Genetic Parameters for Various Semen Quality and Reproductive Performance Traits in Nellore Cattle Including X Chromosome Markers

Abstract Estimating genetic parameters of male and female fertility and reproduction traits is essential for optimizing beef cattle breeding programs. In general, only markers located on autosomal chromosomes are used for genetic evaluations. Sex chromosomes tend to be excluded due to the compensation mechanism of dosage of genes linked to the X chromosome, which is the second largest chromosome in the cattle genome and harbors many genes potentially influencing these traits. Therefore, we estimated genomic-based variance components, including the X chromosome in the variance components analysis for female precocity, fertility, and andrological traits based on the single-step GBLUP procedure and Bayesian inference. The dataset consisted of ~120,000 records for the probability of pregnancy at 14 months (PP14), early rebreeding (REBA), and stability (STAY). From the breeding soundness exam (BSE) of bulls, 20,000 records were obtained for semen quality, andrological biometry, semen defects, and bull fitness (BULL_FIT). The pedigree contained 660,608 animals, where 11,012 animals were genotyped for 770,111 SNPs. All traits have reasonable additive genetic variation and are heritable, with a substantial contribution from the X chromosome, indicating that genetic progress for these traits is feasible. The heritability estimates (h2) explained by autosomal and X chromosome SNPs was low and low-to-moderate, respectively, for female reproductive performance traits, such as REBA [0.17 (auto); 0.07 (sex chr)], PP14 (0.42;0.05), and STAY (0.32;0.01). For male traits, the heritability estimates based on the autosomes and X chromosome, respectively, ranged from 0.02 to 0.04 and from 0.01 to 0.03 for semen quality traits; from 0.10 to 0.68 and from 0.02 to 0.07 for morphological traits; from 0.10 to 0.12 and from 0.01 to 0.02 for semen defects traits; for bull evaluation (BULL_FIT) were observed h2 for autosomes 0.18 and 0.10 for sexual chromosome X. The results observed in this study indicate a substantial contribution of the X chromosome on fertility and reproductive performance traits. These findings will serve as background information for designing breeding programs for genetically improving semen production and quality and reproductive performance in Nellore cattle.