Winter Anestrus
The period of sexual inactivity in a mare, lasting typically from October through April in the Northern Hemisphere.

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.

Hypothalamus Gland
A gland located in the brain that controls the pituitary 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.

Gonadotropins
A hormone produced by the pituitary gland (or later in pregnancy by the placenta). It acts on the ovaries to produce sex hormones.

Follicle-Stimulating Hormone (FSH)
A hormone produced by the pituitary gland. It causes the ovaries to produce egg follicles.

Luteinizing Hormone (LH)
A hormone produced by the pituitary gland. It causes the follicle to mature and ovulate.

The operational breeding season for the mare can be affected by a number of factors:
1. An artificial light program
2. Drug therapy
3. Physical condition

Corpus Hemerrhagicum (CH)
This is the content of the follicle after ovulation. It is filled with blood and then becomes the corpus lateum.

Corpus Lateum (CL)
A growth at the site of ovulation. It produces progesterone.

Diestrus Phase
The phase of the estrous cycle immediately following ovulation. It normally lasts 14 days.

Estrous Cycle
One entire heat cycle. From one ovulation to the next. It normally lasts for 21 to 23 days.

Estrus Phase
The first phase of the estrous cycle. It normally lasts from 4 to 7 days and ends with ovulation.

Follicular Phase
See estrus phase
The process that causes the disappearance of the corpus lateum.

Ovulation
The process of the follicle releasing the egg into the ovaries.

Progesterone The pregnancy hormone.

Prostaglandin (PGF2a) A hormone found in the lining of the uterus.

Fertilization:
The timing of introducing the semen into the ovaries of the mare is critical to the successful fertilization of the egg. Rectal palpation of the follicles is the best method of assuring the semen is introduced at the correct time.
As the follicle begins to expand, it should be monitored regularly, as often as once every 12 hours. Ovulation normally occurs when the follicle reaches about 45mm (with a normal range of from 35mm to 50mm). As ovulation nears, the shape of the follicle changes from spherical to more teardrop.
It is necessary for the semen to be introduced within 12 hours of ovulation or its chances of success are nil. The methods of fertilizing the egg in the mare may be accomplished in a few basic ways:

Natural Cover - The stallion covers the mare.
Natural cover is the good old fashion way. It is the only method you can use with thoroughbreds that you want to register a thoroughbred with the Jockey Club.
Your advantages of using natural cover are:
5. There are no dangers of affecting the motility of the semen in handling as there are in other methods.
6. More semen is normally introduced into the ovaries.

Artificial Insemination - A veterinarian places the semen into the mare.
The semen used for Artificial Insemination may be:
Fresh
Cooled
Frozen

The method you use depends upon what is available. All other factors being equal, the chances of pregnancy is best for the fresh semen and worst for the frozen semen. I have read one study from Equine Reproduction Concepts in Virginia stating that their results with frozen semen was better than with cooled semen. They theorized that there was less chance of damage in shipment with the frozen semen and that a few days delay would have no effect.
Others disagree!
 

In Vitro Fertilization - The egg of a mare is placed in a petri dish with the semen of the stallion,
In vitro fertilization (IVF) is considered an experimental procedure. An egg from a mare is placed in a petri dish with the sperm of a stallion and after the egg is fertilized, it is placed in the recipient mare as described on the embryo transfer (ET) page.
As I hear more about this procedure and find links, I'll post it here.

Factors
Some of the factors that effect the estrous cycle of the mare are:
8. Lights can be used to move up the first cycle of the year.
9. Drugs can be used to hold off a cycle, end a cycle or induce ovulation. Drugs can be used to synchronize the cycle of a mare and a recipient when performing an embryo transfer. And drugs may be used to have ovulation occur when a shipment of cooled semen is scheduled to arrive.
10. 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.

Light
An artificial light program will help shorten the winter anestrus by up to two months.
11. 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.
12. A typical 12' x 12' stall would require about a 200 watt incandescent or 400 watt fluorescent lamp for 16 hours per day.
13. 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.
14. 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.
15. Each mare responds differently. Some may begin ovulation weeks earlier than others and some may require drug therapy.

Drugs
Drugs are used to keep the mare from cycling, synchronizing the cycle and to induce ovulation:
16. 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:
a. Regu-Mate TM

17. 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:
a. Equimate TM
b. Lutalyse TM

18. 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. A.P.L. TM
b. Follutein TM

Embryo Transfer
Embryo transfer (ET) is a process where a mare is inseminated and about 6 to 8 days later, the embryo is flushed from the uterus and transplanted to a recipient mare. The process was made a viable option by work at Colorado State Veterinary School and is now performed by a number of veterinarians throughout the country.
You should check with you breed registry to assure they will register foals that result from ET. The Arabian  Horse Registry of America, does recognize the procedure, but as yet does not allow multiple foals to be registered from the same mare in a single year.
Embryo transfer is a technique that is gaining in popularity for a number of reasons:
19. You can keep your mare in full training.
20. You have the possibility of recovering more than one egg.
21. You may breed your mare more than once per year.
22. You may obtain an embryo, have it frozen for shipment or later transplant.
23. It eliminates all possibility (as with any other AI) of injury to the mare during breeding.
The procedure works like this:
1. The mare is inseminated.
2. Six to eight days after ovulation, the fertilized egg (s) are collected by inserting a catheter into the uterus through the cervix.
3. The quality of the egg(s) recovered are then graded.
4. A fertilized egg is implanted into a recipient mare. The two methods of implanting the egg are by surgically inserting into the uterine horn or by transcervical insertion using a pipette or specially designed insertion device.