1. Universidad Autónoma de Nuevo León
Horse Breeding
Paul R Earl Facultad de Ciencias Biológicas
Universidad Autónoma de Nuevo León
San Nicolás, NL 66451, Mexico

2. Light duration controls the annual sexual cycle
In most herbivores the male is responsible for the protection of the herd against predators. Also, the stallion will fend off competing males, and mares in heat (estrus) will fend off other mares from a stallion. Spring is favorable for births. Let’s pick April for birth. Then if gestation is about 3 months as in sheep, breeding took place in say December. If gestation is 11 months as in horses, breeding took place in April or even in June. Then we have short and long day animals. Goats breed in winter to have kids in spring, just as horses breed in spring to have foals in spring. Also, summer in one hemisphere is winter in the other.

3. A mare needs 16 hours of daylight through October on
After the equinox which is 21 September, as days get shorter goats are turned on sexually and horses off. Horses begin to react on 21 December as the days begin to get longerand so do many other animals. Do chickens lay more eggs with more hours of light ? Which hormones are involved with ovulation ? These climatologic effects can lead to considerations of the biological clock.
You might not believe this. All racehorses have their birthday on 1 January. If yours was born on 11 December, hide it some place ! It has a divinely improved chance to win races if born on or just after 1 January or registered that way. Perhaps another foal was born in August. Tough luck, unless it was reborn next year if you know what I mean. Undoubtedly, trying to negociate the mares with the arbitrary date of 1 January has contributed to the low ferility rate of % in these horses,
A mare needs 16 hours of daylight through October on
to ovulate in January.

4. If a foal is conceived too late as in summer, it likely will not race till it’s 3. The costs up to the first race include the stud fee, the care of the mare and her foal, and training as a yearling and 2-year old. Earlier matings give quicker returns—of course. Most mares are bred midFebruary to midApril. However, today 2 covers or even less not 3 as in 1960 are possible, raising the earning power of the stallions. They too are affected by hours of daylight, by aging and other common factors. Month 5 is April, 10 is October.
The graph above of the days of the estrus cycle are given on the Y axis and the 12 months of the year on the X axis. April to October has shorter cycles than winter of course. R squared is the coefficient of determination, FitStErr is the standard error of the fit and F is the F ratio. The weak F value can be and should be raised by getting a larger sample of records. These are natural values. Values after hormone administration and lengthening the daylight duration should be pursued.

5. Detecting heat (estrus) by using a mare’s responsiveness to a stallion is known as teasing. It is the most important technique in horse breeding. Mares are most successfully bred at maximum receptivity, and this is at ovulation. Rectal palpataion of the uterus and its ultrasonics are 2 supplementary techniques.
When ovulation absolutely
correlates with the time of
sexual intercourse with
the stallion, one cover
will do. Otherwise, returns
make mating much
more costly.
Starting steps to take First note that this lecture is partly based on Horse Breeding by Gary Heusner of the University of Georgia College of Agricultural & Environmental Sciences, Bulletin 945 (1993). All photographs were kindly supplied by Vaughn W. Henry, Also read Ginther, O. J Reproductive Biology of the Mare: Basic and Applied Aspects. McNaughton & Gunn, Inc. Ann Arbor.

6. Is she in heat? Is she in foal? Is the pregnancy normal?
Here are some suggestions: 1/ External genital examination of stallion and mare, 2/ Breeding suitability examination of the stallion, 3/ Palpatation per rectum of complete mare reproductive tract, 4/ Ultrasonographic examination of mare reproductive tract (pregnancy diagnosis via palpation and ultrasonography) and 5/ natural cover or artificial insemination with cool fresh shipped or deep frozen semen.
Is she in heat? Is she in foal? Is the pregnancy normal?
1/ Check mares near delivery often, 2/ Wrap the tail, wash the vulva and udder, 3/ As the allantois ruptures call for help. Call earlier if the mare is straining or rolling, 4/ Provide assistance or traction as needed,1. 5/ Make sure the drug box, clean stainless steel bucket, disinfectant, sleeves, oxygen tank and mask, obstetric chains and handles are available, 6/ After foal is born, clear nasal passages, insure that foal is breathing. Use oxygen to assist foal's respiration. Tank and mask with tubes must be nearby, 7/ Allow umbilical cord to remain intact as long as possible, 8/ Draw foal's blood into a Vacutainer with EDTA anticoagulant (purple top) for compatibility with maternal colostrum and other tests, 9/ Paint the naval stump with iodine, 10/ Is it necessary to provide a heater? 10/ Photograph the foal and fill out a form for its characteristics and 11/ Montitor the temperatures of the mare and foal, and watch for signs of disease like diarrhea.

7. Hormones The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary to secrete Luteinizing Hormone (LH) The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary to secrete LH and Follicle Stimulating Hormone (FSH). These hormones cause the growth and development of ovarian follicles. At the time of estrus, there is a massive release of luteinizing hormone that triggers ovulation, called the preovulatory surge.
As the ovarian follicles increase in size, they produce the hormone estrogen which 1/ causes behavioral estrus (sexual activity and receptivity). 2/ stimulates uterine muscle tone, which aids sperm transport following mating, 3/ stimulates muscle of oviducts, which assists sperm transport and ovum/embryo transport and 4/ induces the preovulatory surge of LH which is a positive feedback.

8. A corpus luteum (yellow body) develops from the remains of the follicle after the egg has been released. Once it has developed, it produces the hormone progesterone. It acts to inhibit GnRH release thereby reducing pituitary FSH secretion and thus prevents follicle maturation. As long as progesterone circulates in blood, estrus is prevented, e. g., pregnancy is so maintained if conception occurred.
Some of this information on the reproductive cycle is repeated later to make sure it is understood. Is testosterone the male equivalent of estrogen ? An easy question !

9. Regumate is synthetic progesterone named altrenogest given orally in vegetable oil. Each ml contains 2.2 mg altrenogest. Dosage is 1 ml/50 kg body weight for 10 consecutive days. Regumate can be used for 1/ inducing an ovulatory estrus, for inducing normal cyclical ovarian activity, 2/ Treatment of the mare in lactation anestrus, 3/for suppression of estrus either during prolonged estrus or in normally cycling mares and 4/ to control the cycle of breeding mares to allow efficient use of the stallion. The majority of mares will return to estrus within 8 days after the last oral dose of Regumate and will ovulate between 7–13 days later.

10. Steps towards successful fertilization then foaling include:
Edema begins 7-8 days before ovulation, peaking at hours before ovulation and declining within 2 days of ovulation. Edema is found on both the day before and at ovulation with a 7.5 MHz ultrasound transducer. The hormones most commonly used to manipulate the mare's estrus cycle are progesterone and its analogues like altrenogest (Regumate). Estradiol is responsible for estrous behaviour in mares, and estrogen concentrations during estrus correlate well with sexual behavior and grossly observable changes in the reproductive tract. In contrast, progesterone inhibits estrus behaviour. Its rise in concentration leads into the next estrus cycle.

11. Estrogen and prostaglandin
For prostaglandin to affect an estrous cycle, it requires an active corpus luteum (CL). Progesterone started on the day of parturition, at regular dosage levels (150 mg/day IM for progesterone or 0.44 mg/kg per day orally for Regumate) for a period of 8 days, will usually result in the mare entering estrus 3 days after treatment ends. Progesterone and estradiol-17 beta combined is an injectable product given at the dosage level of 150 mg progesterone, 10 mg estradiol 17 beta per day IM.

12. Successful breeding accounts for these factors:
1/ A healthy ova (egg). 2/ Ova live 12 hours so must be exposed to sperm within this time. 3/ Most sperm live 3-5 days in the mare that must contact the egg in the oviduct (tube connecting ovary & uterus) at this time. 4/ Oviduct must be clear to allow passage of ova & sperm. 5/ Fertilization must take place within the oviduct. 6/ The oviduct environment must be suitable for egg, sperm & embryo. 7/ Suitable uterine environment for embryo to implant on its wall must be available.

14. The male reproductive tract
Testes. A stallion's testicles each weigh grams, averaging 4-5 cm long, 6-7 cm high and 5 cm wide. The testes produce sperm. Epididymis. The epididymis is attached to the testicle. The epididymis serves to transport spermatozoa out of the testes, concentrate it by absorption of water and provide a place for it to mature and be stored. Vas Deferens. The vas deferens, or spermatic cord, transports sperm from the tail of the epididymis to the urethra.
Accessory Sex Glands. The accessory sex glands include the seminal vesicles, the prostate gland and the bulbourethral glands. These glands secrete or contribute fluids which give the semen a characteristic composition such as buffering fluids and albumin. Penis. A stallion's penis is likely over 50 cm long in the relaxed state with cm lying in the prepuce. During erection the size increases about 2 fold. The penis is roughly divided into the head, body and glans. The glans, or free end of the penis, is bell shaped particularly during erection and ejaculation. The urethral process extends approximately one inch from the surface of the deep depression or fossa glands.

15. The female reproductive tract Ovaries: Mares typically have long erratic estrous periods in January, February and March. During the physiological breeding season-April through August-a large follicle develops during each estrous period and ruptures toward the latter part of the heat period, releasing an egg (ovum). The collapsed follicle is replaced by the CL which produces the hormone progesterone. If the mare does not become pregnant, the CL will have a lifespan of days. If the mare becomes pregnant, several large follicles may develop on either ovary of the mare. These follicles may rupture or luteinize to form accessory or secondary CL as supplementary sources of progesterone.
Oviducts: The oviducts run from the ovaries to the uterus. The ovarian end of each oviduct is fanned out and enlarged,called the fimbria. Fertilization occurs in the upper 1/3 of the oviduct. Within the oviduct are ciliated cells which help propel the egg towards the uterus. Uterus: The uterus contains a body and 2 horns. See illustration. The horns connect the uterus to the fallopian tubes and are about 26 cm long. The body is connected to each horn at roughly 90 degrees and about 20 cm long. The lining of the uterus is generally light pink in color and is made of longitudinal folds. The lining is termed the endometrium. Cervix: It is a muscular structure about 10 cm long. The cervix separates the uterus and the vagina. During pregnancy the cervix of the mare becomes quite firm and hard, and the external opening becomes sealed. Vagina: The vagina extends from the cervix to the vulva and is about 18 cm. The hymen is usually not present. Vulva: The vulva is the terminal part of the reproductive tract. It is about 10 cm long. The external orifice is a vertical slit about 17 cm long with the clitoris at the bottom.

16. The Estrous Cycle
The anovulatory season is a period of sexual quiescence extending from the last ovulation of the breeding season to the first ovulation of the next breeding season. Follicular activity is minimal December-February (Northern Hemisphere) with an increase in activity from March to May. During anestrus the ovaries are usually small and hard. Estrous cycle length and duration of the heat period is longest during the autumn months and shortest between April and October. The number of estrous periods is lowest in winter months. The number of small follicles (<20 mm) increases slowly during January to March. In the latter part of March, the number of small follicles decreases with a concomitant rise in the number of large follicles (>30 mm).
The sequence of hormone concentrations that leads to ovulation and possible fertilization is shown cycling in the illustration. The number of estrous periods is lowest in winter months. The number of small follicles (<20 mm) increases slowly during January to March. In late March, the number of small follicles decreases with a concomitant rise in the number of large follicles (>30 mm).

17. The pituitary hormones involved in the estrous cycle are the gonadotropins: follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and estrogen. Estrogen stimulates 5-7 days heat in mares. LH causes the ovarian follicle to mature and ovulate. After ovulation the follicle is replaced by the CL which secretes progesterone needed to maintain pregnancy up to day 100. Both FSH and LH are under control of the gonadotropin-releasing hormone (GnRH), which is secreted by the hypothalamus gland of the pituitary. Seasonal anestrus is caused by a change in the feedback mechanism between the ovary and the hypothalamus involving a signaling system and cytokinines like inhibin and activin.

18. The estrous cycle is divided into 2 periods
The estrous cycle is divided into 2 periods. The follicular phase is the period during which there is rapid folliclular growth. The mare displays behavioral estrus for 5-7 days, then ovulation. The luteal phase begins with ovulation. During diestrus, the days in between, the CL is formed and progesterone is produced by it. If the mare conceives, the integrity of the CL is maintained. Progesterone is secreted to maintain pregnancy for days. Afterwards, a second rise of progesterone begins due to the formation of secondary CLs.

19. At the beginning of the menstrual cycle, anterior pituitary gonadotrophic cells release FSH. This FSH travels to the ovary where it binds to immature oocytes, initiating their maturation. Such maturing oocytes or follicles make estrogen, which gives negative feedback to the pituitary to inhibit further FSH release. After about 14 days, one oocyte matures and releases a final burst of estrogen that causes the pituitary gonadotrophs to secrete both FSH and LH (luteinizing hormone). This "spike" of LH/FSH induces follicle rupture, leading to a decline in estrogen production. Cells in the ovary that contributed to earlier follicle maturation and now remain following ovulation develop into a structure called the corpus luteum.
This structure becomes a center for the synthesis of progesterone, a molecule that both inhibits pituitary LH production, and promotes the growth of the uterus.  In the absence of fertilization, the corpus luteum involutes, leading to a period of menses with low circulating estrogen and progesterone levels. The lack of these two hormones is a positive signal for new FSH release and the start of a new cycle. Although estrogen is the primary stimulus for the "LH spike", gonad-produced inhibin is believed to limit the amount of FSH released during this spike, thus fine-tuning the response. In addition, during the luteal or post-ovulation period, ovarian-derived inhibin levels rise, helping to suppress pituitary FSH release during this time. Then when inhibin levels fall late in the menstrual cycle, an impediment to FSH production is removed.

20. Maternal progesterone secretion is necessary for some 100 days of gestation. Afterwards, pregnancy can be maintained by progesterons produced by the fetal-placental unit.
If the mare is NOT pregnant, prostaglandin F2 will be secreted by the uterus to lyse the CLs, and the mare will return to estrus.

21. Shorten cycling with prostaglandins
Many hormonal techniques can bring in various desired results. They allow spreading or grouping of mares for breeding, and shortens the waiting period due to missed ovulations. Prostaglandin administered to an diestrus mare lyses the CL causing return to estrus. Inject IM 5-10 mg dinoprost tromethamine (Lutalyse).The CL resists prostaglandin in the first 4-5 days post ovulation, but becomes responsive afterwards. Response depends on degree of follicular development at the time of injection.
In the absence of a dominant follicle, prostaglandin leads to estrus in about 2-4 days and ovulation in 6-12 days. Palpation of mares before prostaglandin injection, and teasing is necessary to cope with variable responses.

22. Breeding management proceedures
Management procedures include:1/ teasing, 2/ rectal palpation, 3/ ultrasound examination, 4/ artificial insemination and 5/ artificial lighting. Teasing and estrous behavior record keeping are the most important parts of a successful breeding program. Positive signs of estrus include: posturing, tail raising, urination and eversion of the vulvar labia (winking the clitoris). Mares not in heat may kick, bite, swish their tail and pin their ears back when they think they are being assaulted. Even when they know you well, they may take shots at you when going in and out of heat !
How many persons with broken hips have you seen ? But now repairing a hip is better so that a person may not be characteristically lame. When she’s in heat, if she winks her clitoris at him, will he notice her, or does she have to back into him ?

23. Separate records should include the year, the name, color and age of the mare, and the farm number. Assign individual mares a numbered neck band. Neck bands help in quick and accurate identification of each mare. Records should also identify the stallion the mare is booked to, the mare’s owner and previous breeding results. Of course records are kept on stallions including fees paid which can be big business.
A form for recording foals
Check List for Foals
Dam's name ___________________________ Sire _____________________
Date of last vaccinations for mare:
influenza ________ rhinopneumonitis ________ tetanus ________
encephalomyelitis ________ distemper ________ rabies _________
wax formation: early ________ late ________ none ________
colostral loss? ____ date mare foaled ________ time of foaling ________
ease of foaling: no assistance ________ slight traction ________
moderate traction ________ severe dystocia ________
oxygen necessary? ____ time & manner of umbilical separation ___________
iodine navel _______ placental expulsion: time _________ weight ________
condition ___________________________________________
attitude of foal _______________________________ first stood ____________
blood/colostrum agglutination: none ___ slight ___ moderate ___
first nursed: _______ meconium passed: _______ urinated: _______
tetanus antitoxin adminstered to foal ___________ enema necessary? _____
antibiotic administered to: dam _____________ foal ____________
weight of foal __________ color of foal __________ sex __________
markings drawn on application? ______ photograph? ______
check foal's vital signs at 4, 12 and 24 hours postpartum:
4 hour vitals: heart ______ respiration ______ temp ______
12 hour vitals: heart ______ respiration ______ temp ______
24 hour vitals: heart ______ respiration ______ temp ______

24. Sperm and artificial insemination
Advantages of artificial insemination can include: / Decreased transport costs, 2/ The stallion can be collected and his ejaculates frozen and 3/ There have been no limitations on the storage time of frozen semen. The disadvantages include: 1/ Wide individual freezing variability, 2/ The techniques are not standardized, 3/ After thawing, frozen semen has a shorter life span in the mare than fresh semen, 4/ The conception rates of artificial insemination with frozen semen are lower than with fresh semen. Conception rates of 60-65% can be reached.

25. A ghost used in stallion collection substituting for a jump mare.
The concentration of spermatozoa should range from million/ml. Second ejaculates collected 1 hour after the first will have a concentration of about 50 % of the first.
The total number of spermatozoa per ejaculate vary with the season of the year. The highest total occurs in July and lowest total in January. For example, the first ejaculate may contain an average of 22 billion spermatozoa in July and 10 billion in January. This means that early in the breeding season a stallion's total spermatozoal output may be about 50 % of his maximum capable spermatozoal output.
A ghost used in stallion collection substituting for a jump mare.

26. A stallion is considered fertile when the semen evaluation meets the following criteria:
1/ Volume ml, 2/ Concentration million spermatozoa/ml, 3/ Progressive motility %, 4/ Morphology % normal and pH Let us say that semen has been diluted and deep frozen as in liquid nitrogen into 200 straws. Cost for managing this ejaculate is say $ 600. Then each straw costs $ 3. Embryo transfer and testtube growth of oocytes (ova) has been performed with very low success levels. A mare could have 25 plus foals by transfers (transplants) and nuclear transplants can be performed as in cloning.