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

There is an early transient rise in gonadotropin secretion in spring-born p repubertal heifers and there is an indication that this pattern is differen t in autumn-born heifers. The:effect of season of birth on age and weight a t puberty is equivocal. This study was designed to compare the temporal pat terns of LH and FSH secretion between spring- and autumn-born heifers and t o determine the effects of season of birth on age and weight at puberty. Bl ood samples from 2 groups of heifer calves born in spring (last week of Mar ch, n = 5) or autumn (last week of October, n = 5) were collected every oth er week from birth to puberty and every 15 min for 10 h at 6, 12,18, 24 and 32 wk of age. Timing of puberty was determined by measuring progesterone i n plasma samples collected every 2 to 3 d starting at 42 wk of age. Age and weight at onset of puberty did not: differ between the 2 groups of heifers (P > 0.05); however, the autumn-born heifers tended to mature in a wider r ange of ages and weights. Based on the 10-h sampling periods, mean serum co ncentrations of LH and LH pulse frequency and amplitude were higher in spri ng-born heifers at 18 wk of age than in autumn-born heifers (P < 0.05). Tn spring-born heifers, LH pulse frequency increased over time to 32 wk of age , and LH pulse amplitude was higher at 12 and 18 wk than at 32 wk of age (P < 0.05). Autumn-born heifers had higher LH pulse frequency at 6 wk and sho wed a decrease in mean concentrations of LH at 12 and 18 wk of age (P < 0.0 5). The FSH pulse frequency of spring-born heifers was higher at 12 wk of a ge than in autumn-born heifers (P < 0.05), FSH pulse amplitude in autumn-bo rn heifers decreased from 6 to 32 wk of age. It was concluded that although the mean age and weight at puberty did not differ between spring- and autu mn-born heifers, the range in age and weight-at puberty was wider in the au tumn-born heifers. The patterns of LH secretion differed between spring- an d autumn-born prepubertal heifers, with spring-born calves exhibiting an ea rly rise in LH secretion, while mean serum concentrations of LH decreased d uring this period in autumn-born heifers. (C) 1999 by Elsevier Science Inc.