Evaluation of Potential Breeding Soundness of the Bull

CHAPTER 31 Evaluation of Potential Breeding Soundness of the Bull

ALBERT D. BARTH

It is estimated that at least one in five bulls in an unselected population would be subfertile owing to inability to serve cows efficiently or to poor semen quality. Although some bulls have such serious impediments to fertility that low pregnancy rates result, many bulls that are less than satisfactory achieve acceptable pregnancy rates under low breeding pressure and over a prolonged breeding season. Disastrously low pregnancy rates are very noticeable and can be financially devastating to the individual producer. In the beef industry, the greatest economic loss that is attributable to subfertile bulls is delayed conception (low first-service pregnancy rate)—which may not be noticed by the producer. It has been estimated that for every 21 days of the breeding season during which a cow remains open, a loss of 50 to 60 pounds of weaning weight can be expected the following year for the calf she finally conceives. Therefore, a subfertile bull could be the cause of economic losses in the range of $1500 to $3000 due to reduced weaning weights a year later, depending on the degree of subfertility, bull-to-female ratios, and weaned calf prices. Additional losses will accrue with the culling of open cows and cows that conceived late. Cows that calve late tend to do so perpetually in following years.

In multiple-sire pasture breeding, a great deal of breeding overlap will occur. As many as 80% of cows may be bred by two or more bulls during one estrous period. Thus, bulls with high fertility can compensate for bulls with poor fertility. Because of the common use of multiple-sire breeding and low bull-to-female ratios, inept bull evaluation often is masked by the excellent performance of highly fertile bulls in a group. Nevertheless, as the economic pressure for highly efficient beef production increases, including the use of short breeding seasons, high bull-to-female ratios, and single-sire breeding groups, the need for thorough, unbiased breeding soundness evaluations also will increase.

Veterinarians often feel pressured to compromise their standards by clients who wish to obtain a “satisfactory” classification for breeding soundness of certain bulls (discussed later in the chapter under “Breeding Soundness Classification”). A veterinarian’s fear of jeopardizing client relationships and lack of self-confidence regarding ability to evaluate bulls may result in approval for use of bulls of questionable fertility. In the long term, taking chances on cow herd fertility is of benefit to no one. Although it may be difficult to be sure that cow fertility will actually be depressed by allowing mating with questionable bulls, only bulls with the potential for high fertility should be classified as satisfactory.

In addition to freedom from disease, a bull requires three attributes to be fertile: (1) good libido, (2) physical soundness, and (3) good semen quality. These three attributes must be held in the forefront of all decisions regarding herd sire selection and breeding soundness evaluations.

LIBIDO AND SERVING CAPACITY

Serving capacity describes the number of matings a bull is willing and able to perform in a test situation. In spite of extensive research showing the importance of libido and serving capacity to fertility, the use of standardized tests has not become part of routine bull evaluation in North America. Furthermore, methods of bull evaluation have in the past almost totally ignored this part of breeding soundness, and producers often have been lulled into a false sense of security by a satisfactory test result. Veterinarians must inform producers when this critical component of bull fertility remains unassessed. It is particularly important to assess libido and serving capacity in yearling and 2-year-old bulls that have had no previous breeding experience. Many factors, however, including injuries and the development of penile deviations, can incapacitate older bulls that have been sound breeders in the past.

It may be necessary to advise producers on how to assess bull libido and serving capacity. For example, the mere presence of an older dominant bull may completely suppress the expression of libido in younger bulls. Yearling bulls often require some time after exposure to estrual females to develop their instincts and learn how to become efficient breeders. Studies have shown that libido or serving capacity tests do not provide reliable results in yearling bulls. Therefore, observations for libido and serving capacity must be diligently undertaken at the beginning of the breeding season and continued for its duration to ensure that estrual females are being bred.

Under certain circumstances, whether to assess the libido and serving capacity of individual problem bulls or for selection of a bull battery for a producer’s herd, practitioners may wish to become involved in libido and service capacity testing. Various testing methods have been investigated, including pasture versus corral settings, use of restrained versus unrestrained cows, use of estrual versus nonestrual cows, test durations ranging from 5 minutes to days, and testing each bull individually versus four or five bulls simultaneously. In Australia, a 3-hour corral test with restrained heifers had a correlation of 0.92 with a 19-day pasture test, but because the first hour of that test had a correlation of 0.91, 1 hour was considered satisfactory. Further testing demonstrated that a 40-minute test had a correlation coefficient of 0.99 as compared with 1 hour. Reduction to a 20-minute test period lowered the correlation to 0.91. Twenty-minute tests seem to be the most practical and are of sufficient accuracy for most purposes.¹

Methods of Testing Libido and Serving Capacity

Bulls to be tested are exposed to restrained cows in a small paddock or pen and observed for expression of sex drive, ability to serve cows, and the number of services completed in a given time period. Strong cows in good body condition are preferred as mount animals. One or more mount cows are restrained in short-sided breeding crates and sedated with xylazine (0.03 mg/kg IV). Cows need not be in estrus, and some practitioners believe that bulls spread out their services more evenly if cows are nonestrual and restrained at least 20 meters apart. When only one or two bulls need to be tested, it usually is advantageous to have an estrous cow to ensure sexual stimulation of the bulls to be tested. The vagina of each mount cow should be lubricated with approximately 30 ml of a sterile lubricating jelly to reduce tissue trauma from repeated breedings.

Bulls to be tested must first receive 10 minutes of sexual stimulation by allowing them to watch another “stimulator” bull serving the mount cow. The stimulator bull may be a test bull himself, and if he performs well initially, without prior sexual stimulation, further testing may not be necessary for him. Generally it is best to select an older, experienced bull to be the stimulator bull. Depending on available human assistance, one or two more bulls than mount cows are turned into the pen with the mount females. For example, with two mount cows, three bulls may be turned into the test pen at one time. Personnel can encourage serving by moving individual bulls to a different cow, separating bulls that are interfering with each other, or removing bulls that appear to be a threat to more timid bulls. Younger bulls should be tested separately from older bulls to prevent social dominance factors from invalidating test results. Some bulls are afraid of people and will not perform in a test situation. Bulls that give questionable results should be retested under different conditions or on different days to ensure that good bulls are not culled erroneously owing to spurious test results. Provided that bulls do not develop health problems, serving capacity test results are highly repeatable. Even after a period of 6 months, bulls had remarkably similar serving capacity scores.

In a 20-minute test, a serving capacity of 0 to 1 is considered low, 2 to 3 is medium, and 4 or more services are considered to indicate high serving capacity. When large numbers of bulls need to be tested, and only bulls of low serving capacity will be culled, bulls that serve a cow twice within the first few minutes can be removed immediately, to conserve the mount cows and speed up the testing process.

PHYSICAL SOUNDNESS

General Health

Breeding soundness examinations are not meant to be health examinations. It is taken for granted that bull health, like the health of all other herd members, is the concern of the producer, who is responsible for detecting any signs of health problems. By virtue of their training, however, clinicians should intuitively be alert for abnormalities in a bull’s attitude, general appearance, body condition, and fecal characteristics that may warrant further investigation into the health of the animal presented for fertility evaluation. Because bulls rely mostly on vision to detect cows in heat, and because the development of squamous cell carcinoma or corneal opacity due to pinkeye is common, special emphasis is placed on examining the eyes. Furthermore, some evidence suggests that squamous cell carcinoma has a heritable basis, and selection against it may be well worthwhile to reduce the incidence of this problem in a cow herd.

The Musculoskeletal System

A great deal of emphasis must be placed on sound conformation of the feet and legs of a bull. Bulls must travel long distances in order to detect females in heat and breed them. During the mating act, the full weight of the bull (commonly 900 to 1100 kg [1980–2420 lbs]) is borne on the hind legs and feet. Clearly, then, any unsoundness in this region will drastically interfere with breeding ability. Many producers fail to realize that although a bull with a conformation defect of the feet or legs may get by for 2 or 3 years, the more serious danger is that the defect will be passed on to offspring. Replacement heifers carrying conformational defects will propagate these problems in the cow herd, reducing longevity, increasing labor in herd management, and increasing veterinary expense. Common foot and leg problems that have a hereditary basis include the corkscrew claw defect, interdigital fibromas (corns), weak pasterns, post-leggedness, and sickle hocks. An effort must be made during the breeding soundness evaluation to watch the bull walk. Too often bulls are seen only standing in a chute, and although most of the onus is on the breeder to cull bulls with poor conformation and on the producer to procure physically sound bulls, it is not uncommon for them to disregard or fail to detect serious abnormalities.

The Scrotum and Testes

Examination of the Scrotum and Its Contents

Bulls must be adequately restrained to allow comfortable and thorough palpation of the scrotum and its contents. If a clinician does not feel safe behind the bull, a careful examination cannot be completed. For most Bos taurus bulls, a strong pole placed at the proper height tightly against the bull’s lower buttocks will allow examination of the scrotum and yet prevent injury from kicking. A height of 30 inches (76 cm) from the floor to the bottom of the pole is most practical and works well for most sizes of bulls. Bos indicus bulls generally require more secure restraint systems.

Letting the bull become aware of the veterinarian’s presence behind him by talking to him and touching him on the rump and thighs before touching the scrotum usually will prevent the bull from reacting to scrotal examination. In general, bulls do not resent palpation of the testes and rarely kick; however, it is common for them to lift their legs at the beginning of the examination. With such movements, the bull’s hock may catch the veterinarian’s arm and knock it against the post above. To prevent this, one arm should be introduced between the bull’s hind legs parallel to the bull’s longitudinal axis at the start of palpation until the bull settles down. When both arms are placed between the bull’s hind legs, the elbows should be held together. Bulls also are likely to flinch or lift their feet during forceful pressing of the testicles to the bottom of the scrotum. If large numbers of bulls must be examined, padding the veterinarian’s wrists and forearms may be helpful. Some bulls are very fractious, and palpating the scrotum may be dangerous because of continuous balking at restraint in the chute. If semen collection is planned, it often helps to do the semen collection first. After the procedure of electroejaculation, fractious bulls usually stand quietly for the scrotal examination.

A visual appraisal of the shape of the scrotum in a warm environment, while the bull is relaxed, reveals valuable information about the thermoregulatory abilities of the scrotum, as well as giving an indication of testis size. Presence of a scrotal “neck” above the testes is of critical importance because this region contains the countercurrent heat exchange mechanism of the testicular cords. In cooler temperatures, the scrotal shape cannot be determined, because the dartos muscle in the scrotal wall and the cremaster muscles will hold the testes closer to the body wall. In cool conditions, the testes must be manually pushed down into the scrotum, stretching the puckered scrotal wall to allow an assessment of scrotal shape. Nevertheless, it is often difficult to reliably assess scrotal shape in cool temperatures. Short scrotums or excess fat in the scrotal neck will prevent normal heat exchange, resulting in abnormal spermatogenesis and possibly testicular degeneration. Less commonly, abnormal scrotal shape may be due to a short caudal frenulum of the scrotum, unilateral testicular hypoplasia, orchitis, scrotal hernia, rotation of the testis, displacement of the cauda epididymidis, or rotation of the scrotum so that the testicles are held in tandem. The scrotum also should be examined for thickness of the scrotal wall, the amount of fat in the neck of the scrotum, and lesions in or on the scrotum.

The testicular cords should be palpated from the body wall down to the top of the testes to detect abscesses, variceles, or a scrotal hernia. The caput epididymidis, located primarily craniodorsally on the testis, usually is palpable and may feel more prominent in some bulls than in others. It is not uncommon to find enlargements in this area due to inflammation or sperm granulomas, which may prevent sperm transport and result in a small, flaccid, empty cauda epididymidis. The body of the epididymis can be palpated on the medial aspect of the testis by first sliding the opposite testis upward; however, it is extremely rare to detect abnormalities in the corpus epididymidis. The cauda epididymidis of a normally functioning testis is turgid and prominent. Differences in size and consistency between the left and the right cauda epididymidis may indicate inflammation on one side or may result from a blockage of sperm transport on the side of the smaller cauda. Segmental aplasia of one or both epididymides probably is an inherited condition.² Occasionally, the ligament that attaches the cauda epididymis to the bottom of the testis is absent or very long, so that the cauda is separated from the bottom of the testis. This condition does not necessarily interfere with semen quality and may not be of concern to a commercial cow-calf producer.

The testes must move freely within the scrotum. Careful palpation of the testes must be done to detect possible abscesses, tumors, hematoceles, or calcification. In some cases, ultrasonography or infrared thermography may be helpful for diagnosis of testicular abnormalities. The consistency of the testis often is difficult to ascertain by subjective palpation. Although the use of tonometers for measuring consistency of testes removes much of the subjectivity, tonometer measurements have not been strongly correlated with semen quality and are not commonly used. In general, yearling bulls have very firm testes compared with those of older bulls. Testes that are obviously soft are most likely to indicate testicular degeneration. This usually can be confirmed by semen analysis.

Scrotal Circumference

The relationship of scrotal circumference to fertility.

Scrotal circumference (SC) measurements are highly correlated with paired testis weight, which in turn is directly and highly correlated with daily sperm production and high semen quality traits.³ Considerable evidence also indicates that SC measurements between 1 and 2 years of age are moderately to highly heritable.

Studies have shown that SC is a more accurate predictor of age at onset of puberty than either age or weight, regardless of breed. Among breed groups, negative correlations greater than 0.9 have been observed between SC and bull age at puberty, age at puberty in half-sibling heifers, and age at puberty in heifer offspring. Good evidence indicates that heterosis in cattle for traits related to size and age at puberty in females and SC of males is due to dominance effects of genes. Furthermore, correlations of 0.66 and 0.97 have been found between breed mean SC and fertility of female offspring.⁴

Strong genetic correlations were reported between SC and age at first breeding (−0.77), age at first calving (−0.66), and pregnancy rate (0.66). Because age at puberty in females is favorably associated with subsequent reproduction, selection for larger SC should improve the reproductive potential of the cow herd.

There is no question that small testes are undesirable. Histologic studies in 14-month-old bulls and in 2- to 3.5-year-old bulls showed that the proportion of seminiferous tubules with normal seminiferous epithelium is significantly lower in small testes (SC less than 32 cm). Yearling bulls with small testes do not exhibit catch-up growth over time and will have small testes at 2 years of age as well. Therefore, yearling bulls with SC below the recommended minimum should be culled.

Effects of nutrition.

In young bulls, scrotal circumference measurements are affected by breed, body weight, and age at onset of puberty. Testis growth rate is maximal during puberty, and the level of nutrition in young growing bulls has a great influence on the age at onset of puberty. High-energy diets with adequate protein, vitamins, and minerals hasten the onset of puberty in bulls.⁵ The early attainment of puberty improves the opportunity for early postpubertal development. This implies greater numbers of and higher-quality spermatozoa available when the bull is first used for breeding.

High-energy intakes up to about 12 months of age in beef bulls usually do not impair future semen quality, provided that their rations from 1 to 2 years of age do not result in fattening. This probably is because young, rapidly growing bulls can put the excess energy into growth, rather than fat. Nevertheless, Angus and Hereford bulls fed a high-energy diet of 80% grain and 20% forage from weaning to 15 months of age showed significantly lower sperm outputs than those in bulls on a medium-energy diet of 100% forage. Bulls in high-energy diet groups had a greater mean SC at 12 months, but not 15 months, of age, than that in bulls in medium-energy diet groups. Furthermore, excessive energy intake in young bulls may result in abnormal foot growth and conformation because of laminitis and possibly epiphysitis. In addition, high-energy diets increase the risk of rumenitis and liver abscesses, which may lead to the development of vesicular adenitis and epididymitis.⁶

Twelve-month-old Hereford and Angus bulls fed high-energy diets (80% concentrate) until 21 months of age had reduced epididymal sperm reserves, lower percentages of progressively motile sperm, and higher percentages of sperm abnormalities compared with bulls fed a medium-energy diet. Furthermore, Hereford bulls on the high-energy diet demonstrated a decline in testicular size that began at approximately 19 months of age, probably from testicular degeneration brought on by obesity.

Effect of breed.

The spermatogenic rates in normal postpubertal bulls of different breeds are similar, and each gram of functional seminiferous tissue contains a similar amount of seminiferous tubule epithelium. Different studies have shown that each gram of normal functioning testicular tissue produces approximately 17 million sperm per day. Testicular shape is remarkably uniform among all breeds. To date, it has not been proved that some breeds with longer, narrower testes can produce larger numbers of sperm than other breeds with equivalent SC measurements.

Significant genetic variation exists among breeds of beef cattle for age at puberty.⁷ In general, faster-gaining breeds of larger mature size reach puberty at a greater weight than that observed for slower-gaining breeds of smaller size. Breeds historically selected for milk production (e.g., Braunvieh, Gelbvieh, Red Poll, Pinzgauer, Simmental) reach puberty at significantly younger ages than those typical for breeds not selected for milk production (e.g., Charolais, Limousin, Hereford). Great differences are recognized between breeds of bulls in average testicular size at any given age. In general, the large milk-producing beef breeds have an earlier onset of puberty and develop larger testicles at an earlier age and at maturity than smaller breeds of cattle that have lower milk production. Double-muscled breeds such as Piedmonte, Belgian Blue, Parthenaise, Blonde d’Aquataine, and Limousin have a later onset of puberty and smaller average testis size at puberty and at maturity. Few breeders have made an effort to select for larger testis size in these latter breeds; consequently, the breed averages for SC are small. Some breeders, however, have capitalized on the high heritability of testicle size and have made remarkable progress selecting for that trait. Breeders and producers alike are encouraged to select for average or above-average SC, just as they would select for calving ease, weaning weight, or yearling weight. SC data for yearling bulls and 2-year-old bulls are available for many of the common beef breeds (Tables 31-1 and 31-2).

Table 31-1 Scrotal Circumference (SC) by Breed in Bulls at 1 Year of Age ^*

Breed	Number of Bulls	Weighted Mean SC (cm)
Simmental	1246	34.7
Brown Swiss	260	33.8
Gelbvieh	261	33.9
Pinzgauer	144	33.7
Charolais	1887	32.5
Limousin	345	29.8
Blonde d’Aquitaine	15	29.7
Salers	45	29.5
Tarentais	14	32.0
Maine Anjou	64	32.2
Hereford	1567	31.9
Angus	1051	33.2
Shorthorn	167	31.9
Red Poll	250	32.5
Galloway	132	30.6

* Values corrected to 365 ± 14 days of age.

Data (for 6 studies in the United States and Canada) from Barth AD: Breeding soundness evaluation of bulls. The Western Canadian Association of Bovine Practitioners. Continuing Education, Western College of Veterinary Medicine, Saskatoon, Canada, 2000.

Table 31-2 Scrotal Circumference (SC) by Breed in Beef Bulls at 2 Years of Age

Breed	Mean SC (cm)
Simmental	38.8
Aberdeen Angus	37.2
Charolais	36.3
Horned Hereford	36.1
Polled Hereford	35.6
Shorthorn	34.9
Limousin	32.2
Texas Longhorn	34.6

Data (for 6 studies in the United States and Canada) from Barth AD: Breeding soundness evaluation of bulls. The Western Canadian Association of Bovine Practitioners. Continuing Education, Western College of Veterinary Medicine, Saskatoon, Canada, 2000.

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