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

摘要

Blood samples were collected from heifer calves born in spring (last week of March, n = 5) or autumn (last week of October, n = 5) every other week from birth to puberty and every 15 min for 10 h at 6, 12, 18, 24 and 32 weeks of age. Timing of puberty was determined by measuring progesterone in plasma samples collected every 2-3 days starting at 42 weeks of age. Age and weight at onset of puberty were similar in the 2 groups; 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 (P<0.05) in spring-born than in autumn-born heifers at 18 weeks of age. In spring-born heifers, LH pulse frequency increased over time to 32 weeks of age, and LH pulse amplitude was higher (P<0.05) at 12 and 18 weeks than at 32 weeks of age. Autumn-born heifers had higher LH pulse frequency at 6 weeks and showed a decrease in mean concentrations of LH at 12 and 18 weeks ofage (P<0.05). The FSH pulse frequency of spring-born heifers was higher (P<0.05) at 12 weeks of age than in autumn-born heifers; FSH pulse amplitude in autumn-born heifers decreased from 6 to 32 weeks 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.