Horse Breeding

Annual Cycle

Horses are known as seasonal breeders.  Their natural breeding season depends on a number of factors, including latitude and length of daylight.  The period of reproductive inactivity is known as winter anestrus. 

The natural breeding season for the mare is usually from April through October in the northern hemisphere.  As the hours of light during the day increases, the retina transmits signals to the pituitary gland that reduce the secretion of melatonin.  The melatonin suppresses the production of gonadotropin-releasing hormone (GnRH) which is normally produced by the hypothalamus gland.

In turn, GnRH causes the pituitary gland to produce and release gonadotropins that are responsible for activating the reproductive cycle.  It also causes the pituitary gland to release follicle-stimulating hormone (FSH) which causes the ovaries to start the follicle enlargement process.

When the production of GnRH reaches a frequency of a pulse every 2 hours, the pituitary gland releases luteinizing hormone (LH) which causes the follicle to mature and release the egg.

The operational breeding season for the mare can be affected by a number of factors:

1. Physical condition
   • Another factor that may effect the success of your breeding is the level of training of your mare. We recommend that the mare be allowed to "come down" a bit if she has been in a strenuous training regimen. A month of relaxing will usually pay dividends.
2. An artificial light program
   
   • An artificial light program will help shorten the winter anestrus by up to two months.
   • The program will take at least 60 days to be effective and produce any follicular activity. This should begin as early as practical if you want to move up your breeding date.
   • A typical 12' x 12' stall would require about a 200 watt incandescent or 400 watt fluorescent lamp for 16 hours per day.
   • Light should fall on the mare's eyes wherever she stands in the stall and you should be able to read a newspaper in the stall.
   • The length of time for the lights to bring on ovulation depends on the initial physical condition of the mare and the time of the year the light regimen was started.
   • Each mare responds differently.  Some may begin ovulation weeks earlier that others and some may require drug therapy.
3. Drug therapy
   Drugs are used to keep the mare from cycling, synchronizing the cycle and to induce ovulation:
   
   • Progesterone is used to shorten the transition between winter anestrus and the natural breeding season. Growth of large follicles may be stimulated by using progesterone. Ovulation typically occurs about 10 days after the last dose of progesterone, although that depends on the mare.  The most popular drug is:
     • Regu-Mateä 
   • Prostaglandin  (or Prostin) is used to shorten or end the luteal phase.  One dose typically causes luteolysis and then ovulation within 4 to 10 days. Side effects may include sweating, diarrhea and mild signs of colic. Two popular forms are:
     • Equimateä
     • Lutalyseä
   • Human Chorionic Gonadotropin (HGC) is used to induce ovulation. An estrous mare with a 35mm follicle should ovulate within 48 hours of receiving the drug. Two commonly used drugs are:
     • A.P.L.ä
     • Folluteinä

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