Genetic associations between pregnancy loss and economically important traits in Nellore cattle

Pregnancy losses (PL) affect calf production and economic efficiency in beef cattle operations. Here, PL was defined as failure from pregnancy diagnosis to calving, capturing a window approximately 60 days after the breeding season. In Zebu breeds, the genetic understanding of this phenomenon remains limited. The objective of this study was to estimate genetic parameters for PL in Nellore heifers and its associations with reproductive, growth, feed efficiency, and carcass traits. The dataset comprised 23,507 Nellore heifers with pregnancy diagnosis records (17,052 genotyped) from the Animal Genetics and Breeding Program (GMA), developed by the Animal Breeding and Biotechnology Group (GMAB) at the University of São Paulo, Brazil. Variance components and genetic parameters were estimated using a two-trait threshold-threshold animal model for PL jointly with the probability of early calving at 30 months of age (PP30) and stayability (STAY), and a two-trait threshold-linear animal model for PL with age at first calving (AFC), scrotal circumference adjusted to 365 days (SC365), cumulative cow productivity (CCP), body weight adjusted to 240 (W240) and 455 days of age (W455), rib eye area (REA), rump fat thickness (RFT), marbling (MAR), residual feed intake (RFI), and dry matter intake (DMI). For all traits, the single-step GBLUP methodology was applied. The two-trait analyses were carried out using Bayesian inference through the Gibbs sampling algorithm implemented in the GIBBSF90+ software. The estimated heritability (h2) for PL was low (0.033), indicating strong environmental influence. Genetic correlations between PL and reproductive traits (PP30, STAY, AFC, CCP, and SC365) ranged from -0.15 to 0.93. For growth (W240 and W455) and carcass (REA, RFT, and MAR), genetic correlation ranged from -0.16 to 0.01. For RFI and DMI, genetic correlations were -0.44, and 0.07, respectively. The direct genetic gain for PL was of low magnitude. The relative efficiency of selection indicates that direct selection for PL tends to be less efficient than indirect selection. Therefore, the results of this study demonstrate that, although PL shows low h2, it is favorably correlated with sexual precocity in females. Selection for growth and carcass traits is not expected to affect PL, whereas the observed antagonistic correlation with RFI suggests a potential increase in PL among more feed-efficient animals, although this effect is small and not conclusive.