Pattern and manipulation of follicle development in sheep and goats

Improving our understanding of the pattern of follicle development and its manipulation by exogenous hormones in sheep and goats will improve our ability to manipulate their fertility. Repeated ultrasonographic observations of the growth of individual follicles have shown that follicles develop in a wave-like pattern during oestrous cycles, with two or three follicular waves per cycle being most common in sheep and three or four follicular waves per cycle being most common in goats. The largest follicles that are present at luteolysis continue development to ovulation and these usually derive from the last follicular waves but in some cases also develop from the penultimate follicular wave. Follicle development is often stimulated by using eCG to enhance the recruitment of small follicles, increase ovulation rates and increase the synchrony of oestrus after oestrous synchronisation treatments. Superovulation treatments also use gonadotrophin stimulation, the success of which seems to be related to the number of small follicles at the start of treatment. Long-term (12 14 days) progestagen treatments used for oestrus synchronisation can promote prolonged growth of ovulatory follicles in sheep but has not been studied in goats. The ovulation of aged follicles in cattle has a detrimental effect on fertility, but this relationship is less clear and seems to be less critical in sheep.