season | conformation | condition | age | stress | breeding soundness
Introduction: Imagine a large group of mares in a broodmare herd. Each mare in the herd with optimal fertility has about a 70% chance of becoming pregnant during each cycle she is bred. After breeding the mares for three cycles, about 96% of the most fertile mares should be bred. However, at the end of 3 cycles, if more than 4% of the mares are still open, the question arises as to why some mares repeatedly fail to become pregnant. Breeding problems in mares arise from a variety of causes. Awareness can be instrumental in addressing these problems.
There are many factors that can reduce a mare’s chances of becoming pregnant. These "mare fertility factors" are most commonly related to season, condition, age, conformation, stress, and breeding soundness. Each one of these topics will be discussed in the following information:
Season: One of the most common fertility problems is related to seasonality. Horses are naturally programmed to begin to become reproductively active in the spring, when milder temperatures and abundance of forage will make supporting a foal easier and less stressful for the mare and foal. For this reason, a mare’s reproductive cycles typically stop during the winter months and start up again in the spring. Unfortunately, it usually takes one or two cycles in the early spring for the mare’s reproductive system, particularly the ovaries, pituitary gland and hypothalamus, to resume normal function to produce estrus periods of normal length. These are often called "transitional cycles." Since it is not unusual for mares to show estrous behavior and grow large follicles that fail to ovulate and regress, these transitional cycles can make the determination of the optimal breeding time problematic. The development of follicles that fail to ovulate also causes the development of a second and sometimes even a third follicle before ovulation finally occurs.
Conformation: Another important fertility factor is related to the conformation of the reproductive tract and its orientation within the pelvis. The reproductive tract has several seals or barriers that prevent disease causing or pathogenic organisms, fluids and air from entering the uterus. For each of these seals to work properly, it is important that they are properly oriented with the pelvic floor. For example, the outermost seal to the reproductive tract is the vulva. It normally is oriented vertically beneath the anus with the upper boundary (or dorsal commissure) located below the floor of the pelvis. This orientation helps prevent fecal contaminants from entering the reproductive tract.
As mares age or after repeated pregnancies, it is common for the posterior (back) portion of the reproductive tract to move forward, thereby changing its orientation with the pelvis. In some mares the anus and rectum will move forward, causing the dorsal commissure of the vulva to slide forward, over the rim of the pelvis. This orientation will allow fecal contaminants access to the vulva.
For most mares, the dorsal commissure of the vulva can be closed using sutures in a procedure known as the Caslicks procedure. When preformed properly, the procedure prevents fecal contamination from entering the reproductive system.
The central seal in the reproductive tract is the vestibular-vaginal junction. It is located over the pelvic floor and between the urethra and the vaginal vault. This barrier prevents air from entering the tract. When air is prevented from entering the vaginal vault, the walls of the vagina are normally tightly opposed, making yet another barrier.
In some mares, the reproductive tract is pulled into the body cavity, thereby changing the orientation of the vestibular-vaginal junction and promoting a condition called "windsucking." This condition allows air and related contaminants to enter the vaginal vault. In severe cases, the urethra is pulled over the pelvis causing urine to collect or "pool" in the vaginal vault. Restorative surgery includes perineal reconstruction and extension of the mare’s urethra to its proper position behind the pelvic floor.
The innermost seal to the reproductive tract is the cervix. This is a thickly muscled tubular opening to the uterus. The function of the cervix is closely tied to the stage of the mare’s estrous cycle. The cervix works by opening and closing and by producing mucus with varied thickness. During estrus the muscular part of the cervix is relaxed and the cervical lining produces a lot of thin, slippery mucus. The flow of mucus helps to keep the vaginal vault clear of contaminants, while providing optimal conditions for breeding by the stallion.
During diestrus, the cervix contracts tightly and produces a relatively small amount of thick, sticky mucus. The thicker mucus adds integrity to the cervical barrier. It also aids in keeping the walls of the vaginal vault together, thus helping to maintain the integrity of the vestibular-vaginal junction.
The cervix is normally positioned horizontally over the anterior (front) portion of the pelvic floor. In some mares, normal wear and tear related to her number of pregnancies can cause the cervix be pulled forward into the abdominal cavity and to become positioned at a tilted angle. With this orientation, the cervix is less able to defend the uterus from contaminants and pathogens. No surgery is currently available to restore the cervix to its proper position.
Condition: Condition, or amount of stored energy in the form of fat, has a great influence on the fertility of mares. Research has shown that mares that have enough fat to make it difficult to see the ribs (condition score 5) are able to conceive earlier in the breeding season, requiring fewer cycles, and once pregnant, have a lower risk of abortion.
It has also been shown that maintaining a mare’s condition is important. For example, fatter mares will have lower fertility if they are losing weight than if they are fed to remain at their present condition. With regard to obesity, mares that are very fat (condition scores 8 and 9) are as fertile as mares with a condition score of 5, but they may be more likely to have problems foaling.
Age: On average, mares that are over the age of 14 require more cycles to conceive and have a greater risk of pregnancy loss than younger mares. One age related problem is simple wear and tear associated with repeated pregnancies. In some mares, the uterus begins to sag downward into the abdomen affecting circulation and the ability for the uterus to eliminate contaminants that enter the uterus during breeding. This type of conformation defect can interfere with the ability of the uterus to successfully accept the embryo when it is deposited from the oviduct on day 5 or 6 after ovulation.
Stress: When considering all of the biological systems that keep a horse alive, (such as the respiratory, circulatory, nervous, and digestive systems), physiologically, the reproductive system of the horse is of relatively little importance. For this reason, any condition or stressor that diverts energy and nutrients to the maintenance of these more critical systems will have a negative effect on reproduction. Resulting reproductive effects include changes in estrous cycle characteristics or even the complete interruption of the estrous cycle altogether. Stress may also cause pregnancy loss in severely affected mares.
Generally, causes of stress are varied and commonly include parasite infestation, illness, harsh climate, lactation, or any condition that would cause the mare to either experience inadequate feed intake or an increase in energy utilization to the point of fatigue. In cases of illness in pregnant mares, progestin treatment during the illness may reduce the risk of pregnancy loss. In mares that are being bred while in competition, there is little reproductive risk when maintaining a regular exercise regime to maintain fitness; however, if increased fitness and performance levels are desired, careful attention should be given to the mare’s diet, body condition score, and level of fatigue resulting from each training session.
Breeding Soundness: Virtually all mares, with the possible exception of the maiden mare entering her first season, have breeding histories. There are many factors that may be related to a mare’s breeding history that can help make a determination of what her chances are of becoming pregnant after each breeding attempt. Following are some questions that may assist in managing mares that have a breeding history that reflects decreased fertility:
Is fluid often detected by ultrasound in the uterus during estrus? The uterus has many functions, among which are sperm transport and evacuation of contaminants. Mares that accumulate fluid during estrus may have impaired function.
Has the mare ever had a uterine infection? Mares that have been previously diagnosed with an infection, should thereafter be considered susceptible to infection and should be bred using only minimal contamination techniques. Post breeding treatments including uterine lavage, oxytocin administration, or intrauterine antibiotic infusion may also be helpful in preventing recurrence.
In addition, if the mare has had a uterine infection, it may also be helpful to know the condition of the innermost tissue of the uterus, the endometrium. Often, if a uterine infection persists without treatment, the endometrium can be permanently damaged, reducing the mare’s chances of becoming pregnant and carrying a foal to term. The condition of the endometrium can be assessed by having a veterinarian send a sample of the mare’s endometrial tissue (a biopsy) to a diagnostic lab. The sample will be given a grade of I, IIA, IIB or III (I is normal, III is severely damaged). Mares with a biopsy score of IIB or worse have a less than 50% chance of carrying a foal to term.
Does the mare have intrauterine cysts? Intrauterine cysts result from the blockage of either a lymphatic vessel or uterine gland. While 1 or 2 small cysts is not usually a problem, large cysts or clusters of cysts may interfere with the movement of the embryo through the uterus as it signals its presence to the uterine lining. Insufficient exposure of the entire uterus to the embryo may result in early regression of the corpus luteum (CL) and subsequent pregnancy loss.
Does the mare often develop multiple large follicles during estrus? Mares that develop more than one follicle, often have multiple ovulations. While multiple ovulations improve a mare’s chance of conceiving, they also have the negative effect of increasing her risk of carrying twins. The development of twin pregnancies is related to a high chance of abortion, which can occur anytime between the second month and term. Twinning in horses is not considered desirable since vigor, size and survival rates of twin foals that are carried to term are very low. For this reason, if multiple large follicles are detected during estrus, care should be taken to have an ultrasound exam performed to search for potential twins between days 14 and 16 after ovulation. If detected, one of the twins should be manually eliminated by crushing.
Does the mare have a history of foaling related problems? Dystocia resulting from large foals, hip fracture, or general weakness is often a repeating problem requiring an attendant at each foaling. Other problems that require special care at foaling include foal rejection, low milk or colostrum production, pre- or peri-parturient loss of colostrum, and retained placenta. Special attention to mares’ breeding histories can be especially helpful in resolving these problems.
Summary: Awareness of problems related to mare fertility has a central role in breeding management. Many reproductive deficiencies can be avoided or overcome with proper management, thus improving many mares’ chances of producing healthy foals.