In bulls, assessment of libido is often not possible during a routine BSE. However, if possible, the bull should be observed serving cows to allow assessment of his desire to breed, ease of mounting, ability to achieve erection and extend the penis, and presence of penile deviation or other abnormalities that may prevent successful service. Libido and serving capacity tests (scoring the number of services achieved during a set time that a bull is in a pen with a restrained cow) have been devised but are time consuming and difficult to standardize under field conditions. In addition, the results are difficult to interpret in light of the variety of stocking conditions used, eg, single versus multiple bulls or small paddocks versus large ranges.

BSE forms have been developed and should be used to ensure the systematic completion of the examination and accurate reporting of results. The bull should be restrained in a chute for the examination. Body condition should be scored, and a general physical examination conducted with special attention paid to the feet, legs, eyes, and sheath. The inguinal rings and internal genitalia should be palpated per rectum for the detection of any abnormalities, eg, seminal vesiculitis. The scrotum should be palpated to evaluate the testes, epididymides, spermatic cords, and scrotal skin. The testes should be symmetrical in size, smooth, resilient, and freely movable within the scrotum. Cryptorchidism is considered an undesirable heritable trait and renders a bull unsatisfactory for breeding even though his semen quality might be acceptable. In ruminants, the epididymides run caudomedially along the testes in a dorsoventral orientation, with the tail most ventrad. The epididymis should have no palpable masses. The scrotal circumference should be measured at its maximal diameter; this should be ≥30 cm for bulls <15 mo old, 31 cm for bulls 16–18 mo, 32 cm for bulls 19–21 mo, 33 cm for bulls 22–23 mo, and 34 cm for bulls ≥2 yr old. Placing the thumb or fingers between the testes during measurement can lead to a false increase in apparent circumference. Scrotal circumference is directly related to the number of sperm the bull is capable of producing, as well as to the percentage of morphologically normal sperm produced.

For the purpose of a routine BSE, semen is most often collected via electroejaculation; semen can be collected with an artificial vagina (AV) in bulls trained to use one (eg, those in bull studs). The penis usually extends during electroejaculation and should be examined at that time for any abnormalities. If it is not extended during electroejaculation, it should be gently exteriorized for examination by grasping the glans with a cotton gauze and, if necessary, by putting pressure on the sigmoid flexure immediately caudal to the scrotum. Preputial wash samples may be taken for isolation and culture of Campylobacter fetus venerealis (using Clark's medium) or Tritrichomonas foetus (using Diamond's medium or a commercially available diagnostic medium kit), in particular when investigating subfertility in bulls ≥4 yr old.

The electroejaculator consists of a rectal probe that has a series of linear banded electrodes connected to a variable current and voltage source. The bull is restrained in a chute, the rectum is emptied, and the entire lubricated probe is inserted rectally with the electrodes oriented ventrally. A hand-operated rheostat permits intermittent pulses of current to be given as the voltage is gradually increased. The response varies considerably, but it is common to use 2- to 4-sec pulses repeated at 5- to 7-sec intervals. After a variable number of such stimulations, erection and protrusion of the penis may be seen, followed by a flow of seminal fluid, or the bull may ejaculate into the sheath without protruding the penis. The semen may be collected by any convenient method; typically a rubber AV cone inserted within a plastic cylinder attached to an 18-in. handle, with a test tube attached to the cone, is used. In some bulls, ejaculation is seen only after a final series of momentary pulses at 1- to 2-sec intervals. Older bulls usually require a higher voltage for ejaculation. In some large bulls, the probe may not reach the correct areas for stimulation; having 2 or more probe sizes is recommended if BSE are to be done on a variety of sizes of bulls. The semen collected via electroejaculation is not representative of a full ejaculate in regard to volume and concentration, and thus the sperm production potential of the bull is based on the evaluation of scrotal circumference. Conversely, sperm quality (motility and morphology) does not differ from that in an ejaculate collected with an AV.

If an AV is to be used, bulls are induced to mount a teaser animal (eg, restrained steer, cow, or phantom), and the erect penis is directed into the AV by the collector as the bull mounts. In preparing the AV, the temperature, which is a critical factor in stimulating ejaculation, is maintained at 105–107°F (40.5–42°C). Temperatures up to 118°F (48°C) may assist collections in untrained bulls. The AV should be lubricated with nonspermicidal jelly. The typical volume of an ejaculate is 4–8 mL, and the concentration 1–1.5 billion sperm/mL. A semen sample can be collected from some bulls by transrectal massage of the accessory sex glands. With this technique, erection seldom occurs. After the rectum is completely emptied, the vesicular glands are massaged with a backward motion until a few mL of fluid drop from the sheath. The ampullae are then massaged, and an assistant collects the semen as for electroejaculation. This method is not always successful, and the quality of the semen emission is usually lower than the ejaculate collected by the other 2 methods.

Sperm concentration, ejaculate volume, and the total number of sperm in the ejaculate are evaluated only if the sample has been collected with an AV. The semen sample should be evaluated immediately for sperm motility. The materials that contact the sperm should be at the same temperature as the sperm (to avoid temperature shock), clean, dry, and nontoxic. Motility evaluation is best when the temperature of the semen is either maintained at ~37°C during the short time before it is evaluated, or when the semen is gently warmed to 37°C before evaluation. Both gross motility (“swirl pattern”) in the semen and motility of individual spermatozoa should be assessed; the individual motility is the more accurate measure and should be used for classification of the bull.

Gross motility is a function of sperm concentration and individual sperm motility and is evaluated in an undiluted drop of semen placed on a slide without a coverslip and examined at low power (~100×). The intensity of wave motion may be classified into 4 categories: very good—intense swirling, rapid dark and light waves; good—slower swirling, waves not as intense; fair—slow movement with fewer waves; or poor—very little or no swirl activity. Individual motility is assessed in a sample diluted with warmed saline or semen extender. A drop of diluted sperm is placed on a slide, covered with a coverslip, and examined at 200–400×. The proportion of sperm that are moving progressively across the field of view is estimated by finding multiple groups of ~10 sperm and estimating how many sperm are progressive versus how many are not.

The environments in which BSE are done are variable, and there is a high chance of temperature shock to sperm before they may be evaluated. Therefore, >30% individual progressive motility, or fair gross motility, is considered acceptable. If the examination is performed under optimal conditions, some associations (eg, the Canadian Bovine Practitioners Association) recommend a minimum of 60% motility for a bull to be classified as a satisfactory potential breeder.

The presence of cells other than spermatozoa in the sample should be investigated while estimating motility. RBC, WBC, and excess numbers of round epithelial cells and developing forms of spermatozoa may indicate a genital tract abnormality. Careful evaluation of the tract, especially the internal genitalia, may indicate the source of WBC. The most common cause is seminal vesiculitis. Round, immature germ cells and spermatozoa with proximal cytoplasmic droplets may indicate immaturity or, alternatively, testicular degeneration.

A sample of the semen should be fixed in a buffered formaldehyde saline solution for evaluation of sperm morphology. This is best done with the fixed, unstained sperm examined under high power (1,000×) using a phase-contrast microscope. Morphology may also be assessed by using a vital sperm stain (eg, eosinnigrosin), but this method may not be as accurate as phase-contrast microscopy. At least 100 sperm should be counted, and the proportion of sperm showing different types of abnormalities noted. However, the only number used in the evaluation of the bull is the proportion of normal sperm, which should be >70%. Use of a live-dead stain is no longer recommended, because the percentage of morphologically normal sperm, scrotal circumference, and physical soundness are more highly correlated with fertility than the live:dead ratio of sperm in an ejaculate.

Bulls are classified as “satisfactory potential breeders” if they have no physical abnormalities that would prevent breeding and if they meet the minimal qualifications for scrotal circumference, sperm motility, and sperm morphology. Bulls that do not meet these criteria are classified as “unsatisfactory potential breeders” or, if the results are marginal or questionable, they are considered “classification deferred” and a retest should be recommended.

Last full review/revision July 2011 by Sylvia J. Bedford-Guaus, DVM, PhD, DACT