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.

Fertility indices for beef bulls.

Young (16- to 30-month-old) beef bulls of 6 different genotypes were assessed for production and reproduction traits at different ages and intervals from single-sire mating. Fertility indices, in the form of multiple regression equations using pregnancy rate as the dependent variable, were derived from these assessments using non-orthogonal analyses of variance and covariance. "Among" and "within" genotype fertility indices showed significant correlations with pregnancy rate. "Within" genotype fertility indices showing significant multiple correlations (p less than 0.01) at 11 (r = 0.75), 8 (r = 0.89), 6 (r = 0.86) and 2 (r = 0.80) months prior to mating. It was found that the most important traits to include in the fertility indices were peripheral LH levels following GnRH stimulation, testicular volume, libido and body weight. In general, the fertility indices showed good correlations with bull reproductive performance and were not significantly affected by bull genotype.

Relationships among bovine male and female reproductive traits.

No significant relationship (p greater than 0.05) was found between age at puberty in heifers and the age and scrotal circumference at puberty in related bulls. There was a significant effect (p less than 0.01) of genotype and sire on age at puberty of heifers and a significant effect (p less than 0.05) of genotype on weight at puberty in heifers. There was a significant effect of genotype on age (p less than 0.01) and weight (p less than 0.05) at puberty of bulls. A significant difference (p less than 0.05) in age at puberty of bulls was found between the 2 methods of assessing puberty. It is possible that the assessment of puberty of heifers at 2-month intervals may not have been precise enough to detect such a relationship and/or that the variation in genotypes and ages in this study were too great to establish such a relationship.

Brahman bull fertility in a north Australian rangeland herd.

Low and variable bull fertility was identified as a constraint on reproductive rates in beef cattle grazed in an extensive, multiple sire mating regimen on Mount Bundey station in the Darwin pastoral district of northwestern Australia. Erratic conception patterns were attributed to a high proportion of bulls with low breeding soundness evaluation scores (BSE), a high proportion of aged bulls (40%>8 yr), and to running bulls of mixed age groups. Liveweight, scrotal circumference (SC) and age were positively correlated. An experiment was subsequently designed to investigate the ability of a number of bull measurements to predict fertility in an extensively-managed, multiple-sire mating system. Blood typing was used to match calves to sires. It proved to be an accurate and useful technique which successfully identified the parentage of 94% of calves examined. Single measurements of serum testosterone after administration of gonadotrophin-releasing hormone (GnRH) were not correlated with fertility. However six of the seven most fertile bulls exhibited high peak serum testosterone levels in summer, and lower levels in the winter. In contrast, the less fertile bulls did not exhibit seasonal variation in GnRH-induced serum testosterone levels. Social dominance ratio was weakly related to fertility (r=0.51: P<0.05). BSE (r=0.51: P<0.05) and SC (r=0.49: P<0.05) prior to, but not subsequent to, mating were correlated with bull fertility. Under the conditions of this experiment, a bull to cow ratio of 1:20 was excessive for bulls with a satisfactory BSE score.

Characterization of LH and testosterone responses to intramuscular injection of GnRH in tropical postpubertal bulls.

Five Zebu x British crossbred bulls 17 months of age and of uniform liveweight (320+/-3 kg) were used to study testosterone responses to single intramuscular doses of exogenous gonadotropin-releasing hormone (GnRH). The eight dose levels used were 0, 31.25, 62.5, 125, 250, 500, 1000, and 2000 ng GnRH/kg live weight. Plasma samples for hormone responses were collected at 30-minute intervals from zero to three hours and at one-hour intervals from three to seven hours postinjection. Luteinizing hormone (LH) and testosterone responses were measured as peak heights or as areas under response curves. Increasing the dosage of GnRH increased the time to reach the peak LH response, the height and duration of the response, and the area under the response curve. The maximum LH peak height was reached by the 1 mug/kg dose. In contrast to LH, testosterone responses reached the same peak heights (two hours postinjection of GnRH) for all doses of GnRH. The only effect of increased dosage was to increase the duration of response. Testosterone responses showed repeatable differences (P<0.01) between animals, but LH responses did not. It was demonstrated that the testosterone status of bulls can be accurately assessed by simply measuring testosterone in a single plasma sample collected two to three hours after the intramuscular injection of 100 mug or more (dose unimportant) of GnRH per bull.

Reproductive performance and productive traits of beef bulls selected for different levels of testosterone response to GnRH.

In two separate studies, one with four Hereford-Shorthorn and one with three Zebu x British crossbred bulls, the efficacy of using the testosterone response to gonadotrophin-releasing hormone (GnRH; Lutal, Hoechst) to predict differences in reproductive performance was assessed. Young bulls (17 or 29 months of age) selected for low to high (3.1 to 10.3 ng/ml) peak plasma testosterone 2 to 2.5 hours after the intramuscular injection of GnRH (62.5 to 2000 ng/kg LWT) were each later individually joined with groups of 19 to 30 cows in which estrus had been synchronized. In both studies, the rankings of bulls for overall fertility (capable cows pregnant), for libido (estrous cows mounted), and for fertilizing ability (mounted cows pregnant) were in close agreement with rankings for testosterone responses to GnRH. In the Bos indicus bulls rankings for both reproductive performance and testosterone response to GnRH were repeatable when measured at two and four years of age. Other reproductive measurements (semen quality, scrotal circumference, pen-type serving capacity tests) were less reliable predictors of reproductive performance. Measurements of liveweight gains and parasite resistance indicated that bulls superior for these characteristics do not always have satisfactory reproductive performance. These studies suggest that the testosterone response to GnRH could be a useful test to ensure that bulls selected for productive traits have adequate reproductive potential.

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.