Inbreeding depression on beef cattle traits: estimates, linearity of effects and heterogeneity among sire-families

Records from up to 19 054 registered cows and 10 297 calves in 155 herds of the Alentejana cattle breed were used to study the effects of individual (Fi) and maternal (Fm) inbreeding on reproductive, growth and carcass traits, as well as assessing the importance of non-linear associations between inbreeding and performance, and evaluating the differences among sire-families in the effect of Fi and Fm on calf weight at 7 months of age (W7M). Overall, regression coefficients of performance traits on inbreeding were small, indicating a minor but still detrimental effect of both Fi and Fm on most traits. The traits with the highest percentage impact of Fi were total number of calvings through life and calf weight at 3 months of age (W3M), followed by longevity and number of calves produced up to 7 years, while the highest effect of Fm was on W3M. Inbreeding depression on feed efficiency and carcass traits was extremely small and not significant. No evidence was found of a non-linear association between inbreeding and performance for the traits analyzed. Large differences were detected among sire-families in inbreeding depression on W7M, for both Fi and Fm, encouraging the possibility of incorporating sire effects on inbreeding depression into selection decisions.