Differences in early patterns of gonadotrophin secretion between early and late maturing bulls, and changes in semen characteristics at puberty

In prepubertal bull calves there is an early transient rise in gonadotrophin secretion between 10 and 20 wk of age, and it has been suggested that this plays a role in the attainment of sexual maturation. To test this, we looked for differences in the gonadotrophin secretory pattern from birth to puberty between early and late maturing bulls. We also characterized the changes in semen morphology that occur about the time of puberty. Blood samples were collected (n=28) every wk from 2 to 20 wk of age and then every 2 wk until 50 wk of age. Semen was collected by electroejaculation at approximately 4-wk intervals from 36 to 49 wk of age. Puberty was defined as the first age at which an ejaculate contained 50 million spermatozoa with a minimum of 10 % motility Bulls were divided into early (n = 14) and late (n = 14) maturing groups based on the age at puberty (41.9 +/- 0.3 and 48.3 +/- 0.7 wk of age, respectively). There was a transient increase in serum concentrations of LH and FSH between 2 and 24 wk of age; LH concentrations were greater in early maturing bulls than in late maturing bulls at 12, 13, 15, 17 and 48 wk of age (P < 0.05). Serum concentrations of testosterone and FSH did not differ between groups (P > 0.05). As the bulls matured there was an increase in the percentage of normal and live sperm cells, cell motility and the number of cells per ejaculate (P < 0.05), and a decrease in the percentage of proximal droplets and knobbed acrosomes (P < 0.05). We concluded that, during the early rise in LH secretion, early maturing bulls had higher circulating LH concentrations than late maturing bulls. During the weeks preceding and following puberty there was an increase in the quality of semen collected by electroejaculation.     

The relationship between secretory patterns of gonadotrophic hormones and the attainment of puberty in bull and heifer calves born early or late during the spring calving season

It was suggested that an early increase in gonadotrophin secretion in calves aged between 6 and 24 weeks might be critical for initiating developmental changes culminating in puberty. An early rise in luteinizing hormone (LH) release appears to be caused by an increase in LH pulse frequency in bull calves and by an increase in LH pulse amplitude in heifer calves. Previously we have found differences in the characteristics of the LH rise between prepubertal beef calves born in spring or fall; however, age at puberty was not affected by season of birth. Here we report the LH/FSH secretory patterns in prepubertal bull and heifer calves (Hereford x Charolais), born in March or April, respectively (i.e., early or late during the spring calving season; six animals of each sex born at each time). The bull calves of both groups reached puberty (defined as an attainment of scrotal circumference of >or=28 cm) at 43.2+/-1.3 weeks of age (P>0.05). Age at puberty for March- and April-born heifer calves (defined as the age at which serum progesterone concentrations first exceeded 0.4 ng/ml) averaged 56.0+/-1.4 weeks (P>0.05). Based on blood samples taken weekly from birth to 26 weeks of age, and then every other week until puberty, bull calves born in March exceeded April-born bull calves in mean serum LH concentrations at 6, 10 and 12 weeks of age (P<0.05). Mean FSH concentrations were greater (P<0.05) in March-born compared to April-born bull calves from 34 to 32 weeks before puberty. Mean serum LH (at 40, 42 and 56 weeks) and FSH concentrations (at 2, 10, 20, 22-26, 30 and 56 weeks of age) were greater (P<0.05) in heifer calves born in April than March. On the basis of frequent blood sampling (every 12 min for 10 h), heifer calves born in April exceeded March-born animals in mean LH and FSH concentrations, at 5 and 25 weeks, and LH pulse frequency, at 5, 10 and 25 weeks of age (P<0.05). None of the parameters of LH secretion (i.e., mean concentrations of LH, LH pulse frequency and amplitude based on frequent blood collection) differed between March- and April-born bull calves in this study (P>0.05). In summary, March-born bull calves had greater mean serum LH and FSH concentrations prior to 24 weeks of age than April-born calves. April-born heifer calves had greater mean serum concentrations of LH and FSH but this difference was not confined to the early postnatal period. Although there were significant differences in absolute amounts of LH secreted, there were no differences in the frequency of LH secretory pulses amongst March- and April-born bull calves and no differences in LH pulse amplitude in heifer calves born in March or April. As these particular parameters of LH secretion, as well as age at puberty, are not affected by the time or season of birth, they may be primary hormonal cues governing sexual development in bulls and heifers, respectively.     

Gonadotrophin secretion in prepubertal bull calves born in spring and autumn

The reproductive development of bull calves born in spring and autumn was compared. Mean serum LH concentrations in calves born in spring increased from week 4 to week 18 after birth and decreased by week 24. In bull calves born in autumn, mean LH concentrations increased from week 4 to week 8 after birth and remained steady until week 44. LH pulse amplitude was lower in bull calves born in autumn than in calves born in spring until week 24 of age (P < 0.05). There was a negative correlation between LH pulse frequency at week 12 after birth and age at puberty in bull calves, irrespective of season of birth, and LH pulse frequency at week 18 also tended to correlate negatively with age at puberty. Mean serum FSH concentrations, age at puberty, bodyweight, scrotal circumference, testes, prostate and vesicular gland dimensions, and ultrasonographic grey scale (pixel units) were not significantly different between bull calves born in autumn and spring. However, age and body-weight at puberty were more variable for bull calves born in autumn (P < 0.05). In a second study, bull calves born in spring received either a melatonin or sham implant immediately after birth and at weeks 6 and 11 after birth. Implants were removed at week 20. Mean LH concentrations, LH pulse frequency and amplitude, mean FSH concentrations and age at puberty did not differ between the two groups. No significant differences between groups in the growth and pixel units of the reproductive tract were observed by ultrasonography. In conclusion, although there were differences in the pattern of LH secretion in the prepubertal period between bull calves born in autumn and spring, the postnatal changes in gonadotrophin secretion were not disrupted by melatonin treatment in bull calves born in spring. Reproductive tract development did not differ between calves born in spring and autumn but age at puberty was more variable in bull calves born in autumn. LH pulse frequency during the early prepubertal period may be a vital factor in determining the age of bull calves at puberty.     

Effects of treatment with GnRH from 4 to 8 weeks of age on the attainment of sexual maturity in bull calves

In bull calves an early transient increase in circulating concentrations of LH occurs between 6 and 20 weeks of age. This has been shown to influence reproductive development and performance later in life. In an attempt to hasten the onset of sexual maturity, bull calves (Hereford x Charolais) were treated (im) with 120 ng/kg of GnRH (n=6) twice every day from 4 to 8 weeks of age; control calves received saline (n=6). Injection of GnRH resulted in an LH pulse in all animals. GnRH treated bulls displayed more rapid testicular growth rates between 22 and 44 weeks of age. Sexual maturity (SC>or=28 cm) was achieved earlier in GnRH treated bulls compared to saline treated bulls (41.7+/-2.22 and 47.0+/-0.45 weeks of age, respectively) and this was confirmed by age of sexual maturity based on ejaculate characteristics (>50 million spermatozoa, >10% motility; 45.0+/-0.86 and 49.0+/-1.13 weeks of age for GnRH and control treated bull calves, respectively; P<0.05). We concluded that treatment with GnRH, twice daily, from 4 to 8 weeks of age, prior to the endogenous early increase in plasma LH concentrations, could increase in plasma LH concentrations, advance testicular development and reduce age at puberty in beef bull calves. This may provide the basis for a simple regimen to hasten sexual development in the bull calf.     

Changes in serum luteinizing hormone (LH) concentrations in response to luteinizing hormone releasing hormone (LHRH) in bull calves that attained puberty early or late

The objectives of this study were to determine if the response to luteinizing hormone releasing hormone (LHRH) could be used to select bull calves capable of early sexual maturation and to establish the optimum route and dose of LHRH. In Trial 1, at 4, 10 and 20 week of age, 20 calves were treated iv with 2 microg/kg body weight of LHRH 1 and 5h after commencing a 9-h period of blood sampling. Bulls were separated into early and late maturing (n=10), based on age at puberty (scrotal circumference (SC) of >or=28 cm). At 4 and 20 week of age, peak serum LH concentrations and area under the LH response curve in response to LHRH were lower (P<0.05) in early- versus late-maturing bulls. In Trial 2, calves at 20 week of age were given LHRH as follows: 2 microg/kg body weight iv (n=6), im (n=6) or sc (n=6); 5 microg/kg im (n=6), or ischio-rectally (ir, n=6) or sc (n=6); and 10 microg/kg im (n=6) or sc (n=6). Serum LH concentrations were at a plateau from 30 to 165 min after treatment with 5 microg/kg of LHRH (im or ir; P>0.05). We concluded that the LH responses to LHRH in calves at 4 and 20 week of age could facilitate the development of a simple test (one blood sample prior to treatment with LHRH and a second during the period of sustained response to LHRH) to select early-maturing bulls.     

Effects of season of birth on the prepubertal pattern of gonadotropin secretion and age at puberty in beef heifers

There is an early transient rise in gonadotropin secretion in spring-born prepubertal heifers and there is an indication that this pattern is different in autumn-born heifers. The effect of season of birth on age and weight at puberty is equivocal. This study was designed to compare the temporal patterns of LH and FSH secretion between spring- and autumn-born heifers and to determine the effects of season of birth on age and weight at puberty. Blood samples from 2 groups of heifer calves born in spring (last week of March, n = 5) or autumn (last week of October, n = 5) were collected every other week from birth to puberty and every 15 min for 10 h at 6, 12, 18, 24 and 32 wk of age. Timing of puberty was determined by measuring progesterone in plasma samples collected every 2 to 3 d starting at 42 wk of age. Age and weight at onset of puberty did not differ between the 2 groups of heifers (P > 0.05); however, the autumn-born heifers tended to mature in a wider range of ages and weights. Based on the 10-h sampling periods, mean serum concentrations of LH and LH pulse frequency and amplitude were higher in spring-born heifers at 18 wk of age than in autumn-born heifers (P < 0.05). In spring-born heifers, LH pulse frequency increased over time to 32 wk of age, and LH pulse amplitude was higher at 12 and 18 wk than at 32 wk of age (P < 0.05). Autumn-born heifers had higher LH pulse frequency at 6 wk and showed a decrease in mean concentrations of LH at 12 and 18 wk of age (P < 0.05). The FSH pulse frequency of spring-born heifers was higher at 12 wk of age than in autumn-born heifers (P < 0.05), FSH pulse amplitude in autumn-born heifers decreased from 6 to 32 wk of age. It was concluded that although the mean age and weight at puberty did not differ between spring- and autumn-born heifers, the range in age and weight at puberty was wider in the autumn-born heifers. The patterns of LH secretion differed between spring- and autumn-born prepubertal heifers, with spring-born calves exhibiting an early rise in LH secretion, while mean serum concentrations of LH decreased during this period in autumn-born heifers.     

Influence of unilateral castration and increased plane of nutrition on sexual development of Holstein bulls. I. Growth and sperm production

Sixty-five Holstein bull calves were used to study the effects of unilateral castration (UC) and increased plane of nutrition on the growth and development of the reproductive system. Bulls were slaughtered at 1 wk., 2, 4, 8 and 16 months. Half of each slaughter group above one week was unilaterally castrated at 7 days of age. Half of the bulls remaining at 6 months of age received 90% of their recommended daily TDN allowance while the remainder received 120%. Compensatory hypertrophy was evident as early as 2 months and the degree of compensation increased for the duration of the experiment (Age x UC, P<.01). By 16 months of age the remaining testis of UC animals was 73% heavier than the average testis weight of intact bulls. While epididymal weight was significantly increased by UC, seminal vesicle weight was not. UC bulls produced significantly more sperm per testis than intact bulls both from the onset of puberty to slaughter and for the 16 week period prior to slaughter. Testis sperm concentration was similar in UC and intact bulls. UC at one weel of age caused greater testis growth and greater sperm production per testis, but did not promote earlier puberty.     

Assessment of development of the testes and accessory glands by ultrasonography in bull calves and associated endocrine changes

The testes, prostate and vesicular glands of 10 bull calves were examined by ultrasonography every 2 wk from 2 to 46 wk of age, at which time the scrotal circumference (SC) of all the calves had reached pubertal size (28 cms). Computer-assisted image intensity analysis (numerical pixel values) was conducted. Blood samples were collected every other week from 2 to 46 wk of age. Testicular diameter increased in a linear manner from 2 to 46 wk of age, but the diameter measured in a transverse plane (caudal) was greater between 10 and 34 wk of age than when measured in a longitudinal (lateral) plane (P<0.05). Growth of the prostate and vesicular glands, based on dimensions, was linear, but vesicular gland length increased more rapidly after 32 wk of age (P<0.05). Image intensity of the vesicular glands and prostate declined from birth or 8 wk of age, respectively, to 14 wk of age, increased to 18 wk and then declined to a nadir at 30 wk, followed by a rapid increase to 34 wk of age for the vesicular glands and to 46 wk of age for the prostate (P<0.05). Image intensity of the testes showed an early increase to 6 to 8 wk of age and a subsequent increase from about 20 wk of age to 46 wk of age, with an inflection at 30 wk of age (P<0.05). There was a transient increase in mean serum concentrations of LH between 6 and 20 wk of age (P<0.05), and LH concentrations appeared to increase again after 36 wk of age (P>0.05). Mean serum concentrations of FSH declined with age (P<0.05). Mean serum concentrations of testosterone increased after 32 wk of age (P<0.05) In summary, numerical pixel values comprising the ultrasound images of the developing testes, prostate and vesicular glands revealed a complex development pattern that may reflect important details of developmental stages.     

Effect of implanting bull calves with testosterone propionate, dihydrotestosterone propionate or oestradiol-17 beta prepubertally on the pituitary-testicular axis and on postpubertal social and sexual behaviour.

Twelve non-implanted crossbred bull calves served as controls and 30 crossbred bull calves (10/treatment) were implanted for 82 days, beginning at 34 days of age, to determine the influence of testosterone propionate (TP), dihydrotestosterone propionate (DHTP) and oestradiol-17 beta (E2) on prepubertal and pubertal pituitary-testicular function and on postpubertal social and sexual behaviour. Compared with control bulls, concentrations of serum luteinizing hormone (LH), follicle-stimulating hormone (FSH) and inhibin concentrations were suppressed (P less than 0.01) in all implanted bulls. Testosterone (T) concentration increased (P less than 0.001) in TP-implanted, but decreased (P less than 0.01) in DHTP and E2 bulls during the implant period. LH response to gonadotrophin-releasing hormone (GnRH) challenge during the implant period (2.5 months of age) was less (P less than 0.01) in TP, E2 and DHTP bulls than in controls. A small but significant T response to GnRH occurred in control bulls at 2.5 months of age. LH and T responses to GnRH challenge at 7 months of age (100 days after implant removal) was similar (P greater than 0.20) in control and implanted bulls. Steroid implants administered prepubertally had no effect (P greater than 0.10) on postpubertal social and sexual behaviours, including number of flehmen responses, abortive mounts, services and competitive order score. Body weight did not differ (P greater than 0.10) between treatment groups, but testis size was reduced (P less than 0.01) during the implant period and up to 10 months of age in treated bulls compared with controls. Testes remained smaller in E2-treated bulls up to the end of the study (23 months of age), but daily sperm production and epididymal weight did not differ (P greater than 0.10) between treatment groups at slaughter. Control bulls reached puberty earlier (P less than 0.01; 270 +/- 11 days of age) than did TP (302 +/- 11 days), DHTP (309 +/- 11 days) or E2 (327 +/- 11 days) bulls. Although puberty was delayed in all implant groups, there was no difference in scrotal circumference at puberty (average 28.4 +/- 0.4 cm) between treatment groups. Our findings indicate that TP, DHTP and E2 implants administered prepubertally result in acute suppression of serum LH, FSH and inhibin during the implant period and in post-implant suppression of testis size and delayed puberty in bulls. The lack of treatment effect on behaviour suggests that steroidal programming of sexual behaviour occurs before 1 month of age in bulls.     

Effects of naloxone on circulating gonadotrophin concentrations in prepubertal heifers.

The pattern and opioidergic control of the secretion of gonadotrophins in prepubertal heifer calves were examined. Ten age-matched Hereford heifer calves were weighed and a blood sample was taken every 2 weeks from 2 to 25 weeks of age and then weekly until 60 weeks of age. At 60 weeks, a fertile bull was introduced and at 75 weeks of age pregnancy diagnosis was performed by transrectal ultrasonography. At 4, 12, 18, 24 and 32 weeks of age, the opioid antagonist naloxone was injected (i.v., n = 5; 1 mg kg-1 body weight) each hour for 12 h. Control heifers received sterile saline at the same ages. Blood samples were collected every 12 min for the 12 h treatment and serum samples were analysed for luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Samples taken once every 2 weeks from 2 to 60 weeks were analysed for LH, FSH and oestradiol, and weekly samples were taken for progesterone determination. There was no effect of naloxone on the age at puberty, which was 56.2 +/- 0.7 weeks at a body weight of 388.5 +/- 8.0 kg. The mean age at conception was 63.4 +/- 0.5 weeks. On the basis of samples taken every other week, serum concentrations of LH were high at 10 weeks and between 40 and 60 weeks of age. From the periods of intensive blood collection, the early rise in mean serum concentrations of LH appeared later at 12 and 18 weeks of age and was caused by a rise in LH pulse amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)     

Age-related changes in secretion of luteinizing hormone and metabolism of hypothalamic amines in bull calves prior to puberty

Effects of age and castration on secretion of luteinizing hormone (LH) and metabolism of hypothalamic monoamines were determined in Holstein bulls. Calves were assigned to be intact or castrated and killed at 8, 12, or 24 wk of age. Animals were castrated and bled every 10 min for 6 h at 96 and 24 h prior to slaughter, respectively. The stalk median eminence (SME), medial basal (MBH), and anterior-preoptic (AHA-POA) hypothalamic regions were obtained at slaughter and assayed for norepinephrine (NE), dopamine (DA), dihydroxy-phenylacetic acid (DOPAC), homovanillic acid (HVA), serotonin (5-HT), and 5-hydroxyindole-acetic acid (5-HIAA) using high performance liquid chromatography with electrochemical detection (HPLC-EC). Concentrations of LH and testosterone in plasma were determined by radioimmunoassay (RIA). In intact calves, LH pulse frequency (pulses/6 h) increased between 8 and 12 wk (1.4 vs. 3.4) and then declined (1.6 at 24 wk of age). Frequency of LH discharges did not change during the first 72 h post-castration in calves 8 (1.4 vs 1.0) and 12 (3.4 vs. 3.8) wk of age, but increased in 24-wk-old calves during this time (1.6 vs. 6.4). The amplitude of LH pulses increased with age (p less than 0.05) and after castration (p less than 0.05). There were marked regional differences in concentrations of monoamines. However, effects of age and castration on concentrations of monoamines were observed only within the SME where DA, DOPAC and NE increased significantly with age. Plasma concentrations of testosterone were correlated with concentrations of NE and DOPAC within the SME. Changes in 5-HT with age were biphasic; at each age, 5-HT increased after castration. From these data, it is concluded that 1) different mechanisms regulate LH pulse frequency and amplitude in calves as early as 8 wk of age, and 2) differences in hypothalamic metabolism of monoamines may be related to maturational changes in secretion of LH in bull calves.     

The effect of nutrition on sexual development of bulls

Most bulls that are managed for sale as yearlings are fed high-energy diets in the post-weaning period to maximize rates of gain in body weight. High-energy diets with adequate protein, vitamins and minerals result in a larger scrotal circumference at 1 y of age, however, part of this increase in size is likely due to scrotal fat. It is unclear whether testis size and spermatogenesis is significantly affected by nutritional intake in the post-weaning period. There are indications of an effect of calfhood nutrition on age at puberty and testis size. Scrotal circumference was smaller in yearling bulls raised by first-parity dams, compared to those raised by older dams. This may have been due to lower milk production by first-parity dams, an in utero effect, or both. The effect of reduced calfhood nutrition may be mediated through gonadotropin secretion. Calves destined to become later maturing bulls with smaller testes had lower amounts of LH secretion during the period of the early gonadotropin rise (8-16 wk of age). Furthermore, augmenting circulating LH concentrations at this time by treating calves with GnRH hastened pubertal development. In addition, FSH treatments in calfhood also increased scrotal circumference and hastened spermatogenesis. In that regard, FSH has been considered a main driver of Sertoli cell proliferation in prepubertal animals. Since Sertoli cell multiplication ceases at 20-25 wk of age in bulls, final testis size in bulls is likely determined in calfhood. Four experiments were done to investigate the effects of calfhood nutrition on pubertal development. These studies confirmed that superior calfhood nutrition augmented gonadotropin secretion (which is probably mediated by metabolic hormones); this resulted in larger testes at 1 y of age and an earlier onset of spermatogenesis.     

Treating heifers with GnRH from 4 to 8 weeks of age advanced growth and the age at puberty

In heifer calves, an early transient increase in circulating concentrations of LH is associated with early follicular development and is thought to regulate the timing of puberty. In an attempt to hasten the onset of sexual maturity, the early rise in LH concentration was advanced by injecting heifer calves with 120 ng/kg of GnRH (n=6) twice daily from 4 to 8 weeks of age; control calves received saline (n=6). Blood samples were collected every 15 min for 10h at 4, 8, 14, 20, 26, 32, 38, 44 and 50 weeks of age. Treatment with GnRH increased mean circulating concentrations of LH at 8 weeks of age (P<0.05), LH pulse frequency at 4 and 8 weeks of age (P<0.05), and reduced the mean age at puberty by 6 weeks (56.8+/-1.7 versus 62.8+/-2.4 weeks of age, for GnRH treated and control calves, respectively; P=0.04). Body weight gain was greater in GnRH-treated calves than control calves (P<0.05), and the rate of weight gain was shown to be a significant covariate within age at puberty. In conclusion, we suggest that the timing of the early rise in LH concentrations is a critical signal involved in the timing of puberty in heifers.     

Production and endocrine role of inhibin during the early development of bull calves.

This study investigated the ontogeny of control of FSH secretion by inhibin during early prepubertal development of bulls by 1) measurements of circulating levels of inhibin and FSH from 1 to 13 wk of age, and 2) immunoneutralization of endogenous inhibin at 7, 21, 60, and 120 days of age. In addition, production and localization of inhibin in testes were examined by immunohistochemistry and Western blots at 7, 21, 60, and 120 days of age. Plasma immunoreactive inhibin levels were relatively low between 1 and 3 wk of age and then showed a tendency to rise (P < 0.1) from 4 wk of age. Circulating concentrations of FSH were low during 3 wk after birth and increased at 5 wk, remained high (P < 0.05) until 16 wk of age. Treatment with inhibin antiserum resulted in a significant (P < 0.05) increase in plasma FSH at 7, 21, 60, and 120 days of age compared to those following injection of control serum; however, the magnitude of the FSH rise after inhibin immunization was greater as bulls aged. There were no significant changes in plasma LH after inhibin immunization. An intense staining of inhibin alpha subunits was found in Sertoli cells within the solid seminiferous cords from 7 to 120 days of age, while no specific immune reaction was found in interstitial cells. Western blot analysis of testicular homogenates isolated from bulls 7-120 days of age revealed presence of a 28.5-kDa molecule that cross-reacted with inhibin alpha subunit and beta(B) subunit-specific antibodies. In this study, before 13 wk of age in bull calves, there was no inverse relationship between plasma concentrations of immunoreactive inhibin and FSH. However, the present immunization study clearly indicates that inhibin participates in the regulation of FSH secretion from infancy to early prepubertal stage, although the endocrine significance of inhibin becomes greater in older bulls. The results also indicate that the major production site of inhibin in the testis is Sertoli cells and that these cells produce inhibin that exerts a negative feedback effect on FSH secretion from early stages of development.     

Effect of long-term immunization against inhibin on sperm output in bulls.

To determine the effect of neutralization of inhibin on sperm output, 12 Holstein bulls were paired by birth date and weight on Day 1 of age. Each bull was actively immunized against bovine inhibin alpha1-26 gly-tyr (bINH) conjugated to human alpha globulin (HAG, n = 6 bulls) or HAG alone (controls, n = 6) at 60 days of age; booster immunizations were administered at 90, 104, 124, 270, and 395 days of age. Body weights and scrotal circumferences were measured at the time of primary immunization and at 10 days after each booster. In addition, jugular blood was obtained at 60, 70, 100, 114, 134, 280, and 405 days of age, during the 3-wk sperm collection period, and during a 6-h blood-sampling period after sperm collection to determine bINH antibody titer and concentrations of FSH, LH, testosterone, and estradiol. Beginning at 405 days of age, sperm output was measured 3 days/wk for 3 wk with two successive ejaculates collected each day for a total of 18 ejaculates per bull. During Days 60-405 of age, the increase in titer of bINH antibodies, scrotal circumference, and serum concentration of FSH was greater (p < 0.01) for the bINH-immunized compared with control bulls. There were significant (p < 0.01) pair x treatment interactions for sperm output and serum FSH and LH concentrations. Specifically, bINH-immunized bulls for four of the six pairs had nearly 50% greater serum FSH concentrations and sperm output. For the remaining two pairs, sperm output was lower and FSH was either lower or only marginally higher in the bINH-immunized bulls compared with controls. Also, the control bulls for the two remaining pairs produced more sperm than all but one bINH-immunized bull, and had markedly higher serum LH concentrations than all other bulls. To summarize, enhancement of sperm output after immunization against inhibin depends on the subsequent increment in FSH concentrations. We conclude that inhibin suppresses spermatogenesis. Thus, methods to immunoneutralize inhibin may have merit as a therapeutic route to enhance sperm production in reproductively maturing bulls.     

Changes in circulating hormone concentrations, testes histology and testes ultrasonography during sexual maturation in beef bulls

Nine groups of bull calves (n = 5 to 6 per group) were castrated every 5 wk from 5 to 45 wk of age, and the stages of spermatogenesis were identified histologically. Prior to castration, the testes of each calf were examined by ultrasonography, and the pixel intensities of the parenchyma were quantitated. Testis ultrasonograms were also recorded every 2 wk from 10 bull calves between 2 and 40 wk of age. Blood samples were collected at weekly intervals until castration. There was an early transient rise in circulating LH concentrations between 4 and 25 wk of age, while circulating FSH concentrations were high initially but decreased between 14 and 30 wk of age. Circulating testosterone concentrations increased gradually from 6 to 35 wk of age and then rapidly to 42 wk of age. There was a progressive increase in the more mature cell types during spermatogenesis as the animals aged, with the most dramatic changes occurring between 15 and 45 wk of age. Outer seminiferous tubule diameter increased between 10 and 45 wk of age, with the most rapid increase occurring from 30 wk of age. Inner tubule diameter increased between 30 and 35 wk of age. The echogenicity of the testes (as determined by ultrasonography) increased between 20 and 40 wk of age. From these data we conclude that testis echogenicity increased during the most active phase of growth of the seminiferous tubules as more mature germ cells were produced. Cessation of the early rise in gonadotrophin secretion immediately preceded this active phase of testicular development. Testosterone secretion rose markedly with the production of mature spermatozoa.     

Influence of season of birth on growth and reproductive development of Brahman bulls

Seasonal effects on reproduction are more dramatic in Bos indicus than Bos taurus cattle. This experiment evaluated reproductive development of fall- (n=7) versus spring- (n = 10) born Brahman bulls to determine if season of birth affects reproductive development. Measurements of growth and reproductive development began after weaning and continued at bi-weekly intervals until each bull reached sexual maturity. Different stages of sexual development were classified according to characteristics of the ejaculate and included first sperm in the ejaculate, puberty (> 50 x 10(6) sperm/ejaculate), and sexual maturity (two ejaculates with > 500 = 10(6) sperm/ejaculate). Average daily increases in all measured traits were similar in fall- and spring-born bulls and there were no differences in age, body weight, scrotal circumference, or paired testis volume between groups at first sperm or puberty. However, fall-born bulls were older (P < 0.05) than spring-born bulls at sexual maturity (553 days versus 481 days, respectively) as the interval between puberty and sexual maturity was longer (P < 0.05) in fall- than in spring-born bulls (82 days versus 54 days, respectively). The prolonged interval between puberty and sexual maturity in fall-born calves coincided with a short photoperiod (winter) whereas the short interval between puberty and sexual maturity in spring-born calves coincided with a long photoperiod (summer). In conclusion, season of birth affected sexual development; photoperiod might be involved in regulating testicular function immediately after puberty in Brahman bulls.     

Depression of follicle stimulating hormone in bulls injected with follicular fluid

Eight bulls were divided into two groups and injected with either charcoal-extracted steer blood serum or charcoal-extracted bovine follicular fluid (bFF). Ten-milliliter injections were given subcutaneous every 12 h for 4 wk. Jugular blood collected before, during and after the injection period was analyzed for follicle-stimulating hormone (FSH) and luteinizing hormone (LH) by radioimmunoassay. All bulls were exposed to restrained, estrual heifers for 15 min every 2 wk for 16 wk starting 4 wk before the first injection. The number of mounts and services by each bull was recorded. Semen was collected with an artificial vagina and evaluated on alternate weeks during the same period. The concentration of FSH in serum decreased (P < 0.05) by 12 h after the first injection and remained 61% lower than that of serum-injected bulls during the injection period. The concentration of FSH increased (P < 0.05) by 3 d after the last injection. Injections of bFF did not affect the concentration of LH in serum. Bovine follicular fluid injections significantly depressed FSH; however, libido, serving capacity, and semen characteristics were unchanged.     

Ultrasonography of the developing reproductive tract in ram lambs: effects of a GnRH agonist

In spring-born ram lambs, the testes (from 2 wk), prostate and vesicular glands (from 4 wk) were examined by ultrasonography every 2 wk up to 26 wk of age. Image analysis was done (numerical pixel values). Ram lambs were treated with a long acting formulation of a GnRH superagonist (Leuprolide acetate; 1.5 mg/kg) at 3 and 7 wk of age. In blood samples taken every 15 min for 8 h, mean serum LH, LH pulse amplitude, and basal and mean serum FSH concentrations were lower at 5 wk of age, and LH pulse frequency was lower at 15 wk of age in animals given Leuprolide acetate compared with those of the controls. There were no differences (P>0.05) in testis, prostate or vesicular gland development between treated and control animals. Testicular diameter of the left and right testes in transverse and longitudinal planes increased slowly to 8 wk of age, more rapidly to 18 wk of age, then more slowly to 26 wk of age (P<0.05). Numerical pixel values of testicular images decreased from 2 to 8 wk of age, increased to 22 wk of age and then plateaued. Width of the prostate increased from 4 to 26 wk of age, but length and width of the vesicular glands increased slowly to 8 wk of age, more rapidly to 18 wk of age and then plateaued (P<0.05). Numerical pixel values for the prostate declined from 4 to 8 wk and for the vesicular glands, declined from 4 to 10 wk of age; numerical pixel values increased to 12 wk and then decreased to a nadir at 18 wk, followed by a steady increase to 26 wk of age (P<0.05). We concluded that developmental patterns of numerical pixel values of the testes, prostate and vesicular glands in ram lambs reflect stages of development, but treatment with a GnRH superagonist at 3 and 7 weeks of age did not affect growth of testes, vesicular or prostate glands.     

Understanding and evaluating bovine testes

The objective is to briefly review bovine testes and how they are assessed, with an emphasis on articles from Theriogenology. Scrotal circumference (SC) is the most common method to assess testicular size; it varies among individual bulls and breeds and is highly heritable. In general, a large SC is associated with early puberty, more sperm, a higher percentage of morphologically normal sperm, and better reproductive performance in closely related females. Consequently, there are minimum requirements for SC for breeding soundness. In prepubertal bull calves, there is an early rise (10–20 weeks of age) in LH, which is critically related to onset of puberty and testicular development. Feeding bulls approximately 130% of maintenance requirements of energy and protein from approximately 8 to 30 weeks of age increased LH release during the early rise, hastened puberty (approximately 1 month), and increased mature testis size and sperm production (approximately 20%–30%). However, high-energy diets after weaning (>200 days) often reduced sperm production and semen quality. A bull's testes and scrotum have opposing (complementary) temperature gradients, which keep the testicular temperature 2 °C to 6 °C cooler than core body temperature for production of fertile sperm (increased testicular temperature reduces semen quality). Infrared thermography, a quick and noninvasive method of assessing scrotal surface temperature, may be beneficial for evaluations of breeding soundness. The primary clinical use of ultrasonography in assessment of reproductive function in the bull is characterization of grossly detectable lesions in the testes and scrotum. In conclusion, testis size and function are critical for bull fertility, affected by nutrition, and readily assessed clinically.