Gary Heusner, Extension Animal Scientist
Horses traditionally have the lowest reproductive efficiency of all species of domestic livestock. National foaling rates are commonly between 50 to 60 percent of the mares bred. There is probably no one specific cause for these low rates. The arbitrary January 1 birthday, causing people to attempt to breed mares during low fertility months, incorrect heat detection of the mare, bacterial infections and low or non-existent culling rates of low fertility mares probably all attribute to the nationally low reproductive rate of mares. Poor, incomplete record keeping also has contributed to low foaling rates. Some knowledge of the reproductive physiologies of the mare and stallion is necessary to achieve success at breeding mares. This is true whether you are a breeding farm manager overseeing the breeding of 300 mares and 20 stallions or the owner of one broodmare that you foal out and send away to be mated.
Some of the important aspects of reproduction in the mare include:
|Puberty Age (Months)||Type of Estrus||Estrous Cycle Interval (Days)|
|Estrus (Heat) Length or Duration (Days)||Time of Ovulation Before the End of Estrus|
|Average Length of Gestation (Days)||Time of Foal Heat After Foaling (Days)|
All of the values given above are averages. Early in the breeding season it is not uncommon for a mare to be in heat for 30 to 40 days without ovulating. This is generally a normal situation you may encounter. It is also not unusual for a mare to have a gestation period longer than 360 days.
Ovaries: Because they produce eggs, the ovaries, which are bean or kidney-shaped, are essential for reproduction. Mares in North American are considered to be seasonally polyestrous because from November to March it is not unusual for their ovaries to become small and inactive and for mares to cease cycling (anestrous). During the transition period between the non-breeding season and the breeding season the ovary contains numerous small follicles but no corpora lutea. This is why 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 corpus luteum which produces the hormone progesterone. If the mare does not become pregnant, the corpus luteum will have a lifespan of 12 to 15 days. If the mare becomes pregnant, several large follicles may develop on either ovary of the mare and these follicles may rupture or luteinize to form accessory or secondary corpora lutea. These accessory corpora lutea are a supplementary source of progesterone during the 40th to 180th day of gestation.
Ovulation,the rupture of the follicle, is spon-taneous, and the extrusion-release of the egg (ovum) occurs in the ovulation fossa. The ovulation fossa is the indented or recessed area of the kidney bean-shaped ovary.
Oviducts: The oviducts run from the ovaries to the uterus. The ovarian end of each oviduct is enlarged and is termed 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. The egg will arrive in the oviduct approximately six days after ovulation.
Uterus: The uterus contains a body and two horns. The horns connect the uterus to the fallopian tubes and are approximately ten inches long. The body is connected to each horn at roughly 90 angles, the body being approximately eight inches long. The lining of the uterus of the mare is generally light pink in color and is made of longitudinal folds. The lining is termed the endometrium.
Cervix: The cervix is a muscular structure approximately three to four inches long. The cervix separates the uterus and the vagina. There is extensive folding of the muscle layer so that no well-defined cervical canal is present. 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 approximately six to eight inches long. The vagina is divided into the vagina proper and the vestibule vagina. The hymen separates the two parts and is just behind the area where the urethra enters the reproductive tract. The hymen is usually not present but should be examined if it is.
Vulva: The vulva is the terminal part of the reproductive tract. It is approximately four inches long. The external orifice is a vertical slit approximately six to seven inches long.
Heat or estrus is defined as the time at which a mare is receptive to a stallion, allowing for mating. The heat, or estrus, phase is one of two phases that make up the estrous cycle of the mare. The estrous cycle is defined as the interval from the beginning of one heat period to the beginning of the next heat period.
Another definition of the estrous cycle is the period from one ovulation to the following ovulation when accompanied by signs of estrus. During the true physiological breeding season the average cycle is 21-23 days. The follicular phase or estrus period of the cycle is the period in which the ovary has rapid follicular growth, and the mare has behavioral signs of estrus. Behavioral signs of the mare when teased or presented to a stallion may include raising of the tail without switching, spreading the hind legs apart, flexing the pelvis. The labia of the vulva contracts and relaxes, and there is eversion of the clitoris, commonly referred to as "winking." Mares in estrus or "heat" usually urinate quite frequently when teased. Estrus will average five to seven days.
The other phase of estrous cycle is the luteal phase or diestrus period. The luteal phase begins with ovulation and during the physiologically normal breeding season lasts 15 to 19 days. The corpus luteum is formed, and within 24 to 48 hours the mare ceases showing signs of heat to the teaser. A repro-ductive history of a mare for approximately one year will illustrate the approximate durations of each of the two phases of the estrous cycle and ovulation times.
There is a wide range of estrous cycle lengths. Irregular or prolonged estrous cycles may be
caused by several factors. The length of daylight exposure is the most common one. Periods of
long daylight, 15 to 16 hours, such as late spring and summer, stimulate ovarian activity, while
periods of short daylight, nine to 10 hours, such as winter, inhibit ovarian activity. Therefore,
cycles are shorter between the months of April and October and longer between November and
March. Table 1 gives the mean estrous cycle length of 11 mares studied over a two-year period
by Hughes et al in California. If you start the breeding season in February follicles may develop,
but may regress before ovulation. This commonly occurs in the mare during the transition period,
from winter anestrus to the breeding season.
TABLE 1. Average Length of the Estrous Cycle
|Month||Mean Cycle Length (Days)|
A prolonged corpus luteum is another factor that may influence the estrous cycle length. The luteal phase may range from 35 to 90 days instead of the normal 14 to 16 days. The prolonged corpus luteum continues to secrete enough progesterone to suppress signs of estrus.
Also, a mare may have normal ovary function, but show no signs of heat. This is often referred to as a silent heat. Often, mares that have nursing foals are very possessive and protective. These mares may be cycling normally, but will not show signs of heat.
Inadequate or improper teasing methods are by far the most common causes of low reproductive efficiency on a breeding farm. Various methods can be used to detect heat in mares. The type of method utilized depends on the size of the breeding operation and the types of mares handled.
Some teasing methods include the tease rail, stall teasing, paddock teasing and chute teasing. The tease rail is simple to construct and mares can be positioned close to the teaser. The disadvantage is that the use of the tease rail is time consuming and labor intensive because each mare must be handled individually.
Stall teasing is a method in which the stallion is brought to the mare's stall. The teasing procedure is fairly rapid. However, two people are required, one to position the mare and one to handle the stallion. The mare is more likely to injure herself, and it may be difficult for the stallion to obtain a reaction. Another stall method would be to bring the mares to the teaser's stall.
Paddock teasing is used more commonly on large breeding farms. Several mares are run into a small paddock and exposed to a stallion. Paddock teasing is easier with mares that have foals. In addition, one person can observe and record reactions. The disad-vantages of paddock teasing are that not all mares in heat may approach the stallion and/or there may be one or two mares that will attempt to dominate the teaser and chase other mares away. These problems can be partially overcome by placing the pen containing the teaser in the middle of the paddock or by using two stallions at different locations in the paddock.
The chute method of teasing uses a chute that is 146 feet long and 28 inches wide. The chute holds nine mares at intervals of 16 feet. The teaser is then led to all mares individually. The chute method allows a large number of mares to be teased at one time. However, it does not work very well with mares with foals.
Whatever method of teasing chosen, it is important to have diligent personnel responsible for heat detection. The personnel most directly affected by the outcome of the breeding program should be those who are involved in heat detection. Mares should be teased at least every other day all during the breeding season.
Record Keeping: Besides teasing daily or every other day it is very important to keep accurate, up to date records. Figure 6 is an example record kept at The University of Georgia. The records contain all the important information for identification. A daily tease chart is kept for each mare. The breeding dates are recorded in a separate space. The back of the record contains a space for recording results of palpations and any treatments the mare may need and the costs of all treatments. This record is considered a permanent record. Many farms will use a daily tease sheet for recording mare reactions and transfer these daily tease sheet results onto the permanent record.
Determine the optimum time to breed a mare by palpation and the use of complete, accurate records. If you do not pasture breed and do not palpate your mares, breed them every other day starting the second day of heat until they go out of heat. As mentioned previously, early in the breeding season this may lead to a number of unnecessary breedings. The mare may be in heat 40 days which may mean 19 to 20 breedings. This is not only time consuming but increases the chances of injury to the mare and stallion. Exposing the mare that many times in one cycle also increases the risk of internal trauma and bacterial infection.
Nonpregnant mares that were bred in a previous breeding season should have a vaginal-cervical exam, intrauterine culture, a cytologic smear, and possibly, a biopsy of the uterine tissue endometrium to determine if the tissue can maintain an embryo/fetus until the birth of a foal. Also, mares that have not conceived after breeding through two estrous cycles should have an intrauterine culture and uterine cytologic exam.
During a vaginal-cervical exam a speculum is inserted into the vagina to determine whether fluids or lesions are present within the cervix and vagina. Urine in urine poolers is one of the fluids that may be seen around the cervix. A uterine culture is done by passing a swab into the uterus to obtain an endometrial mucosal sample. The culture is used to determine what, if any, bacterial organisms are present and their antibiotic sensitivity. Alone, culturing is not a reliable indicator of the presence of infectious organisms or of a mare's suitability to breed. The culture results should be used with the other examination procedures to determine first whether the mare needs treatment, second how she should be treated and third the probable success of getting the mare to conceive and carry to term. Organisms that may be cultured from the uterus are Streptococcus spp., Escherichia coli, Klebsiella pneumonia, Pseudomonas aeruginosa, Staphy-lococcus spp., and Cornybacterium.
Cytology is the study of cells, their origins, structures, functions and relationships to disease. A uterine cytologic exam is more successful in diagnosing infections than culturing for bacteria. Cytology identifies the presence of the mare's disease fighting cells and a culture identifies the organisms triggering the mare's natural defenses.
If it is determined through palpation, speculum examination, and uterine culture and cytology that a bacterial infection is present, intrauterine therapy is generally used. Intrauterine therapy most frequently includes infusions with antibiotics suspended in large volumes 100-500 milliliters of sterile saline. The infusions are repeated once daily for three to five days. The antibiotic used is one that is chosen from antibiotic sensitivity testing from the culture submitted to the laboratory.
The primary purpose of the uterine biopsy is to histologically evaluate the uterine endometrium to decide if the uterine tissues can maintain an embryo/fetus until the birth of a foal.
Candidates for uterine biopsy are mares that have been barren for one or more years without a diagnosis; older mares, 15 or older, not in foal being evaluated as broodmares and mares with genital lacerations needing a fertility evaluation prior to expensive surgery. A biopsy is obtained by actually taking a small section of uterine tissue with an instrument. This sterile instrument is passed through the vagina and cervix into the uterus. The instrument has a small "alligator like jaw" that "bites" a section of uterine tissue. At the laboratory a qualified histologist evaluates the tissue (endometrium) potential to support gestation. The uterine biopsy is graded according to changes in the tissue according to the Kenny Grading System. The tissue is graded a 1, 2 or 3. Following is a brief description of each grade and what they mean:
Endoscopic intrauterine examination or visual examination with a fiber optic instrument has proven helpful as supplemental information to palpation and speculum exams. Viewing of the uterus is not routinely performed. Ultrasound is a non-invasive diagnosis tool for helping to evaluate the condition of the reproductive tract. Acute and chronic endometritis results in visible fluid accumulation in the uterus. Other abnormalities of the ovaries and uterus can be detected with ultrasound which generally cannot be determined by palpation.
Palpation, cultures, cytologies, biopsies, and endoscopic exams should be performed by a competent, trained professional.
The question "When do you breed?" is probably best answered by palpation, and use of ultrasound if available, coupled with interpretation of teasing records. Palpation and the use of an ultrasound probe is done rectally in the mare. Ovary size and, specifically, follicle texture (firm or soft) are determined. Follicles may be of ovulating size at 30 mm or more and usually reach maximum size the day before ovulation. Uterine tone also is determined, and the size and condition are noted also. Cervical size can be determined and may be recorded as one, two, three or four fingers in width. The cervix drops from a position in the middle of the anterior part of the vagina to a position on the floor of the vagina due to relaxation and softening as estrus progresses. Even after the mare is bred, continue teasing. If the mare does not conceive, she will return to estrus (heat) in approximately 15 days from the last heat shown. If the mare does not return to estrus 18 to 20 days after breeding, a competent palpator or ultrasound exam will provide an indication of whether the mare has conceived. Teasing should still continue. Palpation should be done around days 25 to 30 and 50 to 60 after breeding to confirm pregnancy. Many large breeding farms will not send mares home with a confirmed pregnancy until they have been palpated and possibly ultra-sounded pregnant between 50 to 60 days. The key is religious teasing after breeding and knowing which mares may be difficult to detect in heat.
After foaling a mare will return to heat in nine to 13 days. This heat is foal heat. If the mare had a normal foaling, and the reproductive tract appears normal upon palpation and speculum examination the mare may be bred during foal heat.
Prostaglandin. The most widely used hormones are the prostaglandins. Prostaglandin (PG) F2 is used because of its luteolytic effect on the corpus luteum; that is, PGF2 causes the corpus luteum to regress and cease production of progesterone. Therefore prostaglandins are only effective during the time a mare has a functional corpus luteum and is cycling normally.
Prostaglandin may be used in several situations. One may be for treatment of a prolonged or maintained corpus luteum. Mares that exhibit extended diestrus, 25 to 30 days, and have not been bred or mares that have not conceived are candidates for prostaglandin treatment. A second use of prostaglandin would be to shorten diestrus. For example instead of breeding during foal heat a mare would be injected with PGF2 on day five or seven of diestrus. The estrous interval will then be shortened by seven to ten days. Another example would be a case of several mares that need to be bred to one stallion. Some of these mares would be bred while others would be allowed to go out of heat. The mares not bred would be injected on day five or six of diestrus to return to heat in two to four days.
A third use of prostaglandin is to minimize heat detection and synchronize estrus. Estrous detection in mares can be time consuming, tedious and often difficult in mares. Using PGF2 can help in catching mares with difficult-to-determine estrous status such as shy teasers.
HCG. HCG or Human Chorionic Gonadotrophin is a hormone that is used to help a mare ovulate. It is most commonly used when a follicle reaches ovulatory size but does not ovulate in a reasonable period. Some farms are using the HCG 24 hours after the estrus begins. The mares are bred 24 hours after the injection, and ovulation usually occurs before 48 hours after injection.
Estrogens. Estrogens are considered to be the hormones associated with signs of estrus. Estrogen compounds have been used to induce estrus in "jump mares," mares used for collection of a stallion in an artificial insemination program. Estrogen compounds have been used in mares that develop a large ovulatory-sized follicle but do not exhibit any signs of estrus. Estrogens have also been used to induce uterine tone in older barren mares, and to cause relaxation of the cervix during induction of parturition.
Progesterones. Progesterone therapy is presently used to treat transitional-phase ovaries, to suppress signs of estrus, to supplement endogenous progesterone for maintenance of pregnancy and to aid in the treatment of uterine involution and retained placentas.
Hormone therapy is very useful in a broodmare operation but should be used very carefully. You can do more harm to a normally cycling mare with hormones than you can correct in a breeding season because some of these drugs can induce infections. Know why you are using the drug and what effect it will have, and monitor the animal closely after you've administered the drugs.
Here are some routine management practices you should follow to keep a mare at a high level of fertility:
Because a stallion is both valuable and important in the breeding program, the stallion manager should know about the anatomy and physiology of a stallion's reproductive organs.
Testes. A stallion's testicles each weigh approximately 225 to 300 grams, average four to five inches long, two to three inches high, and two inches 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 approximately 20 inches long in the relaxed state with approximately six to eight inches lying in the prepuce. During erection the size increases about twofold. 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.
A breeding soundness evaluation and semen evaluation of the stallion should be done prior to purchase and the start of the breeding season. A soundness exam should include a general physical examination. The reproductive tract should be examined by palpation for any abnormalities. There should be no injuries to the hind leg that may impair a stallion from mounting. The stallion should not be overweight or underweight. Finally, the stallion's general temperament should be observed as well as his libido while teasing or servicing a mare.
A semen evaluation is done by collecting from a stallion twice. The second ejaculate should be collected about one hour after the first. Before declaring a stallion infertile make several evaluations. Several criteria are used to evaluate stallion semen. The ejaculate volume should be 25 to 100 milliliters, but may be as great as 300 milliliters; the season of the year will influence seminal volume. Research has shown that gel-free semen volume is lowest in January and February and highest in May and June. The amount of gel in the semen, which is related to season of the year and individual stallion variation, will also influence total volume.
The concentration of spermatozoa, another factor in evaluating semen, should range from 30 to 800 million per milliliter. Second ejaculates collected one hour after the first will have a concentration of about 50 percent of the first. The total number of spermatozoa per ejaculate also vary with the season of the year. The highest total has been shown to occur 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 percent of his maximum capable spermatozoal output.
A third criterion used to evaluate semen is motility, or progressive movement, of spermatozoa. Stallion spermatozoa should move in a straight line. The percentage of progressively motile spermatozoa in an ejaculate is usually 60 to 100 percent-70 percent is considered quite good. Season of the year does not affect motility.
The morphology of the spermatozoa is a fourth criterion used to evaluate semen. The percent abnormal sperm will usually range from 20 to 30 percent. Abnormal sperm cells include those with deformed heads, and deformed-twisted, abnormally curved, and double-tails.
The pH of stallion semen is fifth criterion used in semen evaluation. The normal pH range of stallion semen is 7.4 to 7.6. A pH outside of this range may indicate incomplete ejaculation or the presence of other fluids in the semen such as blood or urine.
A stallion is considered fertile when the semen evaluation meets the following criteria:
The fertility of a stallion should be questioned when a semen evaluation yields the following results:
Several ways or methods exist for breeding horses; i.e., hand breeding, corral breeding, pasture breeding and artificial insemination. Following is a brief discussion of the various methods.
Hand Mating. Hand mating is probably the most widely used method of breeding horses. Usually one person handles or holds the mare to be bred while another one to two people handle the stallion. The stallion is allowed to breed the mare naturally. Since both the mare and stallion are handled, the two are not as likely to be injured as they would if they were simply turned loose together. The mare should have been teased previously and shown to be in heat and receptive to the stallion. The mare's hind legs are usually hobbled in some manner so she cannot kick the stallion and stallion handlers. The mare should also have had her tail wrapped and her external genitalia cleaned. Wash external genitalia with a mild liquid detergent such as a liquid dishwashing soap. The genitalia along with the buttocks should be soaped and thoroughly rinsed three times. The stallion's penis should also be thoroughly soaped and rinsed about three times. Controversy still exists about proper hygiene for a stallion. In light of recent reports, it appears a stallion's penis should not be washed every time he is used. Some non-pathogenic disease-preventing bacteria are removed when a stallion is washed, so if the penis is washed every day or every other day, the incidence of pathogenic bacteria and diseases may actually increase. Current recommendations are to wash the stallion's penis about once a week. Once a week cleansing with a mild liquid dishwashing soap will also prevent irritation to the penis which affects the stallion's libido.
Corral Mating. Another method used to breed horses is referred to as corral mating. The mare to be bred is turned into a small paddock or "corral" when she is ready to be bred. The stallion is then turned in with the mare. The mare and stallion are then taken out after they have successfully mated. This procedure is used more commonly with young, inexperienced stallions who aren't quite sure of their roles. Corral mating is generally not used with valuable mares or stallions because of the increased risk of injury as compared to hand mating or artificial insemination.
Pasture Mating. Pasture mating is used by many breeding farms. The stallion is turned out with a band of broodmares for the entire breeding season. Conception rates are usually higher than for other methods of breeding, but the stallion and mares are more likely to get injured than in hand breeding or artificial insemination. Probably the biggest disadvantage of pasture breeding is that it is more difficult to establish breeding dates and in turn foaling dates for mares.
Artificial Insemination. Artificial insemination is increasingly used with horses for several reasons. The number of mares a stallion can service during the breeding season can be increased. "Problem mares," such as those with long heat periods where natural service will increase the chance of infection, sutured mares and those which show no external signs of heat, but are cycling and ovulating, can be bred more easily. Venereal disease is more easily controlled with A.I. Stallions that have behavior problems and/or injuries can service an artificial vagina. A disadvantage of artificial insemination is that it is time-consuming to collect, prepare and inseminate semen. Extra labor and equipment such as an artificial vagina, an incubator and inseminating equipment are needed. A final disadvantage of A.I. is that some breed associations do not allow artificial insemination, and those that do usually require semen collected and inseminated fresh on the same premises.
The procedure for A.I. is similar to that of hand mating. A "jump mare" or a phantom -- a padded frame about the size of a horse -- can be used to collect semen from the stallion. A phantom is preferred because of safety and the fact that a stallion can be collected without having a mare in heat. The semen that is collected can be inseminated fresh or extended. Usually, extenders are used when the volume of the semen per mare is less than 10 cc, or when a stallion has a lowered fertility. In some cases they are used to provide antibiotic treatment to the sperm to control bacterial contamination. Extenders are added to the semen in a 1:1 to 1:3 semen-extender ratio. Several formulas are available including cream-gelatin, skim milk and milk-gelatin extenders.
Insemination is done either manually with a gloved hand or through the use of a speculum. The hand guides the insemination fussette directly into and through the cervix or the fussette is passed through the speculum into the cervix.
The stallion reaches puberty between 11-15 months of age. The earliest advisable age to breed a
stallion is during the two-year-old year. Puberty in a stallion is when the ejaculate contains 1 x
108 spermatozoa with 10% progressive motility. If a 2 year old is used as a breeding stallion the
number of services should not exceed two per week and only 5 to 10 mares should be bred
during the breeding season. The 3 year old stallion can be used four to six times per week. The
mature stallion can be bred daily but he should get one day of rest every 7 to 8 days. If necessary,
mature stallions can be used twice a day for 3 to 4 days each week. An overused stallion will
begin to develop behavior problems such as savaging (excessively biting) a mare, slow reaction
and mounting times, and a dislike for the handler, especially after mating with a mare. When
stallions begin to display abnormal behavior signs they should be rested for a period of five to
seven days. Make sure they get outside and are allowed free exercise. Following is the
recommended numbers of mares to hand breed to various aged stallions during the breeding
|Age (Years)||No. of Mares/ Breeding Season (Hand Mating)|
Here are some routine management practices for keeping the stallion at the highest level of fertility:
Bulletin 945/Revised April, 1993
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