Repeatability of LH responses by lambs to monthly challenge with synthetic gonadotrophin releasing hormone (GnRH)

The aim of this experiment with ram and ewe lambs was to test the hypothesis that there are consistent individual differences in Luteinising Hormone (LH) response to Gonadotrophin Releasing Hormone (GnRH). Pre-puberal Border-Leicester × Merino lambs (15 of each sex) aged 9 weeks were challenged with either 0, 30 or 60 μg synthetic GnRH each month for 7 months (December to June). The lambs were then rested from this monthly routine until they were challenged an eighth time in September at 48 weeks of age. Luteinising Hormone response (area under LH release curve) was measured each month and the repeatability of individual LH responses calculated. There was a significant interaction (P < 0.01) between treatment month and sex reflecting a fall in LH response by ram lambs after a peak in February, while at the same time responses by ewe lambs increased to peak again in May. LH response also increased with GnRH dose (30 vs. 60 μg; P < 0.05). Responses by individual lambs were ranked 1 to 5 each month within sex and GnRH doses (30 and 60 μg only). Highest ranked lambs had LH responses 1.4 to 7.0 times larger than lowest ranked lambs. Repeatability of rank between months was poor in all groups except ewe lambs given 60 μg GnRH, where three of the five lambs repeated a particular rank at 5 of the 8 sample months. However, the repeatability of response in this group was not considered to be sufficient to reject the null hypothesis. It was concluded that if consistent individual differences do exist they may be subtle and easily masked by factors such as GnRH dose, sex, age and season.