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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.

Milk progesterone profiles were used to study the high incidence of embryonic mortality between 30 to 80 days after insemination in 91 cows in a single herd. The progesterone levels dropped to basal values at 46.7 +/- 1.37 days after service in 22 per cent of apparently pregnant cows. Embryo loss occurred significantly more frequently following services by one bull (Z). The mortality rates were 14/32 (44 per cent) in cows conceiving to Z compared with 8/73 (11 per cent) in cows conceiving to other bulls. There was no significant effect of the age of the cow or yield on the incidence of embryonic mortality.

Recent findings on the development and natural outcome of twins from Day 17 (immediately after fixation) to Day 40 are reviewed. Incidence of embryo reduction was increased significantly when the vesicles became fixed unilaterally, rather than bilaterally, and when the vesicles were unequal in diameter. Of 68 mares with twins on the day of fixation, post fixation embryo reduction occurred in 41 (60 per cent). The incidence of reduction was 41 of 48 (85 per cent) following unilateral fixation; reduction occurred in all of 22 mares with vesicles of dissimilar size (4 mm or more difference in diameter) and in 19 (73 per cent) of 26 mares with vesicles of similar size (0 to 3 mm difference). Embryo reductions were complete (vesicle no longer visible ultrasonically) by Day 20 (59 per cent of the reductions), during Days 21 and 30 (27 per cent), or Days 31 to 38 (14 per cent). In 80 per cent of the early reductions (by Day 20) the eliminated vesicle disappeared within one day. Reductions that occurred after Day 20 were preceded by a gradual decrease in size. As the number of days after Day 17 increased, the frequency of reduction decreased and the time required for completion of reduction increased. When the twins were dissimilar in diameter (4 mm or more), they were more likely to undergo reduction by Day 20. In summary, dissimilarity in diameter increased the likelihood of unilateral fixation, increased the incidence of reduction for unilaterally fixed vesicles, hastened the day of occurrence of reduction and shortened the interval from initiation to completion of reduction. The deprivation hypothesis proposes that embryo reduction occurs when a major portion of the three walled area of the yolk sac or the vascularised wall of the yolk sac or allantoic sac is in apposition with the wall of the adjacent vesicle rather than with the endometrium; the vesicle is deprived of adequate embryonal-maternal exchange and therefore regresses.

Embryonic resorption is reported as the main reason for early pregnancy losses in the West German Thoroughbred mare. An analysis of breeding records for the last 11 years showed a resorption rat of 8.4 per cent. Evidence was obtained that these early embryonic losses were associated with too early conception after foaling and with the state of lactation.