Previous studies have shown that autumn-born lambs are lighter (by 0.4 to 1.0 kg) at birth than spring-born lambs, an effect which is associated with impaired placental development in ewes mated to lamb in the autumn. As previous studies have focused on near-term fetal weights and birth weights, the objective of this study was to determine the pattern of growth throughout gestation in lambs destined to be born in the autumn or spring, and to examine associated changes in placental development. Mature N.Z. Romney ewes were randomly assigned to groups mated in December (n=23) or March (n=28) and managed on pasture in both seasons for similar live- weight profiles from mating to day 140 of gestation. Ewes were slaughtered at day 56, 84, 112 or 140 of gestation, and measures of fetal and placental weight were determined. Fetal weights (kg) of December- versus March- mated ewes were: d56 (0.040

SA, McCoard, SW Peterson, CMC Jenkinson, JW Campbell, and SN McCutcheon

Proceedings of the New Zealand Society of Animal Production, Volume 56, , 297-300, 1996
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