Plasma LH and testosterone responses to synthetic gonadotrophin-releasing hormone (GnRH) or dexamethasone-GnRH combined treatment and their relationship to semen output in bulls

Abstract

The sperm output of each of 54 dairy bulls of the Montbeliarde breed was carefully investigated from 15 weekly semen collections at 51-65 weeks of age. Assessment of classical characteristics of semen output led to classification of bulls as good, medium and poor semen producers. At 12 and 13 months of age, bulls were injected i.m. with 0.25 mg GnRH (N=25) or with 20 mg dexamethasone + 0.25 mg GnRH (N=29). Peripheral plasma LH and testosterone responses to these injections were evaluated from frequent sampling. In the two groups, the mean areas under the curve of LH (μg/ml x 150 min) were significantly different between bulls (P<0.05) and 0.57 after dexamethasone-GnRH (P<0.05). This latter value indicates a predominant individual influence over the LH response to such a combined treatment. No significant individual differences were seen from the testosterone responses to either of these challenges. Correlations between individual LH responses and any of the semen output criteria were not significant (P>0.05) and there were no significant differences in terms of mean LH responses between bulls in the 3 categories of semen production.