Joint breeding value evaluation for fertility in Finnish and Norwegian blue foxes

We developed a multiple-trait animal model for blue fox fertility evaluation and estimated genetic parameters simultaneously for seven traits: first three litter sizes (LS), pregnancy rate (PREG), whelping success (WHELP), grading size (gSI) and fur quality (gQU). Grading size and quality were included into the new multiple-trait model as correlated traits. Litter size of the first parity had the highest unfavourable genetic correlations with gSI (-0.57) and gQU (-0.56). Thus, selection for higher gSI and gQU slows down the genetic gain in fertility traits. WHELP had moderate to fairly high negative genetic correlations with gSI and gQU (-0.44 and -0.36, respectively), indicating that larger animals are more likely to lose their pups during pregnancy or immediately after birth. Our new model corrected the slight overestimation of estimated breeding values (EBVs) especially for first litter size. The accuracy of LS, PREG and WHELP estimation is likely to benefit from the new multiple-trait animal model. PREG and WHELP improved steadily from 1998 to 2014, and LS traits have shown a moderate genetic trend since 2007, whereas the positive genetic trend for gSI has levelled down, at least temporarily. The fairly high effective population size (150) allows to increase selection intensity, and the new fertility evaluation enables further improvement of fertility traits.