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.