Plasma LH and FSH concentrations in prepubertal beef heifers before and in response to repeated injections of low doses of Gn-RH.

Abstract

Blood samples were taken at 15-min intervals from prepubertal beef heifers, 6 aged 4 months and 6 aged 10 months, to determine plasma LH and FSH concentrations. Two animals of each age group were then subjected to 9 consecutive injections of 0.5, 2.0 or 5.0 micrograms Gn-RH at 2-h intervals. LH episodes were evident in all animals before treatment, and in 11/12 heifers occurred at a rate of between 1 and 4 per 24 h. One 10-month old heifer, later detected in oestrus, displayed 10 episodes/24 h. Mean FSH concentrations did not differ significantly between age groups. In all animals, Gn-RH injections induced an episodic pattern of LH secretion which was significantly different between Gn-RH doses, but not between age groups. A distinct episode-like response in FSH concentrations occurred to each Gn-RH injection in only one heifer. These results provide further evidence that the prepubertal period is associated with a lack of pituitary stimulation, rather than an inability to respond to Gn-RH.