Heritability of litter size in the Swedish population of Shetland sheepdogs

Abstract: The Shetland sheepdog is one of the most common canine breeds in Sweden and the demand for puppies is arguably high on the Swedish market. However, the continuously small litter sizes (mean of 3.0-3.4) in the population have been a cause for concern. Previous studies have presented low to medium heritability estimates of litter size in other dog breed populations, but there is no such estimate for the Shetland sheepdog. In this retrospective study, data from 10 443 Shetland sheepdog litters born and registered in the Swedish kennel club between 1980 and 2021 was used to estimate litter size heritability and analyse environmental variables affecting the trait. The study showed that dam age and parity had significant effects on litter size. Sire inbreeding had a significant positive effect (+2.7, SE=1.2) when also adjusting for the inbreeding coefficients of the litter and dam. Litter inbreeding had a significant negative effect (-1.6, SE=0.6) on litter size when included separately in the model, while no significant effect of dam inbreeding was found. The heritability of litter size was estimated between 0.14 (SE=0.02) and 0.22 (SE=0.05), with the most reliable estimate being 0.15 (SE=0.02). The results indicate that a breeding progress to increase litter sizes in the Swedish Shetland sheepdog population is possible with the implementation of best linear unbiased prediction (BLUP) based breeding values, if the trait is prioritised when selecting breeding animals.