Patterns of puberal development in Sahiwal and Brahman cross bulls in tropical Australia. I. Growth and semen characteristics

Reproductive development was monitored in 13 1 2 Brahman (BX), 20 3 4 BX, 8 1 2 Sahiwal (SX) and 11 3 4 SX crossbred bulls between one and two years of age. Liveweight increased during wet season (0.7 kg/day) and decreased during dry season conditions (-0.25 kg/day). Scrotal circumference (SC) increased from 18.6 to 25.0 cm, but decreased slightly during the later stages of the dry season. First ejaculates containing 50 x 10(6) spermatozoa were obtained from 25% of the bulls at 15.3 months of age and from 84% by two years of age. Sperm motility and morphology showed no consistent pattern, but number of sperm per ejaculate increased and seminal fructose concentrations decreased with advancing age. Significant genotype differences were noted for SC and seminal characteristics. Estimated age and liveweight at puberty were 570, 563, 578 and 542 days, and 251, 265, 247 and 256 kg for 1 2 BX, 3 4 BX, 1 2 SX and 3 4 SX bulls, respectively.     

Growth and reproductive development from weaning through 20 months of age among breeds of bulls in subtropical Florida

To determine the effect of breed on growth and reproductive development, weaned bulls in each of 2 yr were managed as a single group for approximately a year. In Year 1, the study group consisted of 24 Angus, 24 Brahman, 20 Hereford and 14 Senepol bulls, while in Year 2, it contained 25 Angus, 17 Brahman. 13 Romosinuano and 9 Nellore x Brahman bulls. Body and testicular growth measurements were recorded at 6-wk intervals. At approximately 1 yr of age and quarterly thereafter (4 periods), bulls were evaluated for libido, pubertal status, and GnRH-induced LH and testosterone secretion. Significant breed-by-age interactions occurred for most growth measurements. Brahman bulls (Bos indicus ) were (P < 0.05) older and heavier at puberty than Angus, Hereford, Senepol and Romosinuano bulls (Bos taurus ). Libido scores were lowest for Brahman and Nell ore x Brahman bulls (Bos indicus ). highest for Angus and Hereford bulls (temperate Bos taurus breeds) and intermediate for Senepol and Romosinuano bulls (tropical Bos taurus breeds; P < 0.05). Differences were not consistent among breeds or between years for GnRH-induced LH secretion. In both years, basal testosterone concentrations and areas under the GnRH-induced testosterone curve were higher (P < 0.05) for Angus and Hereford bulls (temperate breeds) than for Brahman, Senepol, Romosinuano and Nellore x Brahman bulls (tropical breeds). In conclusion, reproductive development of Senepol and Romosinuano bulls (tropical Bos taurus breeds) was more similar to Angus and Hereford bulls (temperate Bos taurus breeds) than to Brahman and Nellore x Brahman bulls (Bos indicus ).     

Plasma luteinizing hormone and testosterone concentrations in different breeds of young beef bulls in the tropics

Plasma concentrations of luteinizing hormone (LH) and testosterone were measured at 3, 8, and 11 months of age in 48 Africander cross (AX), 24 Brahman cross (BX), 21 Hereford-Shorthorn, selected (HSS) and 14 Hereford-Shorthorn, random-bred (HSR) bulls. In all breeds plasma LH was lower (P less than 0.01) at 8 months (1.7 ng/ml) than at 3 months (2.6 ng/ml) or at 11 months (2.6 ng/ml). Over all ages there were no differences among breeds in mean plasma LH (AX 2.4, BX 2.4, HSS 1.8, HSR 2.2 ng/ml) and no breed X age interactions. In contrast, plasma testosterone increased significantly (P less than 0.01) with age at a faster rate in the AX breed, resulting in a significant (P less than 0.05) breed X age interaction. Testosterone concentrations, though similar among breeds at 3 months of age (0.45 ng/ml), were much higher (P less than 0.01) by 11 months in AX (2.56 ng/ml) than in BX (1.30 ng/ml), HSS (0.78 ng/ml) or HSR (0.66 ng/ml) bulls. Although LH did not differ among the breeds studied, the more pronounced increase in testosterone with age in the Africander cross bulls is consistent with the higher level of fertility commonly observed in this breed when compared to Brahman cross and Hereford-Shorthorn breeds during natural mating in Queensland.     

Temporal relationships among peripheral blood concentrations of corticosteroids, luteinizing hormone and testosterone in bulls

Interrelationships among peripheral blood concentrations of corticosteroids (CS), luteinizing hormone (LH) and testosterone (T) were evaluated over a 24-hr period in four Angus bulls (18 months of age and 450 kg in body weight). Concentrations of LH and T were determined by radioimmunoassay and concentrations of CS by competitive protein binding assay of blood samples collected via jugular cannula at hourly intervals for 24 consecutive hr. A positive temporal relationship was observed between LH and T as significant positive correlations were obtained between concentrations of LH at one hour and concentrations of T at the subsequent hour in 3 of 4 bulls. Although LH peaks preceded T peaks by 1 hr, variation in this temporal relationship was observed as LH peaks occurred which were not accompanied by T peaks in some bulls. LH peaks were usually preceded by basal or declining concentrations of CS and prolonged elevations in concentrations of CS were often coincident with basal concentrations of LH and T. Negative correlations were obtained between concentrations of CS at one hour and concentrations of LH and T at the subsequent hour. These data describe the positive regulatory role of LH in testicular T production in the bull and suggest that alterations in endogenous concentrations of CS may influence peripheral concentrations of LH and T in the bull.     

Cortisol and prolactin concentrations during three different seasons in relocated Brahman and Hereford bulls

The objective of this study was to evaluate seasonal changes of cortisol and prolactin (PRL) concentrations in Brahman and Hereford bulls moved to locations that differ in geographical and environmental conditions. Postpubertal Hereford bulls from Montana (n = 15) and Nebraska (n = 15) and Brahman bulls from Texas (n = 18) were located in or relocated to Montana, Nebraska or Texas so that each location had 5 Montana Herefords, 5 Nebraska Herefords and 6 Texas Brahman bulls. Blood samples were collected at 20-minute intervals for 8 hours in November (Fall 1), April (Spring) and November (Fall 2) of the next year. These dates corresponded to 6, 12 and 18 months, respectively, after relocation in May of the first year. Cortisol concentrations were higher (P<0.05) in Fall 1 than in Fall 2 and were higher (P<0.05) for bulls in Montana than for bulls in Texas. The decrease in cortisol concentrations from Fall 1 to Fall 2 was negatively related (P<0.05) to age and weight. There was a three-way interaction (P<0.05) of breed-type origin, location and season for PRL concentrations. Seasonal patterns of PRL concentrations differed between relocated Texas Brahman and Hereford bulls, and patterns for relocated bulls differed from those of the nonrelocated bulls. Seasonal patterns of PRL were influenced to a greater extent by relocation in Texas Brahman bulls than in Hereford bulls.     

Circadian variations of plasma LH and testosterone in adult swamp buffalo bulls

Three swamp buffalo bulls aged 1.5, 1.10 and 2 years were submitted to frequent blood sampling every 15 m during a period of 25 h using an indwelling infusion set. Plasma LH and testosterone were quantified by radioimmunoassay technique. The levels of the two hormones in each individual exhibited episodic and nonrhythmic patterns. The number of LH peaks varied according to individval, ranging from no peak in one bull to 2 in the other two bulls. The mean LH concentrations during the period of study for each bull were 0.74, 0.33 and 1.17 ng/ml. Whereas the number of testosterone peaks varied between 1-10 and the average testosterone concentrations were 0.1, 0.33 and 0.55 ng/ml for the younger to the older bulls respectively. The testosterone peaks related to the LH peaks in each individual bull.     

The effect of dexamethasone (DXM) on circulating testosterone (T) and luteinizing hormone (LH) in young postpubertal bulls

Testosterone (T) and luteinizing hormone (LH) in the peripheral plasma of 6 young postpubertal (52 weeks of age) bulls were measured by radioimmunoassay. For 3 bulls blood samples were collected at half-hour intervals for 6 hours one day before dexamethasone (DXM) injection (20 mg) and two days after. For the 3 others blood collection occurred two days before injection and two days after. On the days before treatment, T and LH concentrations fluctuated similarly to what was previously observed. After treatment LH decreased rapidly and remained between 0.25 and 1.0 ng/ml until the end of the experiment. We observed a small peak of T (between 1.9 and 6.1 ng/ml depending on the animal) immediately after DXM injection; this peak was followed by a decrease to low values (0.25 to 0.5 ng/ml) as soon as 4 hours after injection. It is concluded that DXM suppresses the testosterone secretion. Since we observed a large decrease of LH, we postulate that DXM lowers LH release and therefore indirectly lowers the T synthesis and/or release.     

Bull selection and use in northern Australia. 5. Social behaviour and management

Calf output of bulls was derived using DNA typing for paternity following multiple-sire mating at two sites in northern Australia. At Swan's Lagoon Beef Cattle Research Station, 12, mixed-age, Brahman cross bulls were continuously mated with an average of 325 females in a 22km2 open-savannah paddock. Water was available in two troughs. Behaviour of the bulls and location of cows were monitored. At Kamilaroi Station, 2- to 2.5-year-old Brahman bulls were introduced to the study. Twenty-four bulls (HIGH%) were mated in an 84km2 paddock for 3.5 months to 411 heifers in 1995/1995 and for 4.5 months to 350 heifers and 320 first-lactation cows in 1995/1996. A second group of 10 bulls (LOW%) selected on reproductive soundness was mated concurrently in a neighbouring 60km2 paddock to 411 heifers in 1995/1995 and to 350 heifers and 298 first-lactation cows in 1995/1996. In each paddock in both years, 300-350 females were expected to cycle during mating. Both paddocks were flat and semi-forested and water was available only at troughs. At both sites, detailed physical and reproductive examinations of all bulls were conducted prior to and post-mating.Calf output of individual bulls was highly variable but repeatable (r=0.6-0.7) between years. Up to 90% of the 270-380 calves resulting from each mating were sired by between 6 and 8 bulls. Reducing from 3.7 to 2.8% bulls:females at Swan's Lagoon did not delay conceptions. At Kamilaroi, reproductively sound bulls achieved an estimated 5-6 conceptions per week over the peak mating period when sufficient cycling females were available. Differences in pregnancy rates between paddocks appeared due to differences in nutrition and it appeared that conceptions were not delayed with LOW% vs. HIGH% bulls. Variance between bulls in calf output was substantially lower when fewer bulls were used. Bull attrition occurred each year in the HIGH% paddock but not in the LOW% paddock. Calf output was unrelated to body condition of bulls.Seven of the 12 bulls in one 2-year period at Swan's Lagoon appeared to restrict their movement range to 250-750ha for 90-100% of the time. These ranges expanded when the bull:female ratio was reduced. Only one of the nine bulls remained within a 500ha home range for at least 85% of the time during peak mating in 1998 at 2.8% bulls:females. In previous years with 3.7% bulls:females, up to eight of the 12 bulls had remained within a 500ha home range for 85% of the time. Bull behaviour related to high calf output included restricted movement range, grazing with females at a majority of observations, stable social behaviour, and social dominance.These observations demonstrate that multiple-sire mating of reproductively sound Brahman and Brahman-derived bulls at 2.5% of cycling females will not jeopardise herd fertility under extensive management in northern Australia.     

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.     

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.     

Testicular development, histology, and hormone profiles in three yearling angus bulls with spermatogenic arrest

This article discusses the interactions between testis criteria and hormone profiles in Angus bulls with spermatogenic arrest. From 2 to 12 months (mo), testis diameter and hormone concentrations (basal and GnRH-stimulated) were evaluated in 27 bulls. At 12 mo, testes were excised. The z statistical test was used to determine whether parameters in three infertile bulls were different (P < 0.05) from those in 24 bulls with normal spermatouenesis. Bull 1 had Sertoli cell-only syndrome and Bull 2 had 90% of the tubules without germ cells and only A1 spermatogonia in the remaining. In Bull 3, germ cells did not advance beyond the primary spermatocyte stage. At 12 mo, testes of Bull 1 (99 g), Bull 2 (105 g) and Bull 3 (32 g) weighed less than those of normal bulls (251.5 +/- 56 g). Sertoli cell numbers/testis in Bull 1 (3.8 x 10(9)) and Bull 2 (4.3 x 10(9)) were not different from those in normal bulls (4.9 +/- 0.3 x 10(9)), but were reduced in Bull 3 (1.6 x 10(9)). The number of Leydig cells per gram of testis parenchyma was higher in Bull 1 (5.4 x 10(7)), Bull 2 (7.3 x 10(7)) and Bull 3 (19 x 10(7)) than in normal bulls (3.6 +/- 0.2 x 10(7)). In Bulls 1 and 2, basal and GnRH-stimulated LH, FSH, testosterone (T), androstenedione (delta4A) and estradiol 17-beta (E2) were within normal ranges at most ages. However, basal FSH and LH were greater in Bull 3 than in normal bulls, probably the causes for higher Leydig cell density. Also in the same animal, GnRH induced lower responses in LH and FSH, consequence of low basal T and E2 at some ages. Basal and GnRH-stimulated delta4A in Bull 3 were greater than in normal bulls after 6 mo, indicating impairment of Leydig cell differentiation. Deficiency in hormone secretion did not appear to be the cause of infertility, which points toward impaired gonadal responses or secretion of intratesticular factors, or genetic defects. Moreover, infertile animals may not always show pronounced changes in hormone secretion, but evaluation of testis growth around puberty can help identify those animals that do not have proper gonadal development.     

Plasma insulin-like peptide 3 concentrations are acutely regulated by luteinizing hormone in pubertal Japanese Black beef bulls

Insulin-like peptide 3 (INSL3) is a major secretory product of testicular Leydig cells. The mechanism of acute regulation of INSL3 secretion is still unknown. The present study was undertaken in pubertal beef bulls to (1) determine the temporal relationship of pulsatile secretion among LH, INSL3, and testosterone and (2) monitor acute regulation of INSL3 secretion by LH using GnRH analogue and hCG. Blood samples were collected from Japanese Black beef bulls (N = 6) at 15-minute intervals for 8 hours. Moreover, blood samples were collected at −0.5, 0, 1, 2, 3, 4, 5, and 6 hours after GnRH treatment and −0.5, 0, 2, 4, and 8 hours on the day of treatment (Day 0), and Days 1, 2, 4, 8, and 12 after hCG treatment. Concentrations of LH, INSL3, and testosterone determined by EIAs indicated that secretion in the general circulation was pulsatile. The frequency of LH, INSL3, and testosterone pulses was 4.7 ± 0.9, 3.8 ± 0.2, and 1.0 ± 0.0, respectively, during the 8-hour period. Seventy percent of these INSL3 pulses peaked within 1 hour after a peak of an LH pulse had occurred. The mean increase (peak per basal concentration) of testosterone pulses was higher (P < 0.001) than that of INSL3 pulses. After GnRH treatment, LH concentrations increased (P < 0.01) dramatically 1 hour after treatment and remained high (P < 0.05) until the end of sampling, whereas an elevated (P < 0.05) INSL3 concentration occurred at 1, 2, 5, and 6 hours after treatment. Testosterone concentrations increased (P < 0.01) 1 hour after the treatment and remained high until the end of sampling. After hCG treatment, an increase of INSL3 concentration occurred at 2 and 4 hours, and Days 2, 4, and 8 after treatment (P < 0.05), whereas in case of testosterone, concentrations remained high (P < 0.01) until Day 8 after treatment. The increase (maximum per pretreatment concentration) of INSL3 concentrations after injecting GnRH or hCG was much lower (P < 0.001) than that of testosterone. In conclusion, secretion of INSL3 in blood of bulls occurred in a pulsatile manner. We inferred an acute regulation of INSL3 by LH in bulls because INSL3 concentrations increased immediately after endogenous and exogenous LH stimulation. The increase of INSL3 concentrations by LH was much lower than that of testosterone in bulls.     

Bull selection and use in northern Australia. 1. Physical traits

A systematic breeding soundness examination, including assessment of serving capacity was conducted on 5/8 Brahman, Brahman and Belmont Red bulls, most aged 2-4 years, at 12 different cattle properties across northern Australia. A subset of bulls (n=235) were subsequently mated in various groups, as multiple-sires, to cows and heifers at bull:female ratios of 2.5-6%. The number of calves sired by individual bulls (calf output) was determined by DNA typing for paternity. Overall, the incidence of physical abnormalities that were judged likely to result in reduced reproductive performance was low (5.6-12% of bulls). Measures of key physical traits scrotal circumference, sheath depth (vertical distance from ventral abdominal wall to preputial orifice), and sheath score were moderately to highly repeatable. Except in Belmont Red bulls, liveweight was positively correlated with scrotal circumference (r=0.36-0.78; P<0.01), and both traits increased with age. In 2-year-old Brahman bulls only, percent normal spermatozoa was correlated with scrotal circumference (r=0.34; P<0.05). However, 12 and 15% of Santa Gertrudis and Brahman bulls, respectively, with greater than or equal to the recommended threshold values for scrotal circumferences, had less than 50% morphologically normal spermatozoa. Size and conformation of the umbilicus was associated with conformation of the sheath, and influenced mating ability in 2-year-old Brahman and 3-year-old Santa Gertrudis bulls. In 2-year-old Brahman bulls umbilical cord thickness was positively related (r=0.36; P<0.05) to sheath depth, and negatively related (r=-0.65; P<0.05) to number of mounts and serves in a serving capacity test. In 3-year-old Santa Gertrudis bulls navel (external umbilical scar) score was negatively related to the number of serves (r=-0.53; P<0.01) in a serving capacity test. None of the physical traits measured were consistently included in the final regression model for calf output for each breed.     

Pattern of gonadotropin secretion and ultrasonographic evaluation of developmental changes in the testis of early and late maturing bull calves

This was a study that retrospectively analyzed serum gonadotropin secretion and the ultrasonographic appearance of the testis during development in prepubertal bull calves to determine whether there were differences between early and late maturing bulls. Blood samples were taken every other week from 2 wk of age until puberty. Samples were also taken at 12 minute intervals for 12 hours at 4, 10, 20, 25, 30, 35, 40 and 45 wk of age. The GnRH treatment was administered 10 hours after the start of each period of frequent blood sampling. Bull calves fell into two distinctive groups, with one group maturing between 36.6 and 44.2 wk (n = 12) and the other between 46.4 and 48.9 wk of age (n = 8). In samples taken every other week mean serum LH concentrations were greater in early maturing bulls than in late maturing bulls at 12, 14 and 16 wk of age (P<0.05). In blood samples taken every 12 minutes for 10 hours early maturing bull calves had higher mean serum LH concentrations at 4 and 10 wk of age (P<0.05) and higher LH pulse frequency at 10 and 20 wk of age (P<0.05). Mean serum LH concentrations at 4, 10 and 40 wk of age and LH pulse frequency at 10 and 20 wk of age were negatively correlated with age at puberty in bull calves. Mean pixel units of the right and left testis were higher from 34 to 40 wk of age in early maturing than in late maturing animals (P<0.05). It seems possible that hormone measurements and ultrasonographic characteristics of the testes could be developed into powerful tools for studies on the regulation of reproductive development and may aid in the prediction of reproductive potential.     

The relationship between 13,14-dihydro-15-keto PGF2 alpha and LH secretion in bulls

Half-life (t1/2), volume of distribution (Vd) and total body clearance (TBC) of 13,14-dihydro-15-keto PGF2 alpha (PGFM) were measured in order to determine optimal sampling frequency for accurate measurement of PGFM. Three yearling Holstein bulls (349.2 +/- 6.7 kg) and 3 yearling Holstein steers (346.7 +/- 7.0 kg) were utilized in a 3 X 3 Latin square design. Animals were given 0, 25 or 50 micrograms PGF2 alpha I.V.; blood samples collected every 2 min and plasma PGFM determined. The t1/2, Vd and TBC of PGFM were 2.3 +/- .2 min, 43.3 +/- 3.3 liters and 13.7 +/- 1.9 liters/min, respectively and were similar for 25 and 50 micrograms doses. To determine the relationship between endogenous PGFM and LH secretion in bulls, blood samples were collected every 2 min for 12 h in 4 yearling Angus bulls (489.1 +/- 11.6 kg). All animals elicited at least one LH surge and PGFM concentrations were measured in samples coincident with the LH surge. Mean plasma PGFM concentrations were greater prior to the LH surge than during the LH surge. In addition, mean plasma PGFM concentration and frequency of PGFM peaks appeared to increase prior to the LH surge suggesting an association between PGFM and pulsatile LH secretion in the bull.     

Patterns of development of gonads, sex-drive and hormonal responses in tropical beef bulls

The development of different traits was studied in tropical beef bulls of seven genotypes (Brahman, Africander, British and combinations of these) from approximately 500 to 910 d of age. Bulls were raised under pasture conditions without supplementation. At each examination, approximately 2 mo apart, bulls were weighed, palpated (including scrotal and testicular measurement), electroejaculated, and subjected to two libido tests with estrus-induced females. At alternate examinations, plasma luteinizing hormone (LH) was measured at 30 and 150 min post gonadotrophin releasing hormone (GnRH) injection (LH - 30 and LH - 150) and testosterone (T) was measured at 150 min (T - 150). In general, nutritional and environmental stressors appeared to impede bull reproductive development. Scrotal circumference increased nonlinearly, apparently influenced by puberty and average daily gain (ADG). Libido increased overall, albeit nonlinearly also. No apparent marked differences in development of either trait could be attributed to genotype differences, although Brahman bulls tended to display lower sexual interest. The LH-30 level was relatively high (>14 ng/ml) at 500 and 640 d of age, but then dropped markedly at 760 d followed by a slight recovery. The LH-150 level followed a similar pattern, although it was very low at 500 d of age. The T-150 level showed a reverse pattern, being lower initially and higher in the latter part of the study. No apparent genotype differences occurred. Possible contributory influences on these patterns, including the onset of puberty and sexual maturity, season and nutrition, are discussed herein.     

Serum luteinizing hormone, 13,14-dihydro-15-keto-prostaglandin F2alpha and cortisol profiles during postpartum anestrus in Brahman and Angus cows

Pluriparous suckled Brahman and Angus cows were utilized to evaluate the effect of breed, day after calving and endogenous opioid peptides (EOP) on hormonal profiles during postpartum anestrus. On Days 17 and 34 after calving, blood samples with and without heparin were collected at 15- and 30-min intervals, respectively, for a 7-h period via jugular cannula. Two hours after the start of blood sampling, cows of each breed were administered either 1 mg/kg iv naloxone or saline. Three hours later, all animals received 10 ng/kg iv GnRH. On Day 34 after calving cows received 0.2 IU/kg iv ACTH. Mean LH, basal LH and area under the LH curve increased (P < 0.01) from Day 17 to Day 34 after calving. Height of LH pulses increased (P < 0.05) by Day 34 after calving. Brahman cows had higher (P < 0.05) mean LH, basal LH, LH pulse frequency and area under the LH curve than Angus cows. Naloxone increased postchallenge area under the LH curve in treated cows above that of control cows (P < 0.06). Naloxone also increased the postchallenge area under the LH curve above that of the prechallenge level (P < 0.01). No breed differences in the response to the naloxone challenge were observed. The LH response to naloxone challenge occurred earlier on Day 34 than on Day 17 after calving but the amount of LH released was similar between days. The GnRH-induced LH release was greater in Brahman than in Angus cows (P < 0.04). Mean cortisol concentrations and area under the cortisol curve decreased (P < 0.05) between Day 17 and Day 34 after calving. Mean cortisol concentrations and area under the cortisol curve were lower (P < 0.01) in Brahman than in Angus cows. Cortisol secretion after ACTH treatment was similar between Brahman and Angus cows. The cortisol response after ACTH challenge was positively correlated (r=0.68; P < 0.001) to the prechallenge area under the cortisol curve. Under optimal environmental conditions Brahman cows have a greater LH release and their anterior hypophysis is more sensitive to GnRH challenge than the Angus cows.     

Effects of copulation on timing of the LH surge following synchronization of estrus in the bovine

Forty-four crossbred postpubertal bovine females were used to study how mating with a bull affected estradiol-17beta (E(2)) secretion and timing of the preovulatory LH surge. Estrous cycles were synchronized with two injections of prostaglandin-F(2alpha) (PGF(2alpha)) 11 d apart. Females were either isolated from males (NE) or exposed to epididectomized bulls (BE) after the second PGF(2alpha) injection. Females exposed to bulls were allowed to mate once and then were separated from the bull. Blood samples were collected at 2-h intervals from the second PGF(2alpha) injection until 12-h post injection to monitor progesterone (P(4)) and luteinizing hormone (LH) concentrations and at hourly intervals from 12 h to 60 h post-injection to monitor LH secretion and timing of the preovulatory LH surge. Samples were also collected at 4-h intervals until 60 h post-injection to monitor estrogen (E(2)) secretion. LH surges were detected in 16 and 14 of 22 females from the BE and NE groups, respectively, during the 60-h period after PGF(2alpha) injection Mean P(4) concentrations and time of P(4) decline to <1 ng/ml were not different between the two treatment groups (P>0.30). Mean E(2) concentration during the 60-h sampling period was different (P<0.003) between BE and NE groups, and a significant treatment effect (P<0.002) occurred 48 h, 52 h and 60 h after the second PGF(2alpha) injection. However, mean LH concentration before the LH surge, duration of the LH surge and peak LH concentration during the surge were not different between the BE and NE groups (P>0.40). Mean time for the second PGF(2alpha) injection to the beginning of the LH surge was 51.6 +/- 1.5 h (X +/- S E) for the females not exposed to bulls and 48.5 +/- 1.4 h for females exposed to bulls (P>0.14). In this study, the presence of and/or mating by a bull did not affect LH secretion or timing of the preovulatory LH surge after PGF(2alpha) administration.     

Prepubertal changes in the hypothalamic-pituitary axis of Holstein bulls

We investigated the nature and sites of changes in the hypothalamic-pituitary axis associated with the onset of high-frequency, high-amplitude discharges of luteinizing hormone (LH) in young bulls during the transition from the infantile to the prepubertal phase of development. Blood serum and neuroendocrine tissues from bulls killed at 1, 6, 10, 14, or 18 wk of age were evaluated. Concentrations of LH in serum from bulls 1 or 6 wk old averaged less than 0.25 ng/ml and only one episodic discharge of LH was detected for 10 bulls. At 10, 14, or 18 wk, 14 of 15 bulls had episodic discharges of LH. Concentrations of testosterone in serum were progressively higher at 10, 14, and 18 wk, but the concentration of estradiol was maximal at 6 wk. The concentrations of gonadotropin-releasing hormone (GnRH) in the anterior hypothalamus, posterior hypothalamus, or median eminence were not influenced by age. However, concentration of GnRH receptors in the anterior pituitary gland increased 314% between 6 and 10 wk and the concentration of LH increased 67%. Between 6 and 10 wk, concentrations of estradiol receptors in the anterior and posterior hypothalamus declined by 68% and 46%, but the concentration of estradiol receptors in the anterior pituitary gland increased by 103%. For most characteristics, there was no major change between 10 and 18 wk. We postulate that between 6 and 10 wk of age, there is 1) removal of an estradiol-mediated block of GnRH secretion and 2) an estradiol-mediated, and possibly GnRH-mediated, increase in pituitary GnRH receptors. Together, these changes result in greatly increased stimulation of the anterior pituitary gland by GnRH between 6 and 10 wk of age and stimulation of the discharges of LH characteristic of bulls in the early prepubertal phase of development.     

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.