Chapter 11 Reproduction and breeding.
After completing this chapter, you should be able to:
* Discuss breeding periods
* List and discuss the major parts of the female reproduction tract
* List and discuss the major parts of the male reproduction tract
* Describe reproductive hormones during the estrous cycle
* Recognize fertility problems
* Explain gestation and parturition in horses
* Discuss and demonstrate methods of artificial insemination and heat detection
* Explain embryo transfer
* Describe the management of the mare and stallion before, during, and after the breeding season
* Describe the management of the mare, including care at parturition, nursing to weaning, and growing to maturity
artificial insemination (AI)
PHYSIOLOGY OF REPRODUCTION
When it comes to breeding practices and the reproductive process, the mare's reproductive control mechanisms are quite efficient when the animal is left to function in the wild. Manipulation and confinement have reduced the efficiency of reproduction. Several factors contributing to poor reproductive performance include:
* Reproductive anatomy
* Long time period before an embryo can safely implant in the uterus
* Variable hormonal system synchronizing the whole process
Reproductive organs of the stallion and mare, as well as the horse's endocrine system, were introduced in Chapter 5. Figures 11-1 and 11-3 review the reproductive organs of the stallion and mare.
The reproductive organs of the stallion consist of two testes, each suspended by a spermatic cord and external cremaster muscle; two epididymides; two deferent ducts; the penis; and the associated muscles. Accessory sex glands are paired vesicular glands, one prostate gland, and paired bulbourethral glands. The outside of the reproductive tract includes the scrotum, prepuce, and penis (Figure 11-1).
The scrotum is an outpouching of the skin, divided into two scrotal sacs by a septum. The two sacs each contain one testis, located on either side of the penis. The testes should descend from the abdominal cavity through the inguinal canal into the scrotum between the last 3 weeks of gestation and the first 2 weeks after birth. If this does not happen before closure of the inguinal ring, the cryptorchid testis (usually the left one) stays in the abdominal cavity.
Mating Process. The process of ejaculation consists of three parts: erection, emission, and ejaculation.
Erection is stimulated by teasing the stallion. During erection, the penis lengthens and stiffens through engorgement with blood. Emission occurs in strong pulsatile contractions.
During emission, semen, which contains spermatozoa and fluid from the cauda epididymis plus fluids from the accessory glands, arrive in the pelvic urethra.
[FIGURE 11-1 OMITTED]
During ejaculation, the semen is expelled through the urethra (Figure 11-2). Reproduction in the stallion is also under hormonal control. The hormones directly involved include FSH, LH, and testosterone. Actions of these hormones are described in Chapter 5 and Table 5-1.
Sperm Production. No spermatozoa production occurs until a stallion is well over a year old, and full reproductive capacity is not reached until the age of 4. The stallion's reproductive capacity will then remain constant until he is about 20 years old. The tendency is to overuse a young stallion and underuse an old stallion.
[FIGURE 11-2 OMITTED]
Sperm output and sperm production is influenced by:
* Testicular size
* Frequency of ejaculation
Although stallions produce spermatozoa throughout the year, they are seasonal breeders. In the Northern Hemisphere, the three best months for testicular size, development, and function are May, June, and July, while from September through February the testes are regressed, especially in November and December. In December and January, the sperm count is 50 percent of that during June and July. Normal semen characteristics are given in Table 11-1.
If the majority of mares have to be bred early in the season, between February and June, an artificial lighting program may be useful. Starting in mid- to late December, the stallion should be exposed to 16 hours of light and to 8 hours of darkness per day; this should be continued until there are 16 hours of natural daylight. However, the stallion has to be normally exposed to the decreasing daylight in the fall to eliminate a photorefractory condition that would prevent his being sensitive to increasing light. Artificial photoperiods do lead to early burnout and a decline in performance at the end of the breeding season.
The mare's reproductive anatomy is characterized by a simple uterus, unusual placenta arrangement, and inefficient cervical closure (Figure 11-3). The uterus has a small uterine body with two long, narrow uterine horns. These structures are suspended within the abdominal cavity via ligaments and connective tissue to the abdominal wall.
[FIGURE 11-3 OMITTED]
The elongated shape of the uterus and uterine horns causes the uterus to drain inefficiently, predisposing this organ to infections. With each subsequent pregnancy, the uterus and supporting connective tissue decrease in tone, and the suspension of the uterus becomes lower and lower in the abdominal cavity. The closure of the cervix maintains pregnancy by retaining the embryo and its membranes within the uterus and preventing entry of bacteria. Maintenance of the cervical seal is vital for embryo survival by preventing infection. The closure of the cervix is controlled by hormonal levels and can be unstable. This has been cited as a possible cause for early embryonic death.
The placenta is attached to the endometrial lining of the uterus by innumerable tiny villi that project into the lining, forming a shallow one-cell-thick fusion through which the placenta transfers the embryo's blood, oxygen, and nutritional needs. This type of placenta does not allow immunoglobulins to pass to the fetus. This attachment is responsible for maintaining embryonic life, and early shedding or detachment of the placenta drastically endangers the pregnancy.
Hormones of Reproduction. All reproductive functions in the mare are controlled by hormones produced in the glands of her endocrine system; hormonal balance controls all phases of reproductive tract stimulation and inhibition. When hormonal balance is not achieved, either because of a natural imbalance or a disturbance, a mare will have problems cycling, conceiving, maintaining pregnancy, delivering a foal, and providing an adequate milk supply. Table 5-1 describes the actions of the hormones involved in reproduction.
Hormonal Cycles. The mare has a strong follicle-stimulating phase of her cycle, which is what causes her to come into heat and ovulate. The luteal phase occurs after she ovulates and is responsible for production of the corpus luteum and its production of progesterone.
If conception does not take place, prostaglandins are released that destroy the corpus luteum, reduce progesterone, and allow the mare to cycle again. If there is conception, no prostaglandins are released, and the corpus luteum remains dominant. The high level of progesterone during the luteal phase maintains pregnancy by keeping follicle-stimulating hormone (FSH) and estrogen in check, thereby preventing the mare from coming back into estrous and disrupting the newly established pregnancy. Progesterone also relaxes the uterus to allow the embryo to implant and the new pregnancy to be established.
The mare is a seasonally polyestrous species, which means that she comes into estrous several times a year but does not cycle all year round. In the Northern Hemisphere, the mare begins cycling somewhat irregularly in January and February as the days get longer. She continues having more regular cycles until her breeding season peaks in June. In September or October, as the days shorten, the mare ceases to cycle regularly. By late November she stops cycling altogether and remains inactive through winter. The times of the year with irregular and subfertile cycles in February and March, and in September and October, are called breeding transition months.
The current tendency of some breeders to mate mares earlier and earlier in hopes of producing foals earlier in the year is a reproductive problem for the mare. While this practice is perhaps economically advantageous when a larger and more developed foal is presented in the sale or show ring, it is incongruent with the mare's natural timing. The result is that reproductive efficiency is sacrificed for the chance to attain greater weanling and yearling sizes. An arbitrary January 1 birth date entices breeders to try to breed their mares before they are ready to accept and maintain a pregnancy or even to conceive.
When mares begin cycling in the spring, their estrous lasts 6 to 8 days. The length of estrous progressively shortens until it is only 3 to 4 days in most mares at the peak of the season in May or June. These durations are only guidelines. Variation is more the rule.
During estrous, or heat, follicles develop in the ovaries. The follicles produce the hormone estrogen, which causes the signs of sexual receptivity. Although several follicles develop simultaneously, usually only one follicle will emerge as dominant. Ovulation, the time when a primary follicle is ready to shed an egg mature enough for fertilization from the ovary, occurs late in the estrous, no more than 2 days before the mare goes out of heat. The exact time of ovulation varies for individual mares. In fact, early in the season, a mare may exhibit signs of estrous and not ovulate at all, just as some mares may ovulate on schedule but not show outward signs of estrous (heat). The incidence of ovulation within the estrous period increases as daylight increases, peaking in late June.
After ovulation, the now eggless cavity in the follicle fills up with a blood clot and is now called the corpus hemorrhagicum. This becomes the corpus luteum that produces the hormone progesterone, which corresponds with the diestrus.
During estrous, the cervix relaxes and is soft and rose pink. Secretions in the vagina during estrous are clear and slimy, and the vagina is red and vascular. In diestrus, the cervix protrudes into the vagina; it is pale pink and tightly constricted; and secretions are scant, viscous, and sticky.
At the time of ovulation, the follicle in large mares may be as large as 65mm in diameter (ranging from 35 to 65mm). By day 20, the follicle may be detected by rectal palpation and by ultrasound. Ultrasound can estimate follicular size and can also differentiate between a young corpus luteum and a soft follicle, even though these structures feel similar during rectal palpation.
Usually only one follicle will ovulate. Occasionally two follicles will ovulate at the end of the estrous phase. This is undesirable because twin fetuses have a high risk of abortion and cause complications such as dystocia (long or difficult labor) and retained placenta. If the fetus is aborted before day 45, the mare continues to show signs of being pregnant, due to special tissue secreting the hormone that maintains pregnancy, until day 120. Therefore, another ultrasound should be performed after day 45 (Table 11-2).
Breeding Habits. Estrous cycles will start in a mare at puberty, which is usually between the age of 15 to 24 months, but can be as early as 1 year of age and as late as 4 years of age. The mare usually goes into winter anestrus between November and February or March. The mare has a normal estrous cycle of 21 to 22 days. The first 5 to 7 days, when the mare displays behavioral signs of sexual receptivity to the stallion, are called the estrous. When teased with a stallion, the mare will raise her tail and urinate, and the labia will open to expose the clitoris ("winking"), while she assumes a mating position.
During the second, or luteal phase, the behavioral pattern is that of sexual rejection of the stallion. This is called diestrus and lasts 14 to 15 days. During diestrus, the mare will switch her tail, pin her ears back, kick, and move away from the stallion when she is teased.
Fertilization. The ovum leaves the ovary and enters the oviduct, where fertilization will take place. The ovum is viable for 8 to 12 hours, while the spermatozoa coming up the oviduct can live for 24 to 48 hours (sometimes for several days) inside the mare's reproductive tract. The time involved for the spermatozoa to travel through the oviduct and reach the ovum is 4 to 6 hours. Based on these time constraints, breeding is recommended within 1 or 2 days before ovulation. After the egg is fertilized, it travels down the oviduct and enters the uterus in 5 to 6 days. Once in the uterine horn, the embryo is very mobile; it bounces around and may move from one uterine horn to the other. By day 16 to 18, the embryo settles in one part of the uterus, where it implants.
From ovulation to parturition, the average length of gestation is 335 days, plus or minus 2 to 4 weeks. This depends on the season, nutritional status of the mare, and sex of the fetus.
Abortion is the premature termination of pregnancy before 300 days of gestation, while termination after that time (when a foal may be born alive) is considered stillbirth. Abortion can be caused by both bacteria and viruses. If a mare aborts, a veterinarian should be consulted.
Checking the Mare for Breeding
Considerations for breeding include:
* Hereditary disorders
Factors that relate to the mare's reproductive potential are:
* Past breeding records
* Previous athletic use
* General health
Age. From puberty to old age, a mare can conceive and carry a foal to term. The 2- to 3-year-old may have some abnormal cycling patterns. After multiple foals, mares may have anatomical changes in the vulva and vagina, predisposing for pneumovagina (air in the vagina) and urine pooling.
Breed. Breed organizations differ in policy on natural versus artificial insemination, semen transport, and preservation of semen, as well as on embryo transfer. Miniature breeds and the very large draft breeds show a greater tendency to reproductive failure.
Status. A maiden mare is one that has never been bred. A barren or open mare is one that was either not bred the previous season or did not conceive in the previous season. Unless she was not bred, this implies failure of conception or failure to maintain pregnancy. A wet mare has foaled during the current breeding season and is nursing the foal. Past Breeding Records. Previous foaling data, such as gestation length, any complications, cycling patterns from previous years, previous reproductive surgery, previous uterine infection and treatment, as well as evidence of early embryonic death in previous seasons are all helpful information in reproductive evaluation.
Previous Athletic Use or Performance. After an athletic career, reproductive performance may be compromised because of injuries, diseases, or treatment with anabolic steroids.
General Health. Previous medical events such as chronic obstructive pulmonary disease (heaves) leading to coughing and difficulty breathing, cardiac disease, or pain from laminitis or tendinitis will all influence reproductive potential.
Nutrition. Mares in poor condition may not cycle or breed. Improving the nutrition of a mare before breeding can increase the chances of a successful mating. More information on this topic can be found in Chapter 13.
No Absolutes in a Biological System As a rule, most mares are seasonally polyestrous. This means that during a specific season of the year, mares experience several reproductive cycles. In the Northern Hemisphere, as the days get longer, mares show behavioral estrus or heat beginning in February and extending through July. The same trends occur in mares in the Southern Hemisphere for the corresponding seasons. Mares kept on grass normally go into anestrus--no cycles--in the winter. Near the equator, where the amount of daylight remains fairly constant, the length of the estrous cycle shows little variation. But as a whole population, mares can be classified into three major categories: * Defined breeding season. Wild horses breed during the time of year that corresponds to the longest days of the year. So, the foals are born during the spring of the year when feed is apt to be the best. * Transitory breeding season. Some domestic breeds and some individual mares show estrous cycles throughout the year. These mares breed, but matings in the winter months are not fertile since ovulation does not occur during the cycle. Actual ovulation occurs with estrus (heat) only during a breeding season defined by the increasing amount of light. Again foals are born during a limited foaling season. * Year-round breeding. Some domestic breeds and some individual mares have estrous cycles accompanied by ovulation all year. Foals are born any time of year. Of all the domestic animals, the reproductive cycle of the mare shows the greatest variation-- there are no absolutes in biology.
Ultrasound has been used for pregnancy diagnosis since the early 1980s and is used in addition to rectal palpation. Ultrasound may also provide diagnosis of conditions that cannot be felt by rectal palpation.
The ultrasound probe is inserted into the rectum and moved across the reproductive tract. Ultrasonography is useful in studying the normal reproductive cycle, diagnosing diseases of the ovaries and uterus, early detection of pregnancy, diagnosing twins, diagnosing embryonic death, and determining length of gestation.
In nonpregnant mares, ultrasound study of the ovaries can distinguish between follicles, corpus hemorrhagica, corpus lutea, ovarian cysts, and tumors. In the uterus, cysts or an infection can be diagnosed.
With ultrasonography, the diagnosis of pregnancy is possible as early as day 14 of gestation. The fetal heartbeat can be detected as early as day 22 of gestation, and should be routinely looked for from day 25 on. Between day 60 and 70, it is possible to determine the sex of the foal.
CARE AND MANAGEMENT OF THE STALLION
Yearlings should not be depended on for breeding. Two-year-olds may settle 10 mares; 3-year-olds, 30; and mature stallions, 50 mares when hand mated. About half of this number can be pasture mated. A short breeding season will reduce the number, and sexual individuality of the stallion will greatly affect his siring ability.
The breeding stallion should be fed like a horse at hard work. An estimate is 11/2 pounds of grain and 1 pound of hay per 100 pounds body weight. If he is worked under saddle, more feed will be required. Because of diverted interests, a ration high in palatability may be necessary for some stallions to get adequate intake. Grazing of good grass, even for short periods of time, is recommended.
Regular exercise usually results in increased sexual vigor (libido) and fertility. For safety, fences should be strong and tall when stallions are grazed loose; and mares should not be in adjoining pastures unless extremely tall fences are used.
Methods of Mating
Two methods of mating are used when breeding horses: pasture mating and hand mating.
Pasture mating reduces labor, affords convenience to the owner, "catches" shy breeding mares, and creates an opportunity for a high settling percentage. It has the disadvantage of reducing the number of mares a stallion can serve, and it obscures breeding dates. Some risk to the stallion exists.
Stallions should be hand mated a few times as 2-year-olds, then turned loose in a large pasture with a few older mares when they are to be used in a pasture breeding program. Even so, stallions are likely to carry some scars from their experience. For this reason, pasture mating is seldom used with breeds whose owners discriminate against blemishes. It is extensively practiced with stock horses in the range country.
A combination of hand mating followed by pasture mating will extend the number of mares bred and increase settling percentage (Table 11-3).
Hand mating is practiced under a wide variety of conditions, ranging from rather casual selection of mares and sanitation conditions to operations that are highly supervised with a veterinarian in attendance.
Stallions used with hand mating should be adept at teasing mares. This may be done at a teasing pole or over a stall door or any other sturdy fixture that does not injure the horses or attendants. A teasing stall and breeding stall are shown in Figures 11-4 and 11-5.
[FIGURE 11-4 OMITTED]
[FIGURE 11-5 OMITTED]
CARE OF THE PREGNANT MARE
For the pregnant mare, aside from nutritional requirements, attention should be paid to regular dental and hoof care. Broodmares usually do not need shoes, but if they are shod, the shoes should be removed a few weeks before foaling to protect the foal at birth. A good exercise program is recommended.
Parasite control to protect both mare and foal can be effected by deworming and helped along with general measures, such as sanitation and cleanliness. More details on parasite control are given in Chapter 15.
A vaccination program should include vaccinations against rhinopneumonitis (3rd, 5th, 7th, and 9th months of pregnancy), tetanus, equine encephalomyelitis, and influenza 4 weeks prior to foaling. In endemic areas, and under guidance from a veterinarian, additional vaccinations may be given for strangles, rabies, anthrax, and Potomac horse fever. In general, deworming and vaccination programs should be administered under the guidance of a veterinarian.
Since the placenta does not allow transfer of maternal antibodies to the fetus, the foal has to obtain antibodies against infection from the colostrum. The foal's intestine will absorb the antibodies only during the first 24 hours after birth, with the greatest absorption during the first few hours. If the mare loses colostrum in the last weeks of pregnancy when she starts to "bag up," this colostrum should be collected and saved for the foal.
Care at Foaling Time
Proper preparation of the mare for foaling is necessary if an owner is to realize the results of the time, effort, and money invested. The owner must prepare the mare, the environment, and her handlers for foaling.
During the final two months of pregnancy--the 10th and 11th months of gestation--the mare's abdomen takes on the pendulous, enlarged characteristics of pregnancy. Mares that have had several foals tend to take on this appearance sooner. The mare develops a wider stride to compensate for the increased weight she is carrying, and her ribs may appear more "sprung." This is also when the mammary glands begin to develop.
The pituitary hormone, prolactin, stimulates the udder to produce milk. If no mammary gland development is noted prior to foaling, the owner should be suspicious of hormonal inadequacies. This is a common occurrence if the mare is consuming fescue grass. An endophyte in some contaminated fescue seed heads blocks the action of prolactin. Normally, however, the udder slowly enlarges over the final 2 months of pregnancy. It becomes turgid and the teats fill out. Any leakage from the teats prior to foaling should be collected to avoid any loss of the antibody-rich colostrum.
The mare's tail head, croup, and perineal area (between rectum and genitals) become relaxed several days prior to parturition. This is due to the hormone relaxin, which loosens the ligaments of the pelvis. The amount of relaxation varies with the age of the mare and the number of previous pregnancies. Relaxation can be very slight and difficult to observe in a maiden mare, and quite pronounced, even to the novice owner, in an older mare.
Signs of Impending Parturition
The following checklist will help owners identify many of the signs present prior to foaling:
* Large, pendulous abdomen, or sudden change in position of the foal. Change in gait; occurs 1 to 2 months before giving birth.
* Udder enlargement. Change in its shape, texture, or temperature. A change in milk color from clear, or amber, to cloudy, or chalky white, means that delivery is very near. Calcium concentration in the mammary secretion increases immediately prior to foaling. If the milk calcium concentration is measured, by either the owner or a veterinarian, it can be used to predict the foal's arrival time.
* Waxed teats. Drops of sticky, clear, or amber-colored fluid excreted prior to parturition become dried and hard, coating the ends of the teats, giving them a waxy appearance; occurs 2 weeks to just hours before foaling.
* Relaxation of the tail head, croup, and perineal area; occurs 1 month to 2 weeks before foaling.
* Enlarged abdominal milk veins; occurs 2 months to 2 weeks before foaling.
* Loss of appetite. Does not occur in all mares, but if present, depressed appetite usually occurs during the last month of gestation.
* Change in personality. May separate from the herd if pastured with other mares. May push hindquarters up against a wall. Usually, if present, this behavior will change in the last 4 to 2 weeks before foaling.
Mares vary widely in the degree and length of time they exhibit these signs. The best predictor of foaling time is knowledge of the mare's gestation length and behavior during previous pregnancies.
Equipment Helpful for Foaling
Before foaling, the owner or manager should assemble the following equipment or supplies that will be needed during and/or after foaling:
* Four to five ounces of an iodine solution in a sterile jar
* Tail bandages or 3-inch gauze bandages
* Roll of sterile cotton
* Package of gauze squares (3" or 4" square)
* Adhesive tape (1 inch wide)
* One pint povidone-iodine compound
* Six to eight clean towels
* Enema tube, soap, and lubricant
* Seamless pail
* Large animal thermometer
Labor and Parturition
The mare's labor is intense and rapid. Usually it is over within an hour. Because of this rate, an owner does not have time to develop a wait-and-see attitude. Because all of the foal's oxygen is obtained through the umbilical cord blood supply, a prolonged delivery can quickly endanger the foal. If the mare or the foal appears to be having difficulties, help must be summoned at once. Labor is commonly divided into three stages.
Stage 1. The uterus begins to contract, the foal moves into position to be born, and the cervix relaxes in stage 1. Stage 1 usually is imperceptible and can go unnoticed, even by the watchful observer. Signs of late stage 1 labor, which may be observed in some mares, include restlessness, tail switching, pacing, and sweating over the neck, chest, and flanks. Because some of the symptoms are similar, stage 1 may be confused with colic. The mare may urinate and defecate frequently and carry her tail in an elevated position. The mare's respiration and heart rate may be increased, and her body temperature may be decreased. Stage 1 may be as short as several minutes or last longer than 24 hours. Recognizing this stage is important in order to prepare for the upcoming period of intense labor. To prepare a mare in stage 1 labor for foaling, thoroughly clean the mare's perineal area with a povidone-iodine solution, and bandage the tail out of the way. Clean and disinfect the foaling stall. If the mare has a caslick in place (a suture closing her vulva to prevent infection), remove it now. After completing the preparations, observe discreetly from a distance. Horses do not like to give birth in noisy surroundings, and the mare can shut down the foaling process if she does not feel completely safe.
Stage 1 ends when the allantois, or fetal membranes, are pushed through the cervix by the advancing fetus and rupture, releasing amniotic fluid (breaking water).
Stage 2. This is the time of intense labor contractions that push the foal through the birth canal. Stage 2 usually lasts no more than 30 minutes. Little can be done to slow the labor or make corrections if problems arise at this point. The advancing fetus is enveloped in a white, filmy membrane known as the amniotic membrane. The placenta, a thicker pink membrane, is attached to the uterine wall and should not be visible at this time.
The foal normally is lying on its stomach, positioned upright in the birth canal with its forefeet slightly displaced, one before the other, and with the muzzle resting on top of the knees. The bottom of the feet should be facing toward the mare's hocks; the curving of the foal toward the hocks helps move the shoulders and hip through the mare's pelvis (Figure 11-6).
[FIGURE 11-6 OMITTED]
The mare usually lies down as stage 2 commences. If the mare is startled, she may jump up and give birth standing. This results in quite a fall for the foal and premature rupture of the umbilical cord.
The foal's feet should appear within 15 minutes of the appearance of fetal fluid and membranes. If both feet are not visible, the attendaint should insert a gentle hand into the vulva to find the missing foot and guide it gently to the proper spot. This may prevent a sharp hoof from tearing the vaginal roof and perineal area.
Once the shoulders, the widest portion of the foal's body, are born, the rest of the foal usually follows shortly. Occasionally the hips may lock in the mare's pelvis; gentle, downward tension on the foal's front legs will help. This assistance should be very quiet and calm and timed with the mare's contractions.
If the mare has been pushing for 45 minutes and no sign of a foal is seen, intervention is needed quickly if the foal is to survive. Should the mare require assistance, the attendant must first make sure that the foal's front legs and head are properly positioned. If they are, traction may be used to gently pull the front legs.
Stage 2 is complete when the foal has been born.
Dystocia. Many situations cause dystocia, or difficult birth, in the mare. A mare may become exhausted in the middle of labor and be unable to push the foal out, especially if she is older or in a debilitated condition. A foal will not fit through the birth canal unless it is in the proper position. Any deviation from the front-legs-first, head-facing-down-between-the-knees posture may result in dystocia. The presence of twins is a possibility when labor becomes extended. Usually, however, a leg or the head becomes turned back or tucked in such a way to be pointing against the direction of the foal's movement. The entire foal may also be completely backward, or breech, with the hindquarters presented first, or upside down, or both.
Signs of dystocia require immediate veterinary attention. Until the veterinarian arrives, the attendant may try to place the foal in the correct position by turning, locating, and extricating lodged limbs. If repositioning is attempted, care must be taken to ensure that the attendant's arm is sterile and well-lubricated. Inserting a length of rubber tubing into the mare's trachea and walking may prevent her from bearing down too hard on the foal or your arm. Whether the foal has been born or not, the placenta detaches from the uterine wall within an hour of the start of stage 2. If the placenta detaches before the foal is born, the foal will lose its oxygen supply and die. Time is critical.
Foal. The newborn foal may have a blue tongue and bluish-white nasal mucus. As soon as it is born, the fetal membranes should be cleared away from the foal's head so that breathing can start. The umbilical cord should be allowed to break on its own. Once this has happened, the foal's navel should be immediately treated with an iodine solution to prevent entrance of pathogens through the opening. In some instances, the navel will bleed after the umbilical cord has broken. The cord should be tied shut with a length of sterile umbilical tape, gauze, or string (Figure 11-7a through d).
Stage 3. Stage 3 is complete when the placenta and fetal membranes are expelled. Prior to being passed, the placenta hangs from the vulva. It should be tied up so that the mare does not tear it or become afraid of it. The membranes usually are passed 5 to 45 minutes after the foal's birth. If they have not been passed in 2 hours, the membranes are considered retained, and a veterinarian should be called (see Figure 11-8).
The placenta must be naturally released from the uterus. Pulling may tear the placenta and leave small pieces in the uterus. These small pieces of placenta in the uterus can cause metritis, an inflammation of the uterus. Metritis is an infection that may result in laminitis or death. A mare with this condition may appear normal 48 hours after birth and then develop symptoms. Once serious symptoms develop, it may be too late to prevent permanent damage.
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The placenta should be inspected after it has been passed to ensure it is complete and there are no tears or pieces missing. It is normal for a soft, dark-brown body of tissue called the hippomane to be floating among the membranes.
After the membranes have been properly expelled, the mare's uterus will undergo involution, during which the uterus returns to its nonpregnant size. Without infection or trauma, the uterus will involute within 10 days. However, if a uterine infection is present, the process is delayed. Older mares who have had many foals involute more slowly than younger mares. Involution may cause abdominal pain and some colic-like symptoms. Some mares become extremely agitated, and the attendant should watch carefully to make sure the mare does not endanger the foal by rolling or getting up and down frequently.
[FIGURE 11-8 OMITTED]
The first 24 hours are most critical for both the mare and the foal (Figure 11-9). For this reason, they should be kept in a clean, quiet environment where they can be observed frequently. Many horse owners leave the pair stalled for the first day. After that, weather conditions and temperature should dictate whether the mare and foal are turned out or housed indoors.
[FIGURE 11-9 OMITTED]
Damp or wet environments should be avoided the first few weeks. Some mares and foals will actually fare better, and some foals show slightly greater resistance to adverse environmental conditions if they are kept outdoors.
Postpartum Mare Care
A mare needs special attention during the first week after delivery. The perineal region will be bruised and sore. Defecation may be painful. Bran, beet pulp, more salt, or other laxativetype feeds may loosen the stool and make defecation less painful. Because decreased appetite and water intake will result in decreased milk production, every effort should be made to keep the mare comfortable. The mare should have free-choice access to water and mineralized salt. During the first 3 months of lactation, the mare's energy requirements are double normal maintenance levels, and she needs larger amounts of concentrates and high-quality hay.
A broodmare's stall is normally very dirty because she is eating and drinking larger amount of feeds. Because the foal is very susceptible to disease at this time, every effort should be made to keep the stall as fresh and clean as possible. This problem is easily prevented by housing outdoors.
Normal and Abnormal Postpartum Occurrences
The mare's uterus continues to involute through the first 2 weeks after delivery. During this time a dark-brown fluid may be seen on the vulva. This odorless discharge, called lochia, is normal. However, a foul-smelling discharge signals a uterine infection and requires medical attention. About the 7th or 8th day postpartum, the mare's reproductive tract needs to be examined by a veterinarian if she is to be rebred.
Colic is relatively common during the first week postpartum. A more serious condition occurring immediately postpartum is internal bleeding caused by rupture of the middle uterine artery. This condition, occasionally seen in mares over 15 years of age, is usually fatal. Symptoms associated with a middle uterine artery rupture are colic, pawing, anxiousness, and profuse sweating. The gums may also look pale or white. A veterinarian should be called at once if this condition is suspected. Often a mare with a middle uterine artery rupture is unaware of her surroundings and of her foal. To prevent injury, remove the foal from the stall.
At 6 to 12 days postpartum, most mares will come into heat. This first estrous, called foal heat is part of the uterine involution process. Unless uterine involution is practically complete, conception is unlikely in a mare bred during foal heat.
Any uterine or vaginal bruising, damage, or swelling slows down the involution process, and the uterus will be unable to support embryo life. The conception rate for mares bred during their foal heat is only 40 percent. These mares run a greater chance of developing uterine infections and scarring since the uterus is most susceptible to infection during the first 30 days after foaling. Semen is not sterile; thus, every time a mare is bred, bacteria are introduced into her uterus. Excessive or improperly timed breeding attempts increase the chances of uterine infections.
The fertile egg is released from a follicle on the ovary during the mare's estrous. This usually occurs on about the 7th and again on the 30th day after delivery. A mare usually goes out of estrous within 1 day after ovulation, although there is variation between mares. Some remain in standing estrous for longer periods after ovulation, and some ovulate without showing any external signs of estrous. These mares are said to go through a silent heat. A common cause of silent heat in mares who have just foaled is their concern for their foals. Their maternal instinct is stronger than the instinct to display estrous. If the mare is allowed to see her foal at all times while she is being teased, she may relax and show some evidence of estrous. Older mares who have had many foals may not be completely involuted and ready to conceive, even on the second heat after foaling.
Prolactin, the hormone that stimulates milk production (lactation), may inhibit estrogen and the hormones necessary for ovarian activity. In some mares, no ovulation takes place during lactation. Lactational inhibition of ovarian activity is more common if the mare is on a substandard diet and in a negative energy balance. If this is the case, the mare's plane of nutrition needs to be improved, the foal weaned early, or hormonal treatments used under a veterinarian's supervision.
Transporting the Mare and Foal
Many consider the second heat cycle after delivery to be the most desirable time to breed. If the mare is to be transported to the stallion, she should arrive well before the onset of her second heat. Arriving at the breeding farm 18 days postpartum should leave sufficient time for the mare to adapt to her new surroundings before estrous begins. Loading and unloading should become routine for the mare before the foal arrives. Many less-experienced owners prefer to have the mare foal at the breeding farm if the distance from home is great.
Body Condition of the Mare
A mare's body condition affects her reproductive efficiency and ability to reproduce. She should have an ideal body condition score of 5 (see Table 14-1) before her second heat occurs.
Condition score is a method of quantifying the amount of subcutaneous fat cover. If a mare is not in acceptable condition, her diet must be changed to move her in the direction of a 5 score. Thin mares must gain weight because poor nutrition causes mares not to cycle normally. A score of 4 is termed moderately thin and is characterized by a negative crease down the back, with the vertebrae slightly protruding. A faint outline of the ribs is discernible, some fat can be felt about the tailhead, and neither the withers, shoulders, or neck are obviously thin.
A mare in ideal 5, or moderate condition score, would have a level back with ribs that cannot be visually distinguished but can be easily felt. The fat around the tailhead is beginning to feel spongy, and the withers appear rounded over the backbone. The shoulders and neck blend smoothly into the body.
Condition score 6 is moderate to fleshy. A horse with a score of 6 has a slight crease down the back and spongy fat over the ribs. Fat around the tailhead feels soft, and deposits of fat form along the sides of the withers as well as behind the shoulders and along the sides of the neck.
Artificial insemination (AI) is the process of collecting semen from a stallion and depositing that semen into the mare, without conventional breeding. Advantages are the decreased risk of injury and infection. Also, because the semen can be evaluated and divided in samples, many more mares can be bred to the same stallion.
Some horse breed registries, such as the Jockey Club, do not permit AI. Others, such as those for quarter horses and Standardbreds, will allow AI, but only with fresh semen collected on the premises of the mare. They will not allow transportation or freezing of the sperm. Before deciding on AI, an owner should contact the breed registry.
Once collected, the semen should be immediately poured into a warmed graduated cylinder to be measured. To evaluate the percentage of progressively motile spermatozoa, an extender should be added to the raw semen before it is examined under the microscope. A sperm is called progressively motile only if it moves rapidly across the microscopic field.
When spermatozoa are to be stored or shipped, they have to be cooled. However, this cooling process stresses the cells and can injure them. To preserve the spermatozoa, they should be mixed with an extender, cooled slowly, and preferably kept at a low temperature for less than 36 hours.
Frozen sperm has both advantages and disadvantages. Although some breed registries impose restrictions on this practice, shipping liquid nitrogen containers anywhere in the world is cheaper than shipping a mare. A stallion's breeding season can continue while he is at shows, and his semen can be preserved many years after his death. The disadvantages are lower pregnancy rates and possible inbreeding.
The practice of artificial insemination requires identification of the stallion with photographs, blood type, and possibly DNA fingerprinting to avoid errors in identity and pedigrees. Contact the breed registry for regulations.
An embryo can be nonsurgically removed from the uterus of one mare, transferred, and inserted into the uterus of another mare. Embryo transfer allows reproduction by older, less fertile mares; reproduction by 2-year-old mares; and increased production of foals from genetically superior mares; but this method is very expensive and the yield is not high.
The basic embryo transfer procedure has changed little over the past decade. Success rates have improved due to improved quality control and precise timing of different aspects of the procedure. Embryo transfer is more costly in horses than in cattle and other animal species due to the unavailability of medications to cause horses to regularly produce many embryos to transfer per procedure.
Embryo transfer consists of three phases:
1. Synchronization of the donor and the recipient mare
2. Embryo flushing
3. Embryo transfer procedure
Synchronization of the Donor and the Recipient Mare
The donor is usually a valuable sport or subfertile mare, while the recipient is an inexpensive but healthy mare. Synchronization requires the use of hormones such as progesterone, prostaglandin, and HCG. To ensure good synchronization, as the process is not perfect, two recipients are usually synchronized for each donating mare. The donor must be carefully palpated, cultured, and bred. Ovulation of the donor and the recipients must be timed to within 12 hours for best results, requiring twice-daily ultrasounds.
Flushing is performed 7 or 8 days after insemination or breeding and involves lavaging (washing) the microscopic embryo out of the uterus using a special sterile solution. The embryo is then developmentally sized and graded from 1 (excellent) through 4 (poor quality).
Embryo Transfer Procedure
Embryo transfer is performed nonsurgically by loading the embryo into a uterine transfer catheter in a special nurturing solution and transferring the embryo into the uterus of the most synchronized recipient.
The odds of retrieving an embryo are very good (70 to 80 percent) from a healthy and reproductively sound donor mare. If the mare has a heavy show schedule or is subfertile, the success rate diminishes somewhat. Once recovered, embryos graded "good" or better will have a 60 to 70 percent chance of resulting in a pregnancy. Overall chances per cycle for a successful transfer with a young healthy donor are 50 to 60 percent and 30 to 40 percent with older, problem donors. Breeding and flushing difficult mares on multiple cycles is the best way to ensure at least one pregnancy.
Recipient mares in no way contribute to the foal's genetics. The characteristics of the foal are already programmed at conception with genetic material only from the donor mare and the stallion. The stallion is of great importance in the equation for success. Fresh sperm and the good timing of breeding are imperative.
ARTIFICIAL CONTROL OF BREEDING
Management of equine reproduction involves the use of photoperiods and hormones. The designated birth date of foals of many performance breeds in the Northern Hemisphere is January 1. This creates an economic pressure to start the breeding season in February so foals will be born in January. Since the mare is usually in winter anestrus, artificial lighting is used to induce follicular activity.
A mare requires approximately 60 days of artificial lighting before ovulation occurs. To induce ovulation in early February, the artificial lighting has to start in late November or early December. Mares must be confined to the area where the light (either one 200-watt incandescent or two 40-watt fluorescent bulbs per stall) is located. Also, they should be within 7 to 8 feet of the light source. Automatic timers can be used to provide artificial light, beginning at sunset. Sixteen hours of total daylight, natural plus artificial, and 8 hours of darkness is the correct ratio.
Hormones can be used to make a mare short cycle, or come in season early. Another use of hormones is to ensure that a mare will ovulate within 24 to 48 hours after being bred.
Special lighting techniques to simulate longer days and the hormone injections that are employed to bring mares into heat earlier in the winter require intensive management and often result in improperly timed breedings that are not successful. Mares becoming pregnant early in the year are more likely to lose their fetuses because of extreme fluctuations in hormone levels.
Reproductive organs of the mare, under hormonal control, produce an egg (ovum) during the breeding season. Reproductive organs of the stallion, under hormonal control, produce sperm cells. Successful breeding during estrous--the heat period of the mare--allows a sperm cell to unite with the egg. This brings together the genetic material of the mare and stallion to produce a new, unique horse. At first this new horse is a mass of cells called an embryo. The embryo implants into the uterus of the mare. As the embryo takes on the form of a new horse, it is called a fetus. After about 11 months of developing the fetus in her uterus, the mare goes through the process of labor and parturition, or birth. While the mare's reproductive tract goes through a process of involution in preparation for the next pregnancy she feeds and cares for the new foal.
The reproductive process can be artificially controlled by lighting or hormones. Breeding can also be controlled by artificial insemination and embryo transfer.
Success in any career requires knowledge. Test your knowledge of this chapter by answering these questions or solving these problems.
True or False
1. Photoperiod is when a mare is receptive to photographers.
2. Dystocia means difficult birth.
3. The practice of artificial insemination is accepted by all breed registries.
4. Winking is a sign of estrous in mares.
5. Waxed teats are a sign of impending birth.
6. Artificial insemination is 100 percent successful.
7. List three parts of the mare's reproductive tract.
8. List three parts of the stallion's reproductive organs.
9. What are four signs of estrous or heat?
10. Name the four factors that influence sperm output and production.
11. How soon after parturition can a mare be bred?
12. What hormone stimulates milk production?
13. What is the role of the hormone progesterone in the mare?
14. What are the advantages and disadvantages of both pasture and hand mating?
15. Discuss the meaning of polyestrus in the mare.
16. Explain the difference between abortion and stillbirth.
17. Describe stage 1 and stage 2 of parturition.
18. What is the meaning of condition scoring of the mare?
1. Invite a local veterinarian to visit your class to discuss reproductive problems in horses.
2. Visit a veterinarian's clinic or a horse farm when horses are being diagnosed for pregnancy.
3. Obtain prepared microscope slides of semen, and compare the sperm cells of two different species.
4. Research and diagram the changes in a horse embryo through the fetal stages to the point of parturition.
5. Using Table A-16 in the appendix, contact five breed registries and inquire about their policy for embryo transfer and artificial insemination.
American Youth Horse Council. (2004). Horse industry handbook: A guide to equine care and management. Lexington, KY: Author.
Frandson, R. D., Fails, A. D., & Wilke, W. L. (2003). Anatomy and physiology of farm animals (6th ed.). Philadelphia: Lippincott Williams & Wilkins.
Hafez, E. S. E. (2000). Reproduction in farm animals (7th ed.). Philadelphia: Lippincott Williams & Wilkins.
Kahn, C. M. (Ed.). (2005). The Merck veterinary manual (9th ed.). Whitehouse Station, NJ: Merck & Co.
Kainer, R. A., & McCracken, T. O. (1998). Horse anatomy: A coloring atlas (2nd ed.). Loveland, CO: Alpine Publications.
Kellon, E. M. (1995). Equine drugs and vaccines: A guide for owners and trainers. Ossining, NY: Breakthrough Publications.
McKinnon, A. O., & Voss, J. L. (1993). Equine reproduction. Philadelphia: Lea & Febiger.
Internet sites represent a vast resource of information, but remember that the URLs (uniform resource locator) for World Wide Web sites can change without notice. Using one of the search engines on the Internet such as Yahoo!, Google, or About.com, find more information by searching for these words or phrases:
care of pregnant mare
horse mating (pasture
Table A-18 in the appendix also provides a listing of some useful Internet sites that can serve as a starting point for further exploration.
TABLE 11-1 Quantitative Data on Stallion Semen and Spermatozoa Volume per Ejaculate Sperm per Cubic Total Sperm in Milliliters Millimeter in Ejaculate 60 to 100 150 to 300 million 5 to 15 billion TABLE 11-2 Breeding Characteristics of Mares Range in age when heat 15 to 24 months period begins Recommended minimum age 24 to 36 months to breed Duration of estrus (heat) 3 to 7 days Best time to breed Every other day beginning second day of heat If not bred, estrus recurs in: 10 to 35 days TABLE 11-3 Mating Capacity of Sires Number of Females to Mate in a Breeding Season Age of Sire Hand Mating Pasture Mating 2-year-old stallion 10 5 3-year-old stallion 30 15 4-year-old stallion 35 to 40 20 5-year-old stallion 40 to 75 20 to 25
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|Publication:||Equine Science, 3rd ed.|
|Date:||Jan 1, 2008|
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