Relationships of thyroid hormones, IGF-I and testosterone in breeds of ram lambs with low and high prolificacies

Ten Suffolk (Sfk) and 10 Outaouais (Out) ram lambs were selected to study the relationship between testosterone, thyroid hormones (T(3), T(4)), IGF-I, glucose, semen production and testicular histology at the onset of puberty in lower prolific (Sfk) and higher prolific (Out) breeds. SC, BW, ADG, RG and DM consumption were recorded weekly. Semen was collected by electro-ejaculation from 32 to 35 wk of age and evaluated. Blood was collected weekly by indwelling jugular catheters, every 20 minutes from 0900 to 1520 h to measure the concentration of glucose. T(4), T(3), IGF-I and testosterone. At wk 35, histological sections of testes were evaluated. Number of spermatozoa in the semen of Out was higher than Sfk (P < 0.01). Average BW, ADG, RG, SC, DM consumption and blood glucose of the two breeds were not different throughout the experiment (P > 0.1). However, there were negative correlations between levels of glucose, T(4), T(3) and age of the lambs. Testosterone concentrations and pulse frequencies were higher in Out than Sfk (P < 0.05). There were positive correlations between testosterone concentration, testosterone pulse frequencies and age of the lambs in the two breeds. Moreover, level of T(4), T(3) and IGF-I were higher in Out than Sfk throughout the sampling period (P < 0.01). Although there were fluctuations in the level of IGF-I in the blood, no pulses were detected. Higher levels of testosterone, T(4), T(3), IGF-I and testosterone pulses in Out may be associated with greater prolificacy in this breed.