ESTIMATES OF PRE- AND PERINATAL MORTALITY IN THE NEW ZEALAND ROMNEY MARSH EWE

Abstract

The results of 2 slaughter observations conducted in 1963 and 1964 that involved the natural mating of 993 2 1/2-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, fetal 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 occured in the period immediately preceding the 18th day; this accounted for 50% of the overall mortality in the 2 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 2 groups of ewes respectively. Perinatal loss amounted to 8.3% and 26.7% for the 2 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.