Incidence and morphology of anovulatory haemorrhagic follicles in the mare

During the dioestrous phase of the mare’s oestrous cycle, a cohort of 8-12 ovarian follicles is stimulated to develop by the action of pituitary FSH. During the ensuing oestrus, a “dominant follicle” is selected to continue maturation towards ovulation under the influence of pituitary LH (Evans and Irvine 1975), while the other follicles in the cohort become atretic. Morphological and biochemical changes in the follicular wall result in follicular rupture (ovulation) through the ovulation fossa and the release of the oocyte surrounded by cumulus cells into the oviduct. Ovulation failure has been well documented in the mare (Ginther 1979, Bosu et al. 1982, Ginther and Pierson 1989, Daels and Hughes 1993, Pierson 1993, McCue and Squires 2002), the most common form of which is the development of an anovulatory haemorrhagic follicle (AHF) where the follicle fails to rupture and its cavity becomes increasingly filled with blood. Such haemorrhagic follicles are commonly observed during the transitional phases in Spring and Autumn (Ginther 1992, Nunes et al. 2002), when the secretion rates of both gonadotropins and ovarian steroids are declining (Daels and Hughes 1993). Their occurrence during the breeding season greatly reduces breeding efficiency and the aim of the present study was to document the incidence of AHFs in an experimental herd of mares undergoing repeated pharmacological shortening of their oestrous cycles. Materials and methods