SEASONAL EFFECTS ON BIRTH-WEIGHT IN SHEEP ARE ASSOCIATED WITH CHANGESIN PLACENTAL DEVELOPMENT

Lambs born in the autumn or winter have substantially lower birth weig hts than those born in the spring, but the physiological basis of this difference is unknown. This study examined the effects of season on f oetal growth and placental development in ewes managed under controlle d grazing to minimise the confounding effects of maternal liveweight c hange. Mature Romney ewes pregnant to matings in December (n = 13) or March (n = 13), with similar liveweights at mating, were managed to ac hieve similar liveweights at Day 140 of gestation. At that time, measu res of foetal growth and placental development, adjusted for litter si ze, were (December- versus March-mated): foetal weight (4.15 +/- 0.16 versus 5.07 +/- 0.16 kg, P < 0.001); total foetal weight per ewe (6.42 +/- 0.18 versus 7.14 +/- 0.22 kg, P < 0.01); caruncle number (114.5 /- 4.1 versus 121.0 +/- 4.2, P > 0.10); placentome number (89.4 +/- 4. 2 versus 106.9 +/- 4.3, P < 0.01); number of placentomes per number of caruncles (0.79 +/- 0.03 versus 0.88 +/- 0.03, P < 0.05); and total p lacentome weight (564.7 +/- 34.0 versus 679.0 +/- 34.9 g, P < 0.05). M aternal circulating concentrations of prolactin, IGF-1, and growth hor mone are reported. This study has demonstrated that the marked seasona l differences in foetal growth are associated with seasonal difference s in placental size, the formation of placentomes being significantly reduced in December-mated ewes.