DYNAMICS AND ETIOLOGY OF OVARIAN FOLLICULAR CYSTS IN POSTPARTUM DAIRY-CATTLE

Ovarian follicular cysts affect approximately 7-13% of dairy cattle du ring the post-partum period, prolonging the interval from calving to c onception. This amounts to significant losses to the dairy industry. C ysts are commonly accompanied by the expression of behavioural oestrus , resulting in additional costs because of unsuccessful artificial ins emination. Although follicular cysts have been the subject of extensiv e study, particularly in relation to their diagnosis and veterinary tr eatment, relatively little is known about the mechanisms underlying th eir development. Some reports have shown that cows with cysts are asso ciated with higher milk production than non-cystic herdmates. A number of experimental approaches have been utilised to induce follicular cy sts, including giving erogenous steroids and gonadotrophins and/or alt ering endogenous gonadotrophin secretion. Utilising these approaches, coupled with rectal ultrasound, it has been observed that follicular c ysts are dynamic structures. Follicular cysts can inhibit the growth o f subordinate follicles, although the loss of this dominance is not ne cessarily accompanied by a decline in their diameter. However, their a bility to increase in size is not always associated with dominance. It appears as though follicular cysts continue their growth because they are still responsive to the stimulatory effects of gonadotrophins. Ev aluation of the success of treatment for cysts is complex because some cysts spontaneously regress and misdiagnosis of cysts occurs. Natural resolution occurs by luteinization of the cyst or by ovulation of the follicular cyst. Some treatments incorporate the induction of luteini zation followed by luteolytic treatments. As the improved understandin g of the reasons for cyst formation and the mechanisms involved should lead to the development of improved management and preventative strat egies to reduce their occurrence.