In modern commercial breeding programs, most boars of high genetic merit are housed in site-segregated locations known as boar studs, where they are managed specifically for production of semen doses for use in artificial insemination. Boar studs are typically managed to ensure the highest levels of health and biosecurity. On smaller farms, breeding boars often receive insufficient attention to optimize herd health and semen production.
Regardless of the size of the operation, as with other food animal species, boars should be examined for breeding soundness before selection and use in a breeding program. Problems become evident when boars show lack of libido or inability to copulate or mount a dummy, show repeated poor-quality ejaculates, or if an increased number of females bred by the boar or using his semen return to estrus approximately 3-4 weeks later. At a minimum, a breeding soundness evaluation should include a history, general physical examination (including genital examination), semen evaluation, and behavior evaluation.
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Selection in Boar Management
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 to be used in a breeding program should, at a minimum, be seronegative for brucellosis, porcine reproductive and respiratory syndrome, and pseudorabies (Aujeszky disease) (1). 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 boar stud or breeding herd.
Boars from large litters that reach puberty early (5½–6 months old) tend to produce highly productive daughters who also reach puberty at an early age. Boars that are nursed in smaller litters gain more weight, have heavier weaning weights, grow faster to puberty, and can show greater lifetime sperm production. Performance parameters such as feed efficiency, backfat, and average daily gain are also highly heritable and are highly valued in terminal-line boars (2).
Skeletal conformation and current or potential locomotor dysfunction should be assessed during boar selection, as well as any unsoundness that can interfere with the boar’s ability to approach, mount, and successfully breed/ejaculate (see ). Acute or chronic musculoskeletal conditions can elicit pain that causes the boar to appear uninterested in mounting.
Physical selection of the boar should be on the basis of reproductive and structural assessments.
Courtesy of Robert Knox, University of Illinois.
Boars are usually selected as breeding prospects at 3–6 months old. The genetic background of the boar should be consistent with the intended use as a maternal or terminal sire. 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.
History in Boar Management
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 time spent in isolation and during acclimation with exposure to the microbes present on the premises and in its breeding animals. It should also include, if available, a description of the boar’s behavior, previous libido, mounting behavior, need for prostaglandin when training, conception rates, litter size, and performance of relatives and other boars in the herd. For young boars, observations of sexual behavior can be useful. Litter traits, growth rate, body conformation, and testicle size should also be considered.
Physical and Genital Examinations in Boar Management
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 can 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 pathological 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 old and 185–375 pounds (84–171 kg) body weight (3, 4). Testicular size increases until approximately 18 months old; testicular growth and sperm numbers increase at the greatest rate between 5 and 12 months old. Because age and testicular weight are important identifiers of early sexual development, boars should be ≥ 9 months old before use in a breeding program.
Pearls & Pitfalls
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The testicles can be affected by diseases (eg, brucellosis, actinobacillosis) and are vulnerable to trauma by handlers, other animals, or as a result of injury from improperly designed or maintained facilities. As boars become sexually excited, they can become aggressive and unpredictable, leading to fear and frustration in animal handlers. Aggressive handling that includes kicks, hits, or slamming gates can cause injury to the boar and damage to the testicles that can lead to boar infertility.
The testicles should contain no nodules or soft masses. The initial reaction of testicles to trauma or infection is swelling, and if untreated, the long-term result is testicular atrophy, which can be diagnosed by increased firmness and loss of resiliency. Asymmetry as a result of unilateral atrophy is potentially deleterious to fertility, and semen evaluation can reveal azoospermia, oligospermia, asthenospermia, or morphological changes indicative of testicular damage.
Behavioral Evaluation and Semen Collection in Boar Management
Semen collection allows evaluation of boar libido and ability to breed and also provides a sample of the ejaculate for determining the potential of sperm to fertilize eggs. Boars are most commonly collected using a dummy or phantom and can also be collected using a mature estrual female.
Independent of the method of collection, boars exhibit precopulatory sexual behavior in response to visual and olfactory stimulation. The boar might grunt, chant or bark rhythmically, chomp its jaws, or salivate, and it typically engages in head-to-head contact with the sow or dummy, followed by nuzzling of the sow's flanks or the dummy to test for immobilization. These activities should be observed, because aberrant sexual behavior can result in infertility.
Constant head mounting, a common problem with inexperienced boars when using live females, is prevented by using a dummy with an angled head position higher than the tail end and with properly designed collection pens (see ). The collection area should also be designed to prevent slippage and feet and leg injury.
A boar collection dummy.
Courtesy of Robert Knox, University of Illinois.
Poor libido is likely caused by behavioral rather than endocrinological problems. Fighting and domination by older boars and sows can inhibit libido in young boars. There are also breed and strain differences, with the tendency to be timid, nervous, and nonaggressive influenced by selection in a breed over several generations, which 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 human, or distractions such as available feed.
Once the boar has mounted the dummy, erection and protrusion of the penis occur as the boar instinctively searches for tactile stimulation. Close observation is necessary to identify 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.
The most common methods for boar semen collection include the gloved-hand technique and the semi-automated collection system. While electroejaculation and use of an artificial vagina can work, these methods are seldom used or necessary.
The gloved-hand method is the most widely used approach, because it is simple, enables very good control of collection and semen quality, has well-developed procedures, and allows reproductive behavior to be simultaneously assessed (see ).
Semen collected from a boar using the gloved-hand technique.
Courtesy of Robert Knox, University of Illinois.
Semi-automated collection systems also rely on a boar mounting a dummy. A technician connects the vacuum collection system to the extended boar penis, and then ejaculate collection proceeds without the need for several minutes of direct human digital pressure. Both the gloved-hand and semi-automated systems are in wide use and account for most of the commercial ejaculates processed throughout the world. Both systems provide high-quality ejaculates without appreciable differences in dose production.
With any dummy collection approach, the boar must be trained to mount the collection dummy, extend the penis, and ejaculate within a reasonable time frame and number of attempts. Boars used for artificial insemination are usually trained to mount the dummy starting at around 6 months old, and successful mounting behavior can take a few weeks with multiple training sessions to learn.
The boar should be introduced to the collection pen and allowed time to explore and become excited by and successfully mount the dummy, while allowing time for multiple false mounts. Allow the mounted boar time to extend the penis and initiate thrusting. Once the penis is extended, the boar should be approached quietly from the rear and side without causing any anxiety or fear. Boars in sexually excited states can become aggressive and dangerous, and the design of the pen should take human safety into account (see ).
A technician is protected by a safety wall as semen is collected from a boar mounted on a dummy.
Courtesy of Robert Knox, University of Illinois.
Prior to collecting the boar, the collection area should be cleaned to remove feces and debris and the prepuce trimmed of hair and cleaned with a disposable wipe. After the boar mounts the dummy, preputial fluids should be evacuated while wearing double gloves and massaging the prepuce. The preputial area should again be wiped clean with a disposable paper towel and the outer gloves removed and discarded.
The back of a 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.
The extended penis should be held off the center line and at a slightly downward angle to allow collection of the ejaculate and not allow penile fluids to drip into the collection container. Once the tip of the penis is firmly in the hand and ejaculation has begun, it continues for ≥ 3–7 minutes. 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 to extend the penis once more.
Most experienced boar trainers achieve a > 96% success rate in training boars to mount a dummy and ejaculate. Methods that can improve the training rate for boars include allowing boars in warm-up pens to watch other boars being collected, dripping semen or urine on the dummy, using pheromone sprays on the dummy, allowing a boar to breed an estrual female in the collection pen, or injecting a boar with prostaglandin.
A nervous boar might 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.
When collecting semen, temperature control and prevention of bacterial contamination are priorities. A prewarmed (37°C [98.6°F]) thermos or large foam cup is a convenient and economical collection vessel and should be used with a disposable filter and disposable collection bag.
The presperm fraction, consisting of 5–15 mL of fluid, often contains urine, cellular debris, and higher counts of bacteria and is ejaculated first. This fluid should not be collected but allowed to fall on the floor. The boar then might ejaculate a small amount of gel, which is filtered out of the ejaculate by a filter or double layer of coarse gauze (placed over the mouth of the collection receptacle or the filtered bag), because it coagulates into a semisolid mass that can interfere with semen dose processing and subsequent evaluation of semen quality.
The boar then ejaculates the milky to cream-colored, sperm-rich fraction, which can range from 60 to 150 mL. The final sperm-poor fraction contains the largest volume of fluid and gel (150–250 mL). Boars can be collected for the sperm-rich fraction alone or for the entire ejaculate for processing. However, care should be taken to let the boar complete the ejaculation process, voluntarily withdraw the penis from the hand, and dismount, to avoid boar frustration with the procedure.
Semen collection by electroejaculation is performed infrequently and only on a highly valued and anesthetized boar. An injectable anesthetic that will allow for 15–30 minutes 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 filtered clear plastic bag that envelops the glans penis.
Semen Evaluation in Boar Management
Standard tests used to evaluate boar semen include sperm motility, morphology, agglutination, concentration, total numbers, and ejaculate volume, each of which should be evaluated whether the semen is to be used for artificial insemination (AI) or natural breeding.
Most of the semen produced for use with AI in breeding farms is obtained from larger commercial boar studs or genetic suppliers that specialize in semen production and have invested in high-cost equipment that can quickly provide accurate measures of ejaculate quality within minimal time while enabling efficient packaging of semen doses.
Larger boar studs often invest in computer-assisted sperm analysis (CASA) systems (see ), microscopes with heated stages, sperm cell counters, and densitometers, as well as automated semen dose-packaging systems. However, sophisticated and expensive equipment is not a requirement for production of quality AI doses.
A semen sample is analyzed by a computer-assisted sperm analyzer (CASA) machine tracking sperm motility.
Courtesy of Robert Knox, University of Illinois.
In modern boar studs, the location and design of the facility create barriers for disease entry. Boar housing and collection often occur in separate rooms from where semen processing and storage will take place. The semen processing room is often of specialized design with positive air pressure, separate air flow, and controlled access and entry.
After semen collection is finished, the ejaculate filter should be removed and discarded and the ejaculate bag or container sealed to protect it from contamination. The ejaculate should be protected from changes in temperature and osmotic pressure during delivery to the processing and analysis area. All equipment and materials that come into contact with semen should be clean or new and warmed to 35–39°C (95–102.2°F).
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 using a low-magnification microscope is of limited value for commercial production of semen doses; however, it can be used to pass an ejaculate for use on small farms.
Gross sperm motility is best estimated on prepared samples in which a monolayer of individual sperm can be visualized using light microscopy at higher magnification. Place a 5–10 mcL drop of semen on a prewarmed slide and overlay it with a coverglass. Sample motility is then subjectively estimated to the nearest 5% by viewing several random fields under 20X magnification (2, 5).
Sperm morphology can be a valuable indicator of fertility potential, especially in those ejaculates with a high percentage of abnormal sperm. This assessment can be made using a quality light or phase-contrast microscope. When using bright-light microscopy, staining is necessary to provide adequate contrast to evaluate sperm morphology. When using higher-resolution microscopy (ie, phase-contrast, differential interference contrast), samples preserved in glutaraldehyde or buffered formalin 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) (5, 6).
Sperm can be categorized into three groups: normal, sperm with abnormal heads, and sperm with abnormal tails (midpiece, principal piece, including cytoplasmic droplets) (see and images). The most common sperm abnormalities include proximal and distal droplets and coiled or bent tails. These types of defects can occur with insult or injury, over-collection, heat stress, or illness.
Phase-contrast image of boar sperm cells with proximal and distal droplets. Magnification 400X.
Courtesy of Robert Knox, University of Illinois.
Stained boar sperm showing hairpin loops. Magnification 400X.
Courtesy of Robert Knox, University of Illinois.
Sperm agglutination can often be observed in a sample and might be an indication of dead or damaged sperm or evidence of bacterial contamination. In cases in which fertility problems are suspected, samples can be examined further, with abnormalities classified as major and minor and the acrosome morphology assessed.
Because of the nearly continuous demand for boar semen, boar studs of various sizes often use third-party laboratories to periodically evaluate the quality of semen doses. Boar studs also often retain samples of semen doses shipped to farms as a quality control measure to assess fertility changes related to length of storage.
Assessing visual opacity of a raw ejaculate by direct examination can provide a crude, subjective, qualitative estimation of sperm concentration; however, this approach has limited accuracy and is not recommended for commercial farms. Many who perform this type of assessment simply dilute the ejaculate 1:4 to 1:5 to create usable semen doses. However, more precise determination of sperm numbers is needed to achieve the required number of sperm in AI doses. This becomes more important as the genetic value of the boar increases and the number of sperm per dose is decreased to allow more doses to be produced, thereby extending the use of valuable sire genetics.
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 can vary ± 30% from that of the actual concentration (7). Large errors can be attributed, in part, to improper sampling or dilution technique, sample contamination, and the inherent opacity of the secretions of the accessory sex glands present in the boar ejaculate. Photometric readings can also be inaccurate if the number of sperm in the diluted sample is too low or too high, leading to readings outside the calibration curve or optimal operating range.
A more direct method to measure sperm concentration is with a hemocytometer or counting chamber and a microscope (see ). 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 disposable diluting-pipette system.
A hemacytometer sperm cell counting chamber showing an active counting field.
Courtesy of Robert Knox, University of Illinois.
The hemocytometer should be cleaned with ethanol and allowed to dry before the sample is added. The sample should be allowed to set for 5 minutes, so that the sperm settle into one visual field. Using microscopy, a sperm count is performed and calculated as would be done for an RBC determination.
Determining sperm concentration using a counting chamber is tedious and time-consuming, making its use on a routine basis on a larger-scale commercial boar stud impractical. More sophisticated methods use a cell counter or a CASA machine, which provide quick, accurate counts, albeit at higher cost. Attention should be focused on quality control and operation of the CASA machines, as these too have been shown to be a source of error when used improperly or when not calibrated.
After calculating sperm concentration per milliliter, total sperm numbers in an ejaculate can be determined by multiplying the sperm concentration by the total volume (in milliliters) of the gel-free ejaculate. Ejaculate volume is most often measured using a scale and determining the weight of the ejaculate (with 1 g equivalent to 1 mL) or by use of a warmed measuring apparatus (eg, graduated cylinder, disposable plastic measuring cups).
More frequently for commercial boar studs, computer-automated semen analysis systems are being used to objectively determine sperm motility, morphology, concentration, and total number.
Interpretation of Findings
Semen values can be affected by frequency of boar use, age of the boar, environment and season, disease or health status, level of nutrition, genotype, and method of sperm cell evaluation. Values that are below expected levels can be associated with breed differences, notably for onset of puberty, libido, mating ability, and conception rate. 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. When problems are identified with the ejaculate, repeated collection over days is recommended to clear out sperm in the epididymis, with the boar allowed a week to recover before another evaluation.
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 can have abnormal spermiograms for ≥ 7 weeks after the insult. The most common issues arise with boars that are heat-stressed during the summer, with some problems extending into early autumn.
Exposure to very high temperatures can result in abnormal spermiograms for a longer period of time and can even lead to permanent spermatogenic disruption. Environmental temperatures that exceed 28°C (82.4°F) on sequential days have been linked to sperm quality issues in the next weeks to months (8, 9, 10). Any disease that increases body temperature and thus disrupts thermoregulation of the testes also has the potential to cause temporary sub- or infertility. Most of the issues related to short- or longer-term heat stress affect sperm morphology, motility, and number of sperm produced for a duration of weeks, related to the duration of the stress.
Guidelines for Boar Evaluation in Boar Management
Libido, mating ability, semen quality (see table ), and breeding results (conception rate and litter size) should be considered in boar evaluation. The duration of spermatogenesis and spermatozoa maturation is approximately 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-week intervals to ascertain whether quality has improved over time. Valuable boars with spermiograms that do not improve over 2–3 months are unlikely ever to improve and should be culled.
Suggested Spermiogram Values for Breeding Boars
Parameter | Natural Service | Artificial Insemination |
|---|---|---|
Ejaculate volume | ≥ 80–150 mL for sperm-rich fraction and ≥ 200–400 mL for whole ejaculate | |
Color | Opaque to white (vulvar discharge) | Opaque to white |
Total sperm numbers | > 35 × 109 sperm/ejaculate | > 35–100 × 109 sperm/ejaculate |
Concentration | 200–500 sperm × 106/mL depending on ejaculate volume collected | |
Sample motility | > 60% | > 70–90% |
Abnormal morphology (including cytoplasmic droplets) | < 25% | < 20–25% |
Cytoplasmic droplets (both proximal and distal) | < 15% | < 15–20% |
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 weeks 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 likely heritable.
All results of the fertility examination must be considered in relation to age, disease history, environmental stress, prior breeding usage, breeding system, and the techniques of semen collection and handling.
For More Information
American Association of Swine Veterinarians. Boar stud guidelines: health, hygiene, and sanitation guidelines for boar studs providing semen to the domestic market. J Swine Health Prod. 2003;11(4):204-206.
Schulze M, Ammon C, Rüdiger K, Jung M, Grobbel M. Analysis of hygienic critical control points in boar semen production. Theriogenology. 2015;83(3):430-437.
Schulze M, Jung M, Hensel B. Science-based quality control in boar semen production. Mol Reprod Dev. 2023;90(7):612-620.
Levis DG, Reicks DL. Assessment of sexual behavior and effect of semen collection pen design and sexual stimulation of boars on behavior and sperm output–a review. Theriogenology. 2005;63:630-642.
Schulze M, Kuster C, Schäfer J, Jung M, Grossfeld R. Effect of production management on semen quality during long-term storage in different European boar studs. Anim Reprod Sci. 2018;190:94-101.
Knox R, Levis D, Safranski T, Singleton W. An update on North American boar stud practices. Theriogenology. 2008;70(8):1202-1208.
Flowers WL. Factors affecting the production of quality ejaculates from boars. Anim Reprod Sci. 2022;246:106840.
References
Althouse G. Infectious and noninfectious causes of infertility in boars. In: Youngquist RS, Threlfall WR, eds. Current Therapy in Large Animal Theriogenology. 2nd ed. Saunders Elsevier; 2007;722-725. doi:10.1016/b978-072169323-1.50098-2
Althouse GC. Artificial insemination in swine: boar stud management. In: Youngquist RS, Threlfall WR, eds. Current Therapy in Large Animal Theriogenology. 2nd ed. Saunders Elsevier; 2007;731-738. doi:10.1016/b978-072169323-1.50100-8
Clark SG, Schaeffer DJ, Althouse GC. B-mode ultrasonographic evaluation of paired testicular diameter of mature boars in relation to average total sperm numbers. Theriogenology. 2003;60(6):1011-1023. doi:10.1016/s0093-691x(03)00127-4
Sanglard LP, Leach RJ, Gomez-Léon VE, Serão NVL. Relationship between the testis size in male pigs and reproductive traits in their progeny. Livest Sci. 2019;228:72-75. doi:10.1016/j.livsci.2019.08.002
Herman H A, Mitchell JR, Doak GA. The Artificial Insemination and Embryo Transfer of Dairy and Beef Cattle. A Handbook and Laboratory Manual. 8th ed. Prentice Hall College Div; 1993.
Gadea J, Sellés E, Marco MA. The predictive value of porcine seminal parameters on fertility outcome under commercial conditions. Reprod Domest Anim. 2004;39(5):303-308. doi:10.1111/j.1439-0531.2004.00513.x
Knox RV, Rodriguez-Zas SL, Roth S, Ruggiero K. Use and accuracy of instruments to estimate sperm concentration: pros, cons & economics. In: Reproductive Pharmacology and Technology. Proceedings of the American Association of Swine Veterinarians 33rd Annual Meeting, 2002;23-38.
Ciereszko A, Ottobre JS, Glogowski J. Effects of season and breed on sperm acrosin activity and semen quality of boars. Anim Reprod Sci. 2000;64(1-2):89-96. doi:10.1016/s0378-4320(00)00194-9
Flowers WL. Factors affecting the production of quality ejaculates from boars. Anim Reprod Sci. 2022;246:106840. doi:10.1016/j.anireprosci.2021.106840
Ross JW, Hale BJ, Gabler NK, Rhoads RP, Keating AF, Baumgard LH. Physiological consequences of heat stress in pigs.Anim Prod Sci. 2015;55(12):1381-1390. doi:10.1071/AN15267
