Effects of genotype, dams age, litter size, birth weight and ram on morphological parameters of the placenta in sheep.

It is well established that especially in prolific ewes the mortality rates are high in lambs with low birth weights and that after birth the absolute growth rates are lower in the surviving light lambs than in the heavier la mbs. Differences in the prenatal placental development as the fete-maternal connection could be one of the reasons for this effect. The aim of the pre sent study was to ascertain primary data for the placenta characteristics. Therefore three different genotypes which varied in litter size and weight of all born lambs were studied. In 197 placentas the weight, the number of cotyledones, the diameter of the cotyledones and the weight of ten cotyledo nes were determined. The total weight of all cotyledones and surface area o f the cotyledones (the fete-maternal contact area) were calculated. The eff ects of the breed, the age of the darns (1.5-7.5 years), the litter size (1 -4), the weight of all born lambs (5 classes between 3 and 15 kg) and the r am were tested with the ANOVA/MANOVA procedure (co-variable = weight of the dams before mating). Litter size and weight of all born lambs influenced s ignificantly the morphologicaly parameters of the placenta. In the examined genotypes the placenta weight increased from a minimum of 500 g to 1100 g and the surface of the cotyledones grew from 200 cm(2) to 350 cm(2) with in creasing litter size and increasing weight of all born lambs. This seems to be a maternal compensation mechanism for the raising fetal requirements. W ith the increase of the litter size the surface area of the cotyledones for each lamb decreases because the possible placental compensation mechanism collapses. This could be a reason for low birth weights and increasing deat hs of lambs. A possible influence of the genotype on the characteristics of the placentas is discussed by considering the litter size and the conditio n of the ewe. Paternal effects were not found.