Inbreeding and its impact on the prolific Polish Olkuska sheep population

•Olkuska sheep population is characterized by high levels of inbreeding, which is highly differentiated between single flocks, and small number of founders.•Inbreeding effects on birth weight, weaning weight (at 56 days of age), lamb survival up to 56 days and the litter size of ewes were evaluated.•Significant inbreeding depression in lamb body weight and survival up to 56 day was found.•No significant inbreeding effect on litter size confirms the hypothesis that the effect of single gene is greater than any effect of inbreeding.

The aim of this study was to assess the level of genetic variability in the Olkuska sheep population and evaluate inbreeding effects on birth weight, weaning weight (at 56 days of age), lamb survival up to 56 days and the litter size of ewes. Pedigree information over a 36 year period of Olkuska population (from 1976 to 2012) was used in this study. A small number of important ancestors, with only four individuals explaining over 50% of the total genetic variation of the population resulted in high levels of inbreeding, with mean values exceeding 10% for lambs born from 2007 till 2012. Animal model was used to test the effects of inbreeding depression on lamb weight and survival. Significant inbreeding depression was found for birth weight (P < 0.01), and for body weight at the age of 8 weeks, −0.0071 kg and −0.0325 kg per 1% increase in inbreeding respectively. The effect of inbreeding on lamb survival was statistically significant, but very small. No significant impact of inbreeding on litter size was observed, due to the segregation of the major gene affecting fecundity. The main factors determining lamb weight and survival were its gender and type of birth.