FOLLICULAR POPULATIONS IN THE EWE UNDER DIFFERENT GONADOTROPIN-LEVELS

Successful ovarian stimulation depends upon two factors, the number of follicles in the growth phase and the speed at which these follicles are growing. These two parameters, that is the number and growth rate of follicles in the growth phase were established in two breeds of sheep, which differ widely in their ovulation rate, and some hormonal influences on these parameters were demonstrated. The number of antral follicles was found to be immediately reliant on gonadotrophic support and a study of hormonal levels during the cycle and follicular populations at the following oestrus found that the magnitude of the second peak of FSH on day 2 of the cycle was correlated with the number of antral follicles present some 17 days later. Thus when 500 IU of PMSG was injected on day 2 of the cycle there were significantly more twin-ovulating ewes at the following oestrus but there were no over-stimulated ewes that are normally observed following the normal PMSG treatment. It is concluded that PMSG treatment early in the cycle may be a worthwhile approach to induce twin ovulations.