THE MERCK VETERINARY MANUAL
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Boar Management

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Male fertility in the breeding program more often than not receives too little attention in herd health programs. As with other food animal species, boars should be examined for breeding soundness before use in a breeding program, whenever they show lack of libido or inability to copulate, or if an increased number of females bred by the boar return to estrus ~3 wk later. At a minimum, a breeding soundness evaluation should include a history, general physical examination (including genital examination), a semen evaluation, and a behavior evaluation (see Breeding Soundness Examination of Boars).

When selecting a boar for a breeding program, factors such as origination from a specific disease-free herd, performance, soundness and conformation, age of puberty, and other pertinent parameters related to reproduction should be considered. All boars that are to be used in a breeding program should, at a minimum, be seronegative for brucellosis, porcine reproductive and respiratory syndrome, and pseudorabies (Aujeszky disease). Additionally, all boars should be isolated and acclimatized for at least 45–60 days and be tested/retested for diseases naive to the herd before introduction into the herd. If involved in the selection process, boars from large litters (>10 piglets) that reach puberty early (5½–6 mo) tend to produce highly productive daughters who also reach puberty at an early age. Performance parameters such as feed efficiency, backfat, and average daily gain are also highly heritable.

Skeletal conformation and examination for current or potential locomotor dysfunction should be assessed. Any unsoundness that may interfere with the boar's ability to approach, mount, and successfully breed/ejaculate should be determined. Acute or chronic musculoskeletal conditions may elicit pain that causes the boar to appear uninterested in mounting. Boars are usually selected as breeding prospects at 3–6 mo. The genetic background of the boar should be consistent with the intended use. Selection of boars with heritable defects such as umbilical or inguinal hernias, cryptorchidism, rectal prolapse, and poor underlines can be avoided by careful analysis of the source herd production records.

A complete history should include the age and origin of the boar, source herd health, immunizations, previous disease problems and treatments, exposure to other animals and premises, as well as the time spent in isolation and exposure to the present premises and its breeding animals. It should also include, if available, a description of the boar's previous libido, mating behavior, conception rates, litter size, and performance of relatives and other boars in the herd. For young boars, observations of sexual behavior may be useful.

A general physical examination should be part of every fertility evaluation. Attention should be given to body condition and conformation, including the back and legs, and locomotor function. Osteomalacia, osteoarthrosis, and arthritis, which may result in lameness and reluctance to mount or bear weight on the rear legs, are serious problems.

The testicles, epididymides, and scrotum should be examined and palpated for size, symmetry (<1 cm difference in diameter), consistency, and pathologic changes. An appreciation of normal testicular consistency is necessary to detect subtle changes. The penis and prepuce should be examined for abnormalities during semen collection. Testicular size is directly correlated with genotype, age, and weight of boars between 142–282 days of age and 185–375 lb (84–171 kg) body wt. Testicular size increases until ~18 mo of age; testicular growth and sperm numbers increase at the greatest rate between 5 and 12 mo of age. Because age and testicular weight are important identifiers of early sexual development, boars should be ≥8 mo old before use in a breeding program.

The testicles can be affected by diseases (eg, brucellosis, actinobacillosis) and are vulnerable to trauma by handlers or other animals or as a result of improperly designed or maintained facilities. They should contain no nodules or soft masses. The initial reaction of testicles to trauma or infection is swelling; if untreated, the longterm result is testicular atrophy, identified by increased firmness and loss of resiliency. Asymmetry, as a result of unilateral atrophy, is potentially deleterious to fertility, and semen evaluation may reveal azoospermia, oligospermia, asthenospermia, or morphologic changes indicative of testicular damage.

Collection allows evaluation of libido and ability to mate and ejaculate; in addition, it provides a sample ejaculate. Precopulatory behavior involves visual and olfactory stimulation. The boar grunts or barks rhythmically, chomps jaws, salivates, and typically engages in head-to-head contact with the sow or dummy, followed by nuzzling her flanks to test for voluntary immobilization. These activities should be observed, because aberrant sexual behavior may result in infertility. Constant head mounting is a common problem with inexperienced boars.

Poor libido is likely caused by behavioral rather than endocrinologic problems. Fighting and domination by older boars and sows can inhibit libido in young boars. Breed and strain differences are also seen; the tendency to be timid, nervous, and nonaggressive can be influenced by selection in a breed over several generations and can result in boars with poor libido. Pain from genital lesions or musculoskeletal problems can have a strong negative effect. Libido can also be impaired by an unfamiliar environment, the presence of a feared person, or distractions such as available feed.

Once the boar has mounted, erection and protrusion of the penis occur as the boar searches for the vulva. Close observation is necessary to notice injuries and lesions of the penis as well as improper erection. Congenital and genetic problems include incomplete erection, penile hypoplasia, masturbation into the diverticulum (ie, “balling up”), and persistent frenulum.

There are basically two methods of semen collection in the boar—the gloved-hand method and electroejaculation. A third method using a water-jacketed artificial vagina is no longer in common use. Although satisfactory ejaculates can be obtained using either the gloved-hand or electroejaculation methods, the gloved-hand method is preferable because it is simpler and reproductive behavior can be simultaneously assessed. The boar is allowed to mount an estrous female or collection dummy and attempt to copulate. Boars used for artificial insemination are usually trained from the onset to mount a collection dummy. The boar should then be approached quietly from the rear without being touched or frightened. Preputial fluids are first evacuated by massaging the prepuce to prevent contamination of the ejaculate. The back of the gloved hand is then placed against the ventral abdomen of the boar just cranial to the preputial orifice, and the penis is allowed to thrust into the gloved hand. Digital pressure is applied to the distal 3–6 cm of the penis. If properly stimulated, the boar will fully extend the penis and become very quiet. This is followed immediately by ejaculation. Once the tip of the penis is firmly in the hand and ejaculation has begun, it continues for ≥3–7 min. If the boar dismounts when the attempt is made to grasp the penis, he should be allowed to make several false mounts until he is aggressively attempting intromission again. Most experienced boar trainers achieve a >96% success rate in training boars to mount a dummy and to ejaculate.

A nervous boar may not allow the penis to be locked into the hand, even after several attempts. Semen can be collected from many such boars by allowing them to achieve natural intromission and lock the penis into the sow's cervix to begin the ejaculation, then quickly retrieving the penis and locking it into the hand. The boar will continue to ejaculate, and the major portion of the ejaculate can be collected.

A prewarmed (37°C) thermos or styrofoam cup is a convenient and economical collection vessel. The pre-sperm fraction, consisting of 5–15 mL of fluid, is usually ejaculated first and allowed to fall on the ground (ie, it is not collected). The boar then usually ejaculates a small amount of gel, which is filtered out of the ejaculate by a double layer of coarse gauze (placed over the mouth of the collection receptacle), because it coagulates into a semisolid mass that can interfere with subsequent evaluation of semen quality. The boar then ejaculates the milky to cream-colored, sperm-rich fraction. The final, sperm-poor fraction contains the largest volume of fluid and gel. Care should be taken to let the boar complete the ejaculation, voluntarily withdraw the penis from the hand, and dismount. Some boars will go through two or more complete ejaculations before voluntarily dismounting.

Semen collection by electroejaculation is done only on an anesthetized boar. An injectable anesthetic that will allow for 15–30 min of general anesthesia is recommended. The rectum is cleaned out using a lubricated hand, and a lubricated rectal probe is inserted. The penis is then exteriorized with the aid of Bozeman sponge forceps and grasped with a surgical sponge wrapped around the penis 5–10 cm distal to the glans penis. Electrostimulation of the boar is performed as in the bull or ram, with the ejaculate collected in a clear, plastic bag that envelops the glans penis.

Standard tests used to evaluate boar semen include sperm motility, morphology, concentration, total numbers, and ejaculate volume. The ejaculate should be protected from changes in temperature, osmotic pressure, and pH during handling and analysis. All equipment and materials that come into contact with semen should be warmed to 35°–39°C.

Sperm motility should be evaluated as soon as possible after collection. Estimating sperm motility in an ejaculate by examining the mass activity or swirl motion of a drop of semen on a slide is of limited value and is not recommended. Gross sperm motility is best estimated on prepared samples in which a monolayer of individual sperm can be visualized using light microscopy. To do this, a 5–10 μL drop of semen is placed on a prewarmed slide and overlaid with a coverglass. Sample motility is then subjectively estimated to the nearest 5% by viewing several random fields under 20 × magnification.

Sperm morphology can be a valuable indicator of fertility potential, especially in those ejaculates with a high percentage of abnormal sperm. When using bright-light microscopy, stained slides are necessary to provide adequate contrast to evaluate sperm morphology. When using higher resolution microscopy (ie, phase-contrast, differential interference contrast), glutaraldehyde or buffered formalin preserved samples can be used. A minimum of 100 (preferably 200) sperm should be assessed for morphology of the head, midpiece, and principal piece (ie, the tail distal to the midpiece). Sperm can be categorized into three groups: normal, sperm with abnormal heads, and sperm with abnormal tails (midpiece, principal piece, including cytoplasmic droplets). Samples with a high number of sperm defects can be examined further, and abnormalities classified as major and minor defects. Acrosome morphology should also be assessed if possible.

Several techniques are available to determine sperm concentration in a filtered boar ejaculate. A crude, subjective, qualitative estimation of sperm concentration can be done by assessing visual opacity of a raw ejaculate, either by direct examination or with the aid of a Karras spermiodensimeter. Analytical determination of sperm concentration can be performed by measuring opacity via a calibrated (spectro) photometer on a diluted semen sample. It is essential that the photometer be calibrated for boar semen. Even with a calibrated photometer, estimates of sperm numbers may be ±30% from that of the actual concentration; this can be attributed, in part, to improper technique, human error, and/or the inherent opacity of the secretions of the accessory sex glands present in the boar ejaculate. Photometric readings can also be inaccurate if the reading is outside the calibration curve or optimal operating range. A second, more direct method to measure sperm concentration is with a hemocytometer or counting chamber. In this method, concentration can be determined by diluting a portion of the filtered ejaculate to a 1:200 ratio—most easily done using a Unopette® system. The hemocytometer should be charged, and the charged unit allowed to set for 5 min, so that the sperm settle into one visual field. Using microscopy, a sperm count is performed and calculated as normally done for RBC determination. Determining sperm concentration using a counting chamber is tedious and time consuming, making its use on a routine basis impractical in most commercial operations.

After calculating sperm concentration/mL, total sperm numbers in an ejaculate can be calculated by multiplying sperm concentration with the total volume (in mL) of the gel-free ejaculate. Ejaculate volume can be measured by using a warmed measuring apparatus (eg, graduated cylinder, disposable plastic measuring cups) or by measuring the weight of the ejaculate (with 1 g equivalent tο 1 mL). More frequently, computer-automated semen analysis systems are being used to objectively determine sperm motility, sperm morphology, and sperm concentration.

Interpretation of Findings:

Semen values can be affected by frequency of boar use, age, environment, disease, level of nutrition, genotype, and method of sperm cell fixation. Therefore, boars that do not have acceptable semen values are not necessarily subfertile or infertile. Spermiograms can change dramatically over a short period of time, and boars should not be culled on the evaluation of a single ejaculate. Breed differences in onset of puberty, libido, mating ability, and conception rate have been seen.

Environment can affect fertility over a short period of time, primarily because of disturbances in the thermoregulation of the testes. Boars exposed to cold or hot environmental temperatures may have abnormal spermiograms for ≥7 wk after the insult. Severe exposure may result in abnormal spermiograms for a longer time or may even lead to permanent spermatogenic disruption. Any disease that increases body temperature, and thus disrupts thermoregulation of the testes, also has the potential to cause temporary sub- or infertility.

Ideally, libido, mating ability, semen quality (see Table 2: Pigs:Suggested Minimum Spermiogram Values for Breeding BoarsTables), and breeding results (conception rate and litter size) should be considered. The duration of spermatogenesis and spermatozoal maturation is ~51 days in the boar. If a boar produces an ejaculate of low or marginal quality when examined in vitro, additional ejaculates should be assessed at 1- to 2-wk intervals to ascertain whether quality has improved over time. Boars with spermiograms that do not improve over 2–3 mo are unlikely to ever improve. Boars with azoospermia on two complete ejaculates or that are unable to achieve complete erection should be culled immediately. Those that have penile lesions or blood in the semen should be sexually rested for ≥2–3 wk and reevaluated. For boars with persistent frenulum or that habitually masturbate in the diverticulum, surgical correction is recommended; however, the progeny should not be kept for breeding, because these conditions are most likely heritable. All results of the fertility examination must be considered in relation to age, disease history, environmental stress, prior breeding usage, mating system, and the techniques of semen collection and handling.

Table 2

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Last full review/revision March 2015 by Gary C. Althouse, BS, MS, DVM, PhD, DACT

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