Testis growth and plasma LH concentration following hemicastration and its relation with female prolificacy in sheep.

The mean testis diameter of 20- to 25-week-old Blackface, Finn and Merino rams was ranked in the same order as the ovulation rates of females of their breeds. The removal of one testis at 12 or 16 weeks of age resulted in hypertrophy of the remaining testis. The relative increase in testis growth following hemicastration was greatest in the Merino rams (72%), least in the Finns (42%) and intermediate in the Blackfaces (57%), so that it was inversely related to their breed ovulation rates. This hypertrophy was associated with increases in the concentration of plasma LH in all breed types. The results indicate that differences in testis growth rate are associated with differences in gonadotrophic stimulation rather than in intrinsic growth potential, and it is postulated that these may arise from breed differences in sensitivity to negative feed-back from the testes.