Heritability and Sire Genetic Trend for Litter Size in Swedish Sheep Estimated with Linear and Threshold Models

Litter sizes at birth and at weaning were analysed for the three most common breeds in the Swedish sheep recording programme. Lambing records for 124 571 Swedish pelt ewes (P), 8140 Svea breed ewes (X) and 3299 Swedish landrace finewool ewes (F) were broken down into subsets by parity. Parities 1 to 5 for P ewes, 1 to 4 for X ewes and 1 to 3 for F ewes were analysed with a linear mixed model that included random effects of flock-year and sire of the ewe and a fixed effect of sire birth-year class. When the non-linear threshold model was used on the same material, the fixed effect of thresholds was added to the above effects. Estimates of heritability were substantial and ranged from 0.07 to 0.40 for linear analyses and from 0.10 to 0.62 for threshold analyses. Litter size at birth generally displayed more additive genetic variation than litter size at weaning. Differences between heritability estimates from linear models and threshold models were largest in the earlier parities and for litter size at birth. Threshold estimates were consistently larger than linear estimates. Heritability was largest in the first parity (for litter size at birth in P, 0.35; in X, 0.62; and in F, 0.46) and decreased after that. Averaged over birth-year class, sire of ewe breeding values for litter size at birth increased annually at a rate of 0.04, 0.04 and 0.03 standard deviation units for P, X, and F, respectively. Average breeding values for litter size at weaning showed slightly higher increases for P and F - 0.06 and 0.03 standard deviation units, respectively - while that for X was lower - 0.02 standard deviation units. In all three breeds, genetic trend in first-parity litter size at birth for rams chosen as ewe sires was seen to be higher and more consistent than in later parities.