A comparison of three methods of assessing sex-drive in yearling beef bulls and relationships with testosterone and LH levels

Three methods of assessing sex-drive were compared in 113 yearling beef bulls. These were the serving capacity score (SC), the libido score (L) and reaction time to first service (R). Ovariectomized heifers restrained in service crates were the stimulus for all tests. Bulls were assessed twice by each method. For the first libido and serving capacity tests (L1 and SC1), the heifers were induced to show estrus. For the second tests (L2 and SC2), the heifers were not induced to show estrus. On a non test day, single blood samples were taken from all bulls and assayed for LH and testosterone. Reaction times to first service (R1 and R2) in the two serving capacity tests were not significantly correlated. Although the numbers of services (SC1 and SC2) in both serving capacity tests, were significantly correlated (r = .67), 57% of the bulls did not achieve a service in both the tests with heifers in heat and with heifers not in heat. Libido scores between the test with heifers in heat and heifers not in heat were significantly correlated (r = .67). The libido score method had the advantage that more bulls received a positive score and the test duration was shorter than in the serving capacity test. Of the three scoring procedures compared, libido score appeared to have most advantage in assessing sex-drive in yearling beef bulls. The total number of services achieved in the first serving capacity tests (using 'estrus' heifers) did not differ from that achieved in the second tests (using non-estrus heifers). The total services achieved in 10-min compartments of the 30-min serving capacity tests showed no difference either within or between tests. It is concluded that a 10-min test provides as much comparative information on the sex-drive of yearling beef bulls as longer tests. Further, the use of females in estrus appears unnecessary to satisfactorily assess bull sex-drive, provided proper restraint and presentation of stimulus females is employed. LH and testosterone values were not significantly correlated or were poorly correlated with sex-drive measurements.     

Bull selection and use in northern Australia. 3. Serving capacity tests

Serving capacity studies were conducted on Santa Gertrudis, 5/8 Brahman, Brahman and Belmont Red bulls at 10 sites. Modifications, such as providing prior sexual experience, using females in oestrus and comparing restrained with unrestrained females, were made to the standard Bos taurus serving capacity test in a series of experiments with Santa Gertrudis bulls. Providing sexual experience to virgin bulls prior to testing improved the number of serves and libido scores in a serving capacity test. In serving capacity tests that used restrained females, expressions of sexual behaviour of bulls were not improved by having females in oestrus compared with non-oestrus females. There were more sexual behaviours, particularly mounts, in serving capacity tests involving restrained rather than unrestrained females. Summary statistics were collated from over 1100 serving capacity tests, in the 2 months prior to mating, across the 10 sites. In serving capacity tests using restrained females, in bulls of the same age, there were more expressions of sexual behaviours (except interest) in Belmont Red than in Brahman or 5/8 Brahman bulls with Santa Gertrudis bulls intermediate. The percentage of bulls displaying serves tended to increase with age; 82, 83 and 86% of 2-, 3- and > or =4-year-old Belmont Red bulls, respectively, completed 1 or more serves. This compared with 50, 50 and 66% of 2-, 3- and > or =4-year-old Santa Gertrudis bulls and 31% of 2-year-old Brahman bulls. Between 2 and 9% of bulls did not display any sexual behaviour in serving capacity tests with restrained females. Across genotypes, there were few and inconsistent relationships between either mounts or serves and liveweight, condition score, scrotal circumference, testicular tone and sheath measurements. The exceptions were in 2-year-old Brahmans where mounts were negatively correlated (r=-0.61; P<0.05) with umbilicus thickness and in 3-year-old Santa Gertrudis bulls, where serves (r=-0.53; P<0.01) and mounts+serves (r=-0.49; P<0.01) were negatively correlated with navel score. Measures of sexual behaviour were unable to consistently predict calf output of bulls in multiple-sire mating groups in extensive pastoral conditions. The main value of the serving capacity test in these genotypes is to identify whether a bull is capable of natural service.     

Determination of minimum weaning scrotal circumference in Santa Gertrudis bulls fed a low energy ration

The scrotal circumference (SC) of 374 Santa Gertrudis bulls was measured prior to and following a period of low energy feed intake. A step by step regression was performed to formulate prediction equations in which the future SC of young bulls could be determined. These equations accounted for only 30 to 50% of the variation. Analysis of variance was used to determine the minimum SC at 7, 8 and 10 mo of age needed to obtain a 30-cm SC by 1 yr of age. At 7 mo, the bulls with the largest final SC, were those with an SC larger than 18 cm (P < 0.01). As the 8-mo SC increased so did the 13-mo SC (P < 0.01). As the 10-mo SC increased, the 15-mo SC also increased but bulls with less than a 21-cm SC were smaller (P < 0.01). Differences were also found between the 7 and 8-mo old weights of bulls and the initial SC (P < 0.01). As the initial testicle size increased, the 7, 8 and 10-mo ages and the 10-mo weight tended to increase. Few differences were found in initial SC measurements between the bulls that reached a 30-cm SC and those that did not at the end of one year regarding weight and age within each SC. These findings indicate that the SC at the beginning of a low energy feed period can be useful in determining the minimum SC outcome by the end of the test period.     

Adjusted 200-day scrotal size as a predictor of 365-day scrotal circumference

Data collected from 264 bulls of 13 beef breeds at the Michigan Bull Test Station was evaluated to determine if scrotal circumference (SC) adjusted to 200 d of age could be used to predict scrotal circumference at 1 yr of age. Scrotal circumference of each bull was recorded on arrival at the test station and at the time of breeding soundness examination (BSE) and was adjusted to 200 and 365 d of age, respectively. Bulls with adjusted SC>34.0 cm by 365 d of age averaged a larger SC at 200 d (P < 0.0001) and faster scrotal growth (P < 0.0001) than bulls with a 365 d adjusted SC /= 23.0 cm had a 95% probability of achieving SC > 34.0 cm by one year of age. Calves measuring < 23 cm at 200 d had a 54% probability of achieving > 34.0 cm scrotal size by one year. This information can contribute to the selection of breeding bulls that will achieve desirable scrotal size by one year of age.     

The effect of using the Breeding Soundness Evaluation as a selection criterion for Santa Gertrudis bulls on bulls in subsequent generations

Results of Breeding Soundness Evaluations (BSE) used to select Santa Gertrudis bulls over a 10-yr period were analyzed in retrospect to determine the effect on bulls in subsequent generations. Bulls were tested at three ages: 16 mo (n = 1,936), 20 mo (n = 3,111) and at maturity (≥ 24 mo, n = 3,814). Scrotal circumference (SC) and sperm motility and morphology were evaluated. Sixteen-mo-old bulls were required to have an SC ≥ 30 cm to be tested. During 1977 to 1980, 20-mo-old bulls with an SC < 30 cm were culled, and the minimum SC was increased to 32 cm during 1981 to 1986. Mature bulls during this period were required to have an SC ≥ 35 cm to be tested. A BSE < 60 was considered a failing score, and bulls were culled at this level. There was a positive correlation between SC and sperm motility and SC and sperm morphology in 16-mo-old and 20-mo-old but not in mature bulls (P < 0.0001). The percentage of bulls passing the BSE was positively correlated with SC at all ages (P < 0.0001), and the passing rate increased over the years (P < 0.0001). Scrotal circumference increased over the years in bulls that passed the BSE (P < 0.0001). This increase with years was not as apparent in the semen quality traits measured. By using the BSE and a minimum SC to select prospective herd sires, the proportion of bulls passing the BSE is expected to increase in subsequent generations. Santa Gertrudis bulls can be reliably selected for use as herd sires at 16 mo of age.     

Further studies on the serving capacity test for beef bulls

In four experiments to investigate aspects of the conduct of the serving capacity (SC) test for beef bulls, it was found that (i) the 60-minute yard test could be shortened to 40 or even 30 minutes without sacrificing the test's accuracy (r = 0.98 between SC40 and SC60, r = 0.97 between SC30 and SC40), (ii) generally 2-year-old bulls displayed their inherent SC on their virgin test (r = 0.86 between SC on their virgin test and SC in a later test), and (iii) as long as bulls were tested within their own age group, their social ranking in their testing group did not significantly influence the SC they displayed in the test.     

Relationships among production traits and estimates of sex drive and dominance value in yearling beef bulls

Average daily gain (ADG) and final test weight (FTW) were compared with libido score (L), serving capacity score (SC), reaction time to first mount (RTM), reaction time to first service (RTS) and dominance value (DV) in 90 purebred yearling beef bulls which had just completed an 140-day performance test. The bulls represented 15 breeding lines (12 Hereford, 2 Black Angus and 1 Red Angus) and nine sires within lines. Libido and serving capacity assessments were applied to randomly selected groups of two bulls which were tested for 10-minutes with nonestrous heifers restrained in service crates. RTM and RTS were recorded for both tests. Social dominance was assessed during randomized observations of social interactions between bulls within their pen groups (comprising 9 to 10 bulls) over four days. Two methods of determining social dominance (DV1 and DV2) were employed initially. Differences occurred among both lines and sire/lines in mean and best libido scores (P < .05). Sire/lines differed in L1, while both lines and sire/lines differed in L2 (P < .05). These differences were not reflected in either reaction times or serving capacity scores. Sires and sire/lines both differed in FTW (P < .05 and P < .01 respectively), while DV1 differed with sire/lines (P < .01). DV1 and DV2 were highly correlated (r = .84). Those sex-drive estimates which correlated with DV1 were the mean and best libido scores, mean serving capacity score, L2 and RTS 1 (P < .05). All correlations were negative, indicating that dominance and sex-drive may be unfavourably related in yearling beef bulls. ADG and FTW showed negative correlations with the mean and best serving capacity scores and with SC2 (P < .05). The only positive correlations between the two production traits and a sex-drive estimate was with RTM 2 (P < .05). This indicates that ADG and FTW may be unfavourably related to sex-drive in yearling beef bulls.     

Breeding soundness evaluation of extensively managed bulls in Costa Rica

This paper describes the results of single breeding soundness evaluations (BSE) in 898 Bos indicus, Bos taurus and B. indicus x B. taurus bulls, 1 to 12 yr old, extensively reared in different climatic regions of Costa Rica and representing approximately 2% of the total breeding bull population. Thirty-three percent (n = 296) of the bulls were classified as unsound for breeding owing to clinical problems (9.1%, n = 82), low scrotal circumference (SC) being the most common finding, followed by unsatisfactory sperm morphology (23.9%, n = 214). The prevalence of bulls unsound for breeding was lowest in Bos indicus (29%, P < 0.01), intermediate in B. taurus (41%), and highest in B. indicus x B. taurus (48%). The percentages of abnormal sperm heads, acrosomes and midpieces tended to be higher in the ejaculates of bulls with softer testicular consistency (P < 0.001), a long scrotum (P < 0.01) or a low SC (P < 0.05), and such bulls were more often classified as being unsound for breeding (P < 0.05). Frequencies of sperm abnormalities were higher in bulls < 2 yr of age than in older males (P < 0.01) and were highest in B. indicus x B. taurus bulls (P < 0.001). The results confirm differences between species in their adaptability to a tropical environment and support earlier evidence of an association between SC, testicular consistency and scrotal length clinical parameters, and testicular function in bulls.     

The relationship between scrotal circumference and quantitative testicular traits in yearling beef bulls

A study was designed to investigate relationships between testicle size and histological, sperm production and endocrinological traits in yearling beef bulls at the end of performance test. Twenty-five beef bulls, (Hereford, n=16; Angus, n=4; and Charolais, n=5), with scrotal circumference (SC) measurements ranging from 28.5 to 36.0 cm, were used. Just prior to slaughter at 15 mo of age, SC measurements were taken, semen was collected, and a GnRH response test was conducted. Testicles were processed for daily sperm production (DSP), epididymal sperm reserves (ESR), seminiferous epithelial area (SEA), and degree of germinal epithelial loss (DGEL). There were significant positive correlations between SC and testicular weight (P<0.05), DSP/g (P<0.02), and DSP/bull (P<0.01) and ESR (P<0.01); however, the correlation between SC and SEA was not significant (P=0.4). Scrotal circumference was negatively correlated with DGEL (P<0.05). Degree of germinal epithelial loss was also negatively correlated with DSP/g, DSP/bull and ESR (P<0.01). Morphological characteristics of spermatozoa were diversely related to sperm production traits, and the percentage of normal spermatozoa was positively related to SC (P<0.02) and negatively related to DGEL (P<0.001). Gonadotropin releasing hormone stimulation did not reveal evidence of gonadotropin deficiency in any of the bulls. However, peak testosterone levels were lower in bulls with SC below 31 cm (P<0.05) than those with SC measurements above 31 cm.     

Breeding soundness and libido examination of Belgian Blue and Holstein Friesian artificial insemination bulls in Belgium and The Netherlands

Data on breeding soundness and libido evaluations in Belgian Blue (BB) bulls are scarce. The present study compared results of breeding soundness and libido evaluations of young BB bulls to young Holstein Friesian (HF) bulls prior to acceptance into an AI program. Breed differences for breeding soundness exist between BB and HF bulls, as 93.7% of the young BB bulls failed the breeding soundness examination (BSE) compared to 59.3% of the HF bulls (P=0.0005). Within the BB breed, differences were present between bulls of different ages, and bull selection for better fertility with increasing age apparently influenced the results. The number of reasons for which bulls failed the test differed between the age groups in the BB breed, whereas a tendency for more failure reasons in the BB breed was noticed in the breed comparison. The most important reasons for failure were sperm morphology and scrotal circumference (SC), but far more BB bulls failed for these traits compared to the HF breed (82.8% versus 56.0% and 43.8% versus 17.6% in the BB and the HF breed for sperm morphology (P=0.0005) and SC (P<0.0001), respectively). The high proportion of BB bulls with a substandard SC and poor sperm morphology might suggest an increased prevalence of testicular hypoplasia or degeneration within this breed. Concerning libido, the reaction time did not differ either between breeds or between age groups within the BB breed, whereas mounting enthusiasm, although not different between the two breeds, did decline with increasing age, probably due to the greater mating experience of the older bulls. All in all, libido did not seem to be different between the breeds.     

Assessing sex-drive in young Bos taurus bulls

Objectives in this study were to determine the accuracy of different methods of evaluating libido score (L), service rate (SR) and reaction time to service (RTS) in yearling Bos taurus bulls. Using restrained, non-estrus females, 26 yearling Bos taurus bulls were evaluated a total of eight times (four sessions, two tests per session) over 2 consecutive months for L, SR, and RTS. Individual bull variation influenced both L (P<0.0001) and SR (P<0.003). Repeatability was moderate for L (R=0.64) and low for both SR (R=0.12) and RTS (R=0.04). Under the conditions of this study and characteristics of these tests, variance was reduced to 69-73% for L and 26-23% for SR with four to eight repetitions, respectively. Bulls that scored highly in the first session, however, tended to score highly throughout. Although, three bulls did not serve in any test, RTS was independent of effects. However, the tendency of RTS to decrease, and for L and SR to both increase with consecutive tests, suggests influences other than genetic, such as learning and/or environmental factors. These tendencies were most evident in bulls which obtained low L scores at the first session. It was concluded that, despite the important degree of individual variability observed in L and SR, accurate quantitative evaluation of the sex-drive in young Bos taurus beef bulls would benefit from use of test procedures of greater repeatability.     

Breeding soundness, libido and performance of beef bulls mated to estrus synchronized females

Ninety-three beef bulls and 2316 females were used to determine the relationships between breeding assessments of bulls and subsequent mating performance. Each bull was given a breeding soundness examination (BSE) and two 10-min libido/serving capacity (L/SC) tests. Breeding potential of each bull was classified as satisfactory (BSE score = 60 to 100) or questionable (BSE score = 30 to 59); libido was classified as either high (mean score = 9.0 to 10) or medium (mean score = 6.0 to 8.5). Bulls were then joined, single-sire, with groups of females which had received one of two treatments to synchronize estrus. Bull-to-female ratios ranged from 1:7 to 1:51. Continuous observations were conducted on the mating activity of each group. One bull was removed from the experiment due to a severe breeding problem. Bulls of satisfactory breeding potential (n = 80) did not differ (P > 0.10) from bulls of questionable breeding potential (n = 12) in measurements of mating activity. However, by the end of the synchronized breeding period, bulls classified as satisfactory breeders achieved approximately a 9% higher (P < 0.10) pregnancy rate than did bulls of questionable breeding status (45.6 +/- 2.1% vx 36.5 +/- 5.3%). Bulls with a high libido (n = 69) serviced more (P < 0.01) estrous females (81.3 +/- 3.1% vs 63.5 +/- 4.2%) than bulls with a medium libido (n = 23). However, pregnancy rates achieved by bulls of either libido classification did not differ significantly. Individual components of the BSE as well as mean libido score were poorly correlated with pregnancy rates (r = -0.22 to 0.18). It was concluded that classification of bulls by mean libido score can aid in identifying groups of bulls that service more estrus synchronized females, whereas classification by BSE score aids in identifying groups of bulls that impregnate more females.     

Exposure of prepubertal beef bulls to cycling females does not enhance sexual development

The objective of this study was to determine whether continuous, long-term, fenceline exposure of prepubertal beef bulls to cycling beef females reduced age at puberty and influenced the percentage of bulls that passed an initial breeding soundness examination (BSE). Bulls (Angus, n = 37; Simmental, n = 22; Hereford, n = 10; Simmental × Angus, n = 8) at an average age of 202 ± 21.5 days were given either continuous fenceline and visual exposure to cycling females (exposed, n = 41) or no exposure (control, n = 36). Estrus was induced in cycling beef females so at least three females were in standing estrus each week during the 182 days of exposure to bulls. Scrotal circumference (SC), body weight, and blood samples were collected every 28 days. When bulls had SC of 26 cm or more, semen samples were obtained monthly via electroejaculation until puberty was achieved (≥50 × 10⁶ sperm/mL with at least 10% progressive motility). Behavioral observations were conducted twice monthly: once when females were in estrus and once during diestrus. Homosexual mounting, flehmen responses, and number of times near penned females were recorded for each observation period. Breeding soundness examinations were conducted when the average age of bulls was 364 ± 21.5 days. Normal sperm morphology of at least 70% and sperm motility of at least 30% were required to pass the BSE. Age, body weight, and SC at puberty did not differ between exposed and control bulls (320 ± 28 and 311 ± 29 days; 466.2 ± 12.2 and 437.7 ± 13.5 kg; and 34.4 ± 2.5 and 34.9 ± 2.5 cm, respectively). Percentage of bulls passing their initial BSE did not differ between treatments (exposed, 87.8%; control, 75.0%). Treatment, month, and female estrous stage interacted (P = 0.05) to affect the number of mount attempts and flehmen responses. Exposed bulls entered the cow area more times (P < 0.001) during estrus than diestrus in Months 1, 2, and 3. We concluded that bulls given continuous, long-term, fenceline exposure to cycling beef females do not have enhanced sexual development.     

The development of beef breeding bulls

Management of the bull battery will have a dramatic impact on profitability of the cow/calf enterprise. It is critical that young bulls be selected and developed to maximize longevity and productivity for the eventual buyer. Bulls must be structurally sound, healthy, and have adequate libido in order to service the required number of females. Once bulls complete their first breeding season, special care must be taken in order to ensure that they recover and regain needed body condition and pass a bull breeding soundness examination (BBSE). Mature bulls that have reached their genetic potential for growth require less intensive management, but the health program and annual BBSE cannot be overlooked. Mature bulls are also more likely to carry venereal disease and should be screened according to local disease incidence and state regulations. All bulls, regardless of age, should be observed early during the breeding season to ensure that they are physically capable of mounting and servicing females. The establishment of a complete management program, especially for young bulls, is essential to ensure that ranch resources are used efficiently, including maintenance of a high level of reproductive performance of the cow herd.     

Associations of scrotal circumference with semen traits in young beef bulls

Associations of percent live sperm, sperm number, sperm concentration, sperm motility, and potential breeding efficiency score with scrotal circumference were studied using data from the records of 465 Polled Hereford and 264 Simmental bulls. Data were subdivided by breed and within breed into three groups according to scrotal circumference (SC) size. Group I bulls had SC measurements <32 cm. Group II included bulls with SC measurements between 32 cm and the mean measurement of the breed (i.e., 35 cm for Polled Hereford and 38 cm for Simmentals). Group III was comprised of bulls with SC values above their respective breed averages. Age and body weight differences were also evaluated in the three groups. Average age differences among bulls in the three groups differed by not more than nine days in the two breeds. Average body weight differences among the three groups were statistically significant (P<0.05) with the maximum difference between any two groups within breed being 80 pounds. Average differences in percent live sperm and sperm number were greater (P<0.05) in Groups II and III than in Group I. For sperm concentration ratings, sperm motility ratings and potential breeding efficiency score, and for both breeds, Group I had a significantly lower (P<0.05) number of bulls classified satisfactory and a significantly greater (P<0.05) number of bulls classified unsatisfactory than did groups II and III.     

Methodology for adjusting scrotal circumference to 365 or 452 days of age and correlations of scrotal circumference with growth traits in beef bulls

A retrospective analysis was conducted on data collected from 1983 through 1991, where weight and hip height were measured at start of test and every 28 d on 604 bulls completing a 224-d forage-based gain test. Scrotal circumference (SC) was measured at start of test, and at either end of test or end of the weigh period after individual bulls reached 365 d of age. Over 3 yr of this study, SC was additionally measured every 28 d. Bulls were representatives of 5 breed groups: Angus, Santa Gertrudis, Simmental, Continental (predominantly Charolais), and Zebu (predominantly Simbrah). Adjusted 365-d SC and adjusted 452-d SC were calculated by regression analysis and from formulas based on SC growth of individuals to 140 and 224 d on test, respectively. Breed group differences were observed for age of dam, birth weight, hip height, weight per day of age, average daily gain and SC at start of test, 140 d, and end of test (224 d). Scrotal circumference was positively correlated with all growth traits. Scrotal circumference was related to breed group, age, weight, hip height, average daily gain, weight per day of age, age by year, and age-by-breed group (P<0.05), as determined by regression analysis. However, omitting weight, hip height, average daily gain, and weight per day of age from the regression model did not significantly affect R2 value. Scrotal circumference growth was linear to 140 d on test; however, SC growth to 224 d on test was curvilinear. The 365-d SC predicted from the formula and from regression analysis differed for Simmental and Zebu by 0.3 and 0.4 cm, respectively (P<0.05). The 452-d SC differed for Santa Gertrudis and Zebu by 0.5 and 0.6 cm, respectively (P<0.05). Formulas based on SC growth of individuals are reasonably accurate predictors of SC at 365 and 452 d of age, when compared with more complex regression analysis. Basing SC adjustments on individual growth appears to account for variables known to affect yearling SC.     

Sexual development in early- and late-maturing Bos indicus and Bos indicus x Bos taurus crossbred bulls in Brazil

Two experiments were conducted to evaluate sexual development in early- and late-maturing Nelore (Bos indicus) and Canchim (3/8 Bos indicus x 5/8 Bos taurus crossbred) bulls and to determine predictors of sexual precocity, and pubertal and maturity status. In Experiment 1, 12 Nelore bulls where examined from 300 to 900 days of age. Puberty was characterized by an ejaculate containing > or =50 million sperm with > or =10% motile sperm, and maturity by an ejaculate containing > or =70% morphologically normal sperm. In Experiment 2, 28 Canchim bulls where examined from 295 to 488 days of age and puberty was characterized by an ejaculate containing > or =30% motile sperm. In both experiments, bulls were classified as early- or late-maturing based on age at puberty. Early-maturing bulls were younger (P < 0.05) than late-maturing bulls at puberty (527 days versus 673 days in Experiment 1 and 360 days versus 461 days in Experiment 2) and at maturity (660 days versus 768 days in Experiment 1). In general, early-maturing bulls were heavier and had greater scrotal circumference (SC), testes, and testicular vascular cone diameter than late-maturing bulls during the experimental period. Scrotal circumference adjusted for 365 days of age was a good predictor of sexual precocity; minimum yearling SC of 19 and 24 cm for Nelore and Canchim bulls, respectively, had the best predictive values. Early-maturing bulls were lighter and had smaller SC at puberty than late-maturing bulls; therefore, sexual precocity was not related to the attainment of a threshold body weight or testicular size earlier, but to lower thresholds in early-maturing bulls. When predictors of pubertal status were evaluated, SC had the best sensitivity/specificity relationship in Nelore bulls, and high sensitivity and specificity in Canchim bulls. When predictors of sexual maturity were evaluated in Nelore bulls, age, weight, and SC had similar sensitivity, specificity, and predictive values. At puberty, approximately 60% of the sperm present in the ejaculate were morphologically defective. Changes in semen quality after puberty in Nelore bulls were characterized by increased motility and proportion of morphologically normal sperm, with a decrease in the proportion of major sperm defects. In conclusion, early-maturing bulls were more developed in the pre-pubertal period and attained puberty at earlier stages of body and testicular development than late-maturing bulls. Yearling SC could be used to select bulls for sexual precocity and SC was the best predictor of pubertal status. Age, weight, and SC were equally good predictors of sexual maturity in B. indicus bulls.     

A description of the findings from bull breeding soundness evaluations and their association with pregnancy outcomes in a study of western Canadian beef herds

The primary objectives were to describe beef bulls considered for use and those reported as used in 205 beef herds in western Canada, and to determine whether factors typically assessed during breeding soundness evaluations were associated with reproductive success. More than 100 veterinary clinics reported 2990 breeding soundness evaluations for bulls considered for natural service in client's herds. Differences among clinics explained 5.2% of the variation in scrotal circumference (SC) and 6.9% of the variation in percentage of morphologically normal sperm of all bulls considered for use (after accounting for age, breed, body condition, significant physical abnormalities, month, and year). The percentage of morphologically normal sperm was lower in bulls with an SC ≤34 versus >34 cm (P < 0.006). This study included data from 1384 and 1370 bulls used for breeding in 2001 and 2002, respectively. Most (80%) of the bulls used were Simmental, Black Angus, Charolais, Red Angus, or Hereford, and 80% were ≤4 y of age. Before the breeding season, a veterinarian evaluated 89.5% of all bulls used in these herds. Of the bulls subjected to a breeding soundness evaluation and subsequently used, 93.1% were satisfactory. In 2001 and 2002, injuries were reported in 2.5 and 2.1% of bulls and in 16.6 and 11.4% of herds, and necrobacillosis of the foot was reported in 2.5 and 1.2% of bulls and 11.2 and 6.5% of the herds. The average number of cows exposed to each bull was 26 (both years). Cows exposed to bulls with a smaller SC were less likely to be diagnosed pregnant (P < 0.047) and had a longer median interval from first bull exposure to calving (P < 0.016) than bulls with a larger SC. In conclusion, our findings emphasized the value of breeding soundness evaluations, including measurements of SC, in fertility management of beef cattle.     

Breed and other effects on reproductive traits and breeding soundness categorization in young beef bulls in Florida

Yearling, grass-fed, beef bulls at the USDA Subtropical Agricultural Research Station, Brooksville, Florida, were assessed for physical and semen traits in January, April, July and October of 1991 (Trial 1) and 1992 (Trial 2). Bulls were given a breeding soundness evaluation (BSE) using revised semen and scrotal circumference (SC) criteria. In Trial 1, the bulls consisted of Angus (n = 15), Brahman (n = 14), Hereford (n = 15) and Senepol (n = 14). In Trial 2, the breeds were Angus (n = 15), Brahman (n = 16), Romosinuano (n = 13) and Nellore x Brahman (n = 9). Trial bulls generally showed delayed growth compared with grain-fed bulls in temperate environments. Breed influenced semen traits (percentage sperm motility, normal spermatozoa and those with primary abnormalities) in both trials. Temperate Bos taurus breeds (Angus, Hereford) were generally superior to Bos indicus breeds (Brahman, Nellore x Brahman). Tropically-adapted Bos taurus breeds (Senepol, Romosinuano) were intermediate for those traits tested. In general, tropically-adapted Bos taurus breeds were more similar in reproductive development to temperate Bos taurus than to Bos indicus breeds. Breed by test period interactions occurred and were mainly influenced by delayed sexual maturity of Bos indicus bulls. Qualitative semen traits increased with bull age, particularly from 12 to 18 mo. Scrotal circumference development was slower in the Bos indicus breeds. Bulls of satisfactory BSE status at 18.1 to 22 mo of age were 73.9% in Trial 1 and 58.5% in Trial 2. Brahman bulls had the least satisfactory BSE scores in both years (Trial 1, 44.4%; Trial 2, 22.2%). Most bulls failed to achieve satisfactory BSE status due to a small SC relative to age (Trial 1, 66%; Trial 2, 72%). The most efficacious use of the BSE was > or = 15 mo in Bos taurus bulls and > 18 mo for Bos indicus bulls. Although the BSE has proven to be useful for the assessment of young, pasture-raised bulls in semi-tropical environments, use of SC thresholds linked more with growth traits than with calendar age would improve comparisons of relative reproductive development in such bulls, particularly those of Bos indicus derivation.     

Nutritional influences on growth and onset of puberty in Boran and Boran x Friesian bulls in Ethiopia

Twenty-seven Boran and 37 Boran x Friesian crossbred bulls were weaned at 6 months of age and randomly assigned to either a dry season supplementary feed containing 16% crude protein or control groups. Data were collected on body weight, wither height and scrotal circumference (SC) at monthly intervals. Bulls were also challenged with 100 mug of gonadotropin releasing hormone (GnRH) at 2-month intervals and plasma testosterone responses were determined. Semen was collected at monthly intervals by electroejaculation. Post weaning average daily weight gain was significantly (P<0.001) higher in supplemented than in control groups (302 vs 208 g/day). Scrotal circumference growth rates were also significantly (P<0.001) higher in supplemented than in control bulls (0.38 vs 0.31 mm/day). Bulls fed supplemented diets were younger (430 vs 473 days; P<0.001) and heavier (217 vs 203 kg; P<0.05) and had better semen quality at puberty than the control bulls. There was a trend for larger SC at puberty in supplemented than in control bulls (P=0.06). There were no significant differences in wither height or in testosterone response to GnRH at puberty between supplemented and control bulls. Body and SC growth rates to puberty for Boran and Boran x Friesian bulls were 189 and 321 g/day (P<0.001) and 0.27 and 0.42 mm/day (P<0.01), respectively. Boran bulls were older (513 vs 406 days; P<0.01) and lighter (201 vs 215 kg; P<0.05), and had smaller SC (23.9 vs 26 cm; P<0.01) at puberty than Boran x Friesian bulls. Testosterone responses to GnRH increased (P<0.001) with age and were significantly higher (P<0.05) in supplemented and in Boran x Friesian bulls than in control and Boran bulls. Wither height, testosterone response to GnRH and semen quality at puberty did not differ between breeds. Dry season supplementation improved growth rate and enhanced sexual development and maturity in both Boran and Boran x Friesian bulls, and would be a useful management strategy for enhancing fertility in bulls in the tropics.