Hereford cross beef heifers at pasture were inseminated at a detected oestrus with thawed semen and were slaughtered at the following intervals after insemination: day 3 (60 heifers); day 8 (71); day 14 (65); day 18 (78) and day 28 (29). The recovery rates of ova at days 3 and 8 were 87 and 77 per cent respectively. The percentage of heifers that were pregnant at the respective time intervals after slaughter were 81, 84, 75, 60 and 62. There was no difference in the proportion of normally developing ova or embryos between days 3 and 8, 8 and 14, 14 and 18 or 18 and 28. There were significant differences in the number of animals with variable embryos between days 3 and days 18 + 28 (P less than 0.025); 8 and 18 + 28 (P less than 0.01); and 14 and 18 + 28 (P less than 0.05). Based on the results and other data in the literature, fertilisation failure can account for up to 20 per cent of reproductive wastage. Embryonic mortality accounts for most of the remaining wastage and occurs gradually between days 8 and 18. Pregnancy rates at days 18 and 28 approximate to calving data.
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