, or the transitional period.  At this time, they often exhibit erratic estrus behavior, and while they appear to be in standing heat and accept a stallion, there often is not an associated ovulation of a mature follicle.  Even if a transitional mare does ovulate appropriately, it is also likely that the hormonal sequence necessary to maintain the early critical days of pregnancy will not be in place and the conceptus is lost.  Particularly in the spring, this transitional period is characterized by long, erratic heat cycles without ovulation. 

During the transition period performance horses often exhibit irritable behavior and are difficult to train.  It is during this time that trainers are often looking for ways to suppress reproductive activity so that their mares will behave appropriately in the show ring.  There are many oral supplements available over the counter which claim to improve the demeanor of irritable mares. The effectiveness of these supplements is debatable.  The only way to know if one will help your cranky mare is to try.  For years people have used cattle subcutaneous hormonal implants to control mare's heat cycles, but multiple research trials have been performed using these implants and no one has ever been able to show that they have any real effect on the mare's hormonal regulation.  Injectable progesterone in olive oil can be used intramuscularly to prevent mare's from cycling during the transitional period with variable success.  The only truly reliable means of controlling a mare's reproductive system and preventing cycling is the daily administration of  oral Regumate liquid (a synthetic progesterone).

Once the transitional period is over and mare's are cycling regularly, reproductive efficiency rapidly improves.  The "normal" mare has a 21 day heat cycle.  She is not receptive for 14-15 days (diestrus), then comes into heat for 4-7 days (estrus), ovulating 12-24 hours before behavioral signs of estrus disappear. Regarding performance horses, some mares continue to be difficult during the days close to ovulation, but in general the number of days when undesirable behavior is exhibited are markedly reduced, and can be predicted based on following the heat cycle.

So, when your mare is acting like a maniac and its February or March, remember that part of her behavior may be attributed to the "raging hormone" condition that we all recognize in each other from time to time.  Mares, just like people, are very individual in their reaction to their own internal chemistry.  Some have placid dispositions and do not seem affected by the ups and downs of hormonal transitions while others are truly distressed during these transitional phases and should not be punished when they are at the mercy of Mother Nature's nasty tricks.  If you own a mare you are trying to breed in the early spring, or a performance horse with seasonal behavior problems, speak with your veterinarian about management practices that may improve your breeding success or help your mare's disposition. 

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I believe that animals are the key to compassion.
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