The effects of litter size on growth rate of rats

It is generally accepted that in both laboratory and farm animal species the young in small litters will be heavier at weaning than those in larger litters. This is presumably due to differences in available milk supply to each animal during the lactation period. Another generally accepted idea is that the smaller animals at weaning will exhibit compensatory growth if an adequate level of nutrition is provided postweaning. In a study of performance of rats developed from senile ova, when mating was delayed until 12 hours after initiation of estrus, litter size at birth was reduced to about one-half the control litter size. Despite the smaller number of rats per litter, the weaning weights of the rats from delayed-mating litters were not different from the weaning weights of the control litters. This study to assess the effects of litter size during the lactation period on subsequent growth of rats was initiated as a consequence of the above findings.