Effect of inbreeding on reproductive, growth and morphological traits of Mertolenga cattle in Portugal

Abstract

Inbreeding is a common issue in small breeds under conservation and larger breeds under selection, leading to genetic diversity loss, increased homozygosity, and inbreeding depression. This study estimated the effect of inbreeding (F_i) and inbreeding rate (ΔF_i) on productive, reproductive and morphological traits and assessed the impact of including these parameters in the animal model used to estimate their genetic parameters. Genealogical and production data from the Mertolenga herdbook were analyzed using MTDFREML software and mixed models to estimate the effect of F_i and ΔF_i on different traits and their genetic parameters. Inbreeding depression was evaluated through trait regressions on individual and maternal inbreeding coefficients, with genetic parameters estimated with and without inbreeding as a covariate. Significant negative effects (P < 0.01) of inbreeding were found for reproductive traits, weaning weight, and carcass weight per day. Among morphological traits, F_i affected only MT2 (P < 0.01), while ΔF_i significantly impacted most traits (P < 0.01), except MT16 (P > 0.05). Inbreeding had an unfavorable effect on all the reproductive and growth traits analyzed. The largest differences in estimates of variance components and heritability between models with and without F_i were observed for productive longevity and total number of calvings. The results obtained suggest that ignoring inbreeding may lead to under- or overestimation of heritability and that the ΔF_i can provide a more accurate assessment of the cumulative negative impact of inbreeding over time, particularly where there are some gaps in the pedigree. Livestock improvement programs should balance genetic progress and genetic diversity to mitigate inbreeding effects.