Influence of season and age on sexual maturation parameters in Texel, Suffolk and Ile-de-France rams: 2. Circulating concentrations of follicle stimulating hormone, luteinizing hormone, prolactin and testosterone.
Blood samples were collected at regular intervals from 6 Texel, 9 Suffolk and 9 Ile-de-France rams at 2 and 3 years of age. Weekly mean concentrations of testosterone and FSH were higher (P<0.05) during the 3rd than during the 2nd year of age, especially in spring and summer for testosterone and in autumn for FSH. Testosterone and FSH values decreased (P<0.01) in spring and were highest (P<0.01) at the peak of the breeding season. There was no significant difference in testosterone level among the breeds. FSH values were lower (P<0.05) in Texel rams than in the other 2 breeds. Prolactin levels were higher (P<0.01) in Texel and Suffolk rams than in Ile-de-France and decreased (P<0.01) with age in Ile-de-France rams but not in the other 2 breeds. Prolactin concentration was lowest during the breeding season and highest in spring and summer (P<0.01). Pulse frequencies of testosterone and LH decreased (P<0.01) in spring and summer during the 2nd year, and were highest in autumn. In contrast, the number of prolactin pulses was greater (P<0.01) in the spring and summer than during the breeding season. It was concluded that the delay in sexual maturity of Texel compared with Ile-de-France rams is associated with low circulating FSH concentrations and persistent hyperprolactinaemia in the spring and summer. [For Pt. I see pp. 67-79 of the same number of the journal.]