Fertilization failure and embryonic mortality in parous and nonparous beef cattle

One hundred and two nonparous females, 18 to 24 mo old, and 55 parous or multiparous beef females, 3 to 12 yr old, were mated naturally (d 0) and slaughtered on d 2 to 5, 6 to 8 or 14 to 16 of gestation. Each reproductive tract was flushed with phosphate-buffered saline, and the flushings were searched for an oocyte or embryo. Parous females had a higher (P less than .05) fertilization rate than nonparous females. In nonparous females, reproductive failure was attributed equally to fertilization failure and embryonic mortality, which had occurred or was occurring by d 8 of gestation. In parous females, reproductive failure was attributed entirely to embryonic death. About 67% of this embryonic mortality had occurred or was occurring by d 8 of gestation; the other 33% occurred between d 8 and 16 of gestation. These findings show that in both nonparous and parous beef females, a large portion of the reproductive failure occurs by d 8 of gestation and would not have any noticeable influence on length of the estrous cycle.