Autumn-born lambs have been reported to have lower birth weights than spring-born lambs by 0.4 to 1.0 kg, but the physiological basis of this difference is unknown. This study examined the effects of season on fetal growth and placental development independent of the confounding effects of maternal nutrition. Romney ewes (aged 5 and 6 years) were randomly allocated to groups mated in December (n=13) or March (n=13 with a similar live weight at mating (55.6±1.8 vs 55.7±1.9 kg, mean±SE, P>0.10) and managed for a similar live weight at day 140 of gestation (62.5±1.8 vs 62.9±1.9 kg, P>0.10). At day 140 of gestation, measures of fetal growth and placental development, adjusted for litter size, were (December- vs March-mated): individual fetal weight (4.15±0.16 vs 5.07±0.16 kg, P<0.001); total fetal weight per ewe (6.42±0.18 vs 7.14±0.22 kg, P<0.01); caruncle number (114.5±4.1 vs 121.0±4.2, P>0.10); placentome number (89.4±4.2 vs 106.9±4.3, P<0.01); caruncle occupancy, i.e. number of placentomes/number of caruncles (0.79±0.03 vs 0.88±0.03, P<0.05); and total placentome weight (564.7±34.0 vs 679.0±34.9 g, P<0.05). These results suggest that seasonal differences in birth weight of lambs are mediated not by differences in maternal nutrition but rather by a direct seasonal effect on placental size (specifically the formation of placentomes and hence total placental weight).

CMC, Jenkinson, SW Peterson, DDS MacKenzie, and SN McCutcheon

Proceedings of the New Zealand Society of Animal Production, Volume 54, , 227-230, 1994
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