Genetic relationships among yearling fertility, body composition and weight traits in tropically adapted composite cattle

Context. Reproduction is an economically important trait in both males and females; however, the relationships between fertility and body composition traits are little researched, but essential to breeding programs, as they will help inform selection decisions and allow the greatest opportunity for genetic gain. Aims. Estimate genetic and phenotypic correlations between male and female yearling fertility traits and investigate their relationship with yearling body composition traits, which have an effect on the attainment of puberty. Methods. Genotype and phenotype data were obtained from a tropical composite commercial cattle population and imputed to 27 638 single nucleotide polymorphisms. A series of univariate and bivariate linear mixed models using a genomic relationship matrix were run to estimate genetic parameters, genetic and phenotypic correlations for a series of male and female fertility and body composition traits. These parameters were then compared to help understand the genetic relationships. Key results. Scrotal circumference was favourably genetically correlated with weight (0.34), fat traits (0.06-0.24), muscle (0.24) and heifer days to calving (-0.32). Heifer days to calving was favourably correlated with muscle (-0.18) but not fat traits (0.11 to 0.21). The genetic correlations between heifer days to calving and sperm morphology traits were generally unfavourable (-0.32 to 0.25). Sperm morphology traits were favourably genetically correlated with fat traits (-0.84 to 0.31) and muscle (-0.61 to 0.31) but not weight (-0.15 to 0.09). Conclusions and implications. Yearling sperm morphology traits were unfavourably correlated with heifer days to calving, indicating that they are not good candidates for indirect selection on improving female fertility in the herd. A different trend was found for yearling scrotal circumference and heifer days to calving, identifying it as a good candidate for indirect selection of heifer fertility as it is easy to measure and heritable. The genetic correlations estimated between composition traits with male and female fertility traits allow breeding programs to make an informed selection decision to optimise genetic gain across all traits.