Direct–Maternal Genetic Covariance in Ruminants: Magnitude, Direction, and Implications for Growth Trait Improvement

This systematic review examines the correlation between direct and maternal genetic effects on growth traits in sheep, goats, and cattle, with a focus on tropical production systems. Reported estimates of this correlation exhibit significant variability, ranging from strongly negative to moderately positive, reflecting considerable uncertainty and biological complexity. The sign and magnitude of this relationship are crucial in genetic evaluation, as they influence the estimation of breeding values and ultimately impact selection decisions. Recent literature emphasizes that omitting or inaccurately estimating the covariance between direct and maternal genetic effects can result in biased rankings of selection candidates and diminished genetic gains, particularly for pre-weaning traits where maternal influence is most significant. The variability in estimates is affected by several factors, including breed differences in maternal ability, environmental and management conditions, model specification (e.g., fixed assumptions of zero covariance), and the structure and quality of pedigree and performance data. In tropical systems, where environmental stressors and incomplete records are prevalent, these issues are especially pronounced. This review highlights the necessity for appropriate statistical modeling that accounts for direct maternal covariance to enhance the accuracy of genetic parameter estimation. For animal breeding programs in tropical regions, such considerations are essential for improving the efficiency and sustainability of genetic improvement strategies.