EMBRYONIC ABILITY TO PROLONG THE INTERESTROUS INTERVAL IN VIRGIN AND REPEAT BREEDER HEIFERS

Fourteen virgin heifers (VH) and eight repeat breeder heifers (RBH) were used for repeated nonsurgical embryo collections. The heifers were flushed on a total of 61 occasions, 14 or 17 days after ovulation. The two categories of heifers were compared for degree of embryonic development, the ability of these embryos to prolong the interoestrous interval, the plasma progesterone levels after insemination, and the light and electron microscopical morphological appearance of the embryos. The RBH gave fewer and smaller embryos than the VH. Moreover, RBH embryos presented a higher incidence of morphological deviations than VH embryos. Day 17 RBH embryos were less successful in prolonging the interoestrous interval than the VH embryos. In particular, those 17-day-old RBH embryos which did not prolong the interoestrous interval showed a clearly deviated morphology of the embryonic disc. Plasma progesterone levels were positively correlated with pregnancy before maternal recognition of pregnancy. In the VH group, plasma progesterone levels at Days 6-11 were positively correlated with the length of 17-day-old embryos. Despite the strict control of oestrus and ovulation and the use of additional artificial insemination (AI) when necessary in this study, differences in pregnancy results between the RBH and VH still remained. It is concluded that the uterine environment in RBH does not provide proper support for embryo development up to the time when maternal recognition of pregnancy occurs.