ESTIMATES OF PRE- AND PERINATAL MORTALITY IN THE NEW ZEALAND ROMNEY MARSH EWE
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Summary.
The results of two slaughter observations conducted in 1963 and 1964 that involved the natural mating of 993 2½-year-old New Zealand Romney Marsh ewes, half of which were parous, the remainder non-parous in the previous year, are described. The aim of the observation was to measure the loss through non-fertilization, embryonic, foetal and lamb death throughout pregnancy by slaughtering equal numbers at 2, 18, 40 and 140 days post coitum, and allowing a control group to lamb.
This paper presents the data obtained from those ewes that were mated and conceived to one service. In 1964, from a comparable ovulation rate of 139% and 143% and a fertilization rate of 91·1% and 83·3% for the parous and non-parous ewes respectively, the period of greatest prenatal mortality in those ewes that did not return to service was in the first 30 days of pregnancy, 22% and 16·6% of the fertilized ova being unaccounted for as live embryos during this stage. The period of maximum loss occurred in the period immediately preceding the 18th day; this accounted for 50% of the overall mortality in the two groups of ewes in the first 30 days of pregnancy. A similar pattern was obtained during a preliminary investigation in 1963.
Of the embryos alive at 30 days, 1·3% and 5·3% from the parous and non-parous ewes, respectively, underwent either abortion or mummification. The overall estimate of prenatal death, excluding non-fertilization, was 23·3% and 20·9% for the two groups of ewes respectively.
Perinatal loss amounted to 8·3% and 26·7% for the two respective classes of ewes, this presenting a total estimated overall pre- and perinatal loss of 29·6% and 41·7% for the parous and non-parous ewes respectively, calculated as the percentage of fertilized ova that failed to produce a viable lamb.
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