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Summary. To define gonadotrophin secretion rates in the prepubertal heifer, 12 Hereford × Friesian heifers were blood-sampled at 15-min intervals for periods of 24 h every 4 weeks from 3 weeks of age until puberty. Radioimmunoassay of plasma LH concentrations showed that, although LH episode frequency increased with age, overall mean LH concentrations and basal LH concentrations decreased between 3 and 15 weeks of age and then increased to 35 weeks of age. The validity of these trends in relation to biological activity of plasma LH was investigated using an in-vitro Leydig cell bioassay. Samples were selected from 24-h profile bleeds of 4 heifers at 3, 7, 11, 15, 19, 27 and 39 weeks of age. No significant differences were found in the patterns of change in overall mean LH concentrations, basal LH concentrations or LH episode amplitude when comparing the estimates obtained by radioimmunoassay with those by bioassay from birth over the prepubertal period. These results indicate that the changes with age observed by radioimmunoassay are representative of changes in biologically active hormone.

Keywords: bioassay; radioimmunoassay; LH; puberty; heifer

Summary. Twelve autumn-born Hereford × Friesian heifers were studied to characterize changes in the patterns of LH and FSH secretion occurring from birth through the peripubertal period. A once weekly blood sampling regimen, starting 3 days after birth, was combined with periods of frequent sampling (15-min intervals for 24 h) every month from 3 weeks of age.

Mean plasma LH concentrations decreased over the period from birth to 15 weeks of age, largely due to a decrease in basal LH concentrations. Thereafter, mean plasma LH concentrations increased to 39 weeks of age, mainly as a consequence of increasing LH episode frequency and LH episode amplitude.

Oestrus was detected using an oestradiol-treated steer, and ovulation inferred from progesterone profiles. A 'short luteal phase' oestrous cycle preceded the first observed oestrus, and this was followed in all heifers by a normal length luteal phase. However, no increase in mean LH concentrations, basal LH concentrations, LH episode frequency, LH episode amplitude or change in mean FSH concentration could be directly associated with the onset of puberty. It is therefore concluded that the gonadotrophic stimulus for first ovulation must occur abruptly.

Keywords: puberty; heifers; LH; FSH