Nutritional stress and pre-implantation mortality in Merino sheep, (1964–7). General discussion and conclusions

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Abstract
SUMMARY Collation and re-analysis of the results obtained in separate experiments investigating prenatal mortality in Merino ewes over a period of 4 years have made it possible to draw the following general conclusions: Data from the control groups indicated that in 3 out of 4 years basal prenatal loss averaged approximately 25%, which agrees with a number of other estimates in the literature. Higher losses of ova in 1965 than in the other 3 years could not be partitioned accurately between fertilization failure and prenatal mortality. Submaintenance nutrition apparently caused embryo mortality in 1964, and possibly in 1967, when underfeeding from day 7 to 37 was followed by an unusual number of long cycles; however, there was no evidence of increased mortality in 1965 and 1966. Losses of ova shed as twins were significantly greater than amongst those shed as singles in 1964 and 1967. The higher loss of twin ova was clearly associated with the treated groups in 1964. In each of the 4 years the left ovary produced more ova than the right, and when the 4 years' data were combined, survival of ova shed by the left ovary was significantly better than of those shed by the right. The type of twin ovulation, i.e. whether the two ova came from one ovary or two, had no consistent effect on prenatal mortality. In 3 years there was no significant regression of prenatal mortality on body weight at mating. However, in 1965, when body weights were relatively low, there was a significantly negative regression. It is possible that there is a critical body weight below which poorer prenatal survival can be expected. Ewes which suffered embryonic loss followed by a delayed return to service subsequently conceived less readily than those having a cycle of normal length following fertilization failure or early embryonic death. It is postulated that in some cases this may be because ovulation occurs while resorption is still incomplete. For some ewes having long cycles after embryonic death, anoestrus or removal of the fertile rams precluded subsequent conception in that season. This was an added reason for the season's lambing performance of ‘long cycle’ ewes being much worse than that of ewes re-mated after normal length cycles.