The effect of age of ewes and of different lambing years on fertility expressed as number of lambs born and surviving to 2 months per lambing has been studied in field data collected by the herdbook for Texel sheep in North Holland. The fertility of ewes was compared when first bred at about 8 months or at about 20 months of age and the effect of heredity on litter size was studied.Hereditary effects were determined by the effect of type of birth of sire, dam and ewe on the fertility of ewes, by heritability of litter size, by the effect of sires on fertility of these female progeny and estimating the repeatability of litter size. Effect of age of ewe on fertility was studied.
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