Patterns of variations in FSH, LH and 17beta-estradiol during the postnatal development of sheep

The aim of the present study was to estimate the onset of sexual maturity of F2 lambs born to crossbred ewes (East-Friesian x Black-Head Pleven breeds)xEast-Friesian rams based on measurements of plasma FSH, LH and 17beta-estradiol levels during postnatal development. The hormonal levels were measured by radioimmunoassay in blood samples taken from 107 ewe lambs at the age of 0 to 10 days, 1, 2, 3, 4, 5, 5.5, 6 months and at 1 year from anestrous ewes (birth-Day 0). Starting at a baseline concentration during Days 0-10, FSH rose to a peak at Month 2 and declined after Month 3 to levels equivalent to those seen in yearling, sexually mature ewes. Mean LH concentrations rose from baseline to the highest level in samples taken at 5.5 months and stabilized at 6 months to the level seen in yearling ewes. The preovulatory LH peak was recorded in 5.5 month-old lambs. Neither FSH nor LH declined to baseline concentrations in lambs after the initial 10 days of life. 17beta-estradiol fluctuated, showing an initial rise in samples taken between Days 0-10 and Month 2, followed by insignificant variations between different ages and were near to those in yearling ewes. The data suggest that the sexual maturity in lambs is attained at 5.5-6 months of age. The findings allow us to suggest that these crossbred ewes might be fertilized at an earlier age (11-12 months) if they had reached the necessary body development (body weight: 75-80% of that of adult ewes). They also might be included earlier in estrous synchronization programs in order to give birth to 3 lambs in 2 years.