215
SEXUAL DEVELOPMENT IN BULL CALVES BORN IN MARCH OR APRIL.
Rawlings, Norman1, Aravindakshan, Jayaprakash2, Bartlewski, Pawel3, Cook, Susan1, 1 2 3
ABSTRACT- There is a transient rise in LH secretion between 6 and 20 wks of age in bull calves which appears to be critical for the onset of reproductive maturity. Fall-born bull calves have lower mean LH concentrations and LH pulse amplitude compared to spring-born bull calves during this early pre-pubertal increase in LH release, but LH pulse frequency is apparently not affected by season of birth, and there is no difference in age at puberty. Therefore, LH pulse frequency in the growing bull calf may be the important cue to initiate and govern the rate of sexual development. In the present study we further examined the effect of season of birth on LH secretory parameters, by studying bull calves born early (n=6; March) or late (n=6; April) in the winter/spring calving season. Blood samples were collected every 12 min for 12 h at 5, 10, 15, 20 and 25 wks after birth; a single injection of GnRH (20 ng/kg body mass i.v.) was given after 10 h of sampling. Blood samples were also taken, body weights recorded and scrotal circumference measured weekly, from 1 to 26 wks of age, and then every other week until puberty. Animals of both groups reached puberty (defined as a scrotal circumference of ≥28 cm) at 43.2±1.3 wks of age (range: 38 to 54 wks after birth; P>0.05). Based on blood samples taken weekly, animals born in March exceeded April-born bull calves in mean serum LH concentrations at 6, 10 and 12 wks of age (P<0.05). Peak serum concentrations of LH following treatment with GnRH were significantly higher in early- than in late-born calves 10 wks after birth. In contrast, scrotal circumference was greater in the late- compared to early-born calves at 34 and 36 wks of age (P<0.10). None of the characteristics of gonadotropin secretion based on frequent blood sampling (i.e., mean concentration of LH and FSH, LH pulse frequency and amplitude) nor body weights across the period of sexual maturation, differed between early- and late-born bull calves in this study (P>0.05). We concluded that although absolute amounts of LH secreted by bull calves early in life were greater in calves born in March compared to those born in April, this did not influence age at puberty; again, LH pulse frequency may be critical for sexual development in the bull calf.
KEY WORDS: bull calves, puberty, gonadotropin, testes