Relationships among uterine and placental factors in prolific ewes and their relevance to variations in foetal weight

Abstract

ABSTRACTObservations were made on the distribution of ovulations between the two ovaries, the distribution of foetuses between the two uterine horns, the total numbers of cotyledonary burrs, and the weights and numbers of foetal cotyledons for each foetus in 80 Finnish Landrace × Dorset Horn ewes killed between 50 and 145 days of gestation. While ovulations were randomly distributed between right and left ovaries, viable foetuses were more evenly distributed between right and left uterine horns. Migration of embryos between the horns contributed to this relative balance but early foetal mortality, which increased with ovulation rate, tended to upset it. The mean number of foetal cotyledons per foetus depended on litter size and distribution, and at any fixed litter size was substantially reduced in those ewes with six or more ovulations. Foetal weights were also relatively low in these ewes. The total weight of cotyledons per foetus decreased by about 12% for each increase of one in number of foetuses, as compared with a 20% decrease in number of cotyledons but only a 7 to 11 % decrease in the weight of the individual foetuses. Comparisons between foetuses within the same litter led to similar trends in the ratios of cotyledon numbers, cotyledon weights and foetal weights. It is suggested that the presence of some very small lambs in large litters should not be attributed to maternal under-nutrition in late pregnancy.