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Short-term undernutrition and prenatal mortality in young and mature Merino ewes

SUMMARY Prenatal mortality was examined in 74 primiparous Merino ewes which, after hand mating to fertile rams on the second oestrus following synchronization, were allocated to two groups. One group of 38 ewes was confined to yards and restricted to approximately 30% of their maintenance requirement for 14 days while the other group of 36 remained at pasture. The degree and timing of prenatal loss was assessed by corpora lutea counts, pregnancy diagnosis about day 40, returns to service and lambing records. The mean ovulation rate was 1·06 and the 19-day non-return rate was 87·8%. Pregnancy failed in a higher proportion of the underfed ewes (45·0%) than in the control ewes (30·8%) but this difference did not reach significance. Of the ewes returning to service, significantly more did so with cycles longer than 19 days in the underfed group (9/12) than in the control group (2/8), indicating that in the latter stages, the treatment may have been detrimental to embryo survival. In a second experiment, ovulation rate and prenatal mortality, assessed as in the first experiment, were studied in 85 mature Merino ewes subjected to the following nutritional treatments: HH, control; HL, 30% submaintenance feeding for 14 days following mating; LH, 15% submaintenance feeding for 7 days prior to mating; LL, both submaintenance treatments. Acute undernutrition prior to mating (LH) had no effect on ovulation rate or prenatal mortality. Ovulation rates were low over all groups (mean 1·09) and thus precluded any valid examination of ovulation rate and body weight relationships. The right ovary was significantly more active in producing ova than the left in all groups (P < 0·05). Although ova wastage was apparently higher in. the post-mating undernutrition groups (HL, 44%; LL, 48%; LH, 50%) than in the control group (HH, 35%), the differences were not significant. An increased incidence of extended (> 19 days) first-return cycle lengths in group LL provided some evidence of induced prenatal mortality. In so far as the experiments can be considered together, they indicate that if body weights are comparable, primiparous 1½-year-old ewes are not necessarily more susceptible than mature ewes to prenatal mortality induced by severe short-term undernutrition.