Vital statistics for an experimental flock of Merino sheep. IV. Failure in conception and embryonic loss as causes of failure to lamb

Abstract

Ewes which fail to lamb can be classified as failing to conceive (E_NC) or conceiving but failing to carry at least one embryo to full term (E_NE). Estimates of E_NE have been made indirectly from mating and lambing records involving 2509 observations over 3 years in a flock of medium Peppin Merinos. Estimates of ENE and of individual embryo loss have been made directly from autopsies of 419 ewes in the same Peppin flock and in one of mixed strains. The indirect estimate of E_NE (ewes served and expected to have conceived but failing to lamb, as a percentage of ewes joined) averaged 4.3% for ewes aged 1½–9½ years at joining. Two measures of early embryonic loss were obtained from the autopsies of ewes expected to have been pregnant for approximately 28 days: (i) Ewes predicted not to lamb, as a percentage of ewes expected to have conceived, gave an estimate of 2.8–3.9 % at 1½ years, and 10.1–12.0% at 8½–11½ years. (ii) By relating the number of active corpora lutea to the number of healthy embryos, the estimates of loss obtained were 4.7–5.8% for maidens and 18.6–20.6% for aged ewes. From approximately 28 days to term no significant foetal loss from resorption or abortion could be demonstrated by comparing autopsy predictions with observed performance in ewes proceeding to term, though a very small loss of one of a pair of twins may have occurred in ewes aged 1½ years. Ewes with two active corpora lutea suffered a significantly greater loss of embryos than did ewes with one. With the corpus luteum as the unit, the loss in maiden ewes was 20.0% with two corpora lutea and 2.9–4.1% with one, the corresponding figures for aged ewes being 34.7–36.1 and 9.0–11.5%. These differences are not evident with the ewe as the unit, as a ewe with two corpora lutea which had lost one embryo would remain in the group predicted to lamb. Estimates of E_NE from autopsy were made on ewes selected from the complete flock because they had not returned to service. Estimates for the same ages on the complete flock were 3.6–4.5% for maiden and 8.7–10.2% for aged ewes. Calculations of E_NC have been based on the estimate of E_NE made here and on previously reported figures of the numbers of dry ewes per ewe joined and present at lambing. Ranges of ENC were 13.6–14.5% at 2 years, and 3.3–4.8% respectively. No identical twins were observed at autopsy, and there is no reason to believe that any would have been present at any earlier stage of pregnancy.