The economic and biological significance of embryo mortality in cattle is well recognised. About 90% or more ova are fertilised at oestrus but only about 55% of dairy cows calve to first insemination (reviewed by Sreenan and Diskin, 1985). Of this 35% post fertilisation loss, most (25-30%) occurs within a cycle length therefore not affecting the time of return to the subsequent oestrus. It is also accepted that the maintenance of early pregnancy requires the secretion of progesterone by a viable corpus luteum and that premature luteolysis is the major cause of embryonic loss during these early days of pregnancy.

The corpus luteum forms after ovulation from the cells of the granulosa and theca interna layers of the ovarian follicle. These are thought to differentiate into the large and small luteal cells respectively (Smith et al., 1994). The large cells secrete progesterone and oxytocin and are responsive to prostaglandin E whilst the small ones secrete progesterone and are responsive to LH (reviewed by Wiltbank, 1994).