Genetic parameters for a composite trait of congenital testicular defects in Nellore cattle

Congenital defects of the male reproductive system can impair fertility and reduce the breeding potential of Bos taurus indicus cattle; however, individual defects occur at low frequencies in commercial populations. This study aimed to estimated genetic parameters for testicular defects in Nellore bulls, by grouping these abnormalities into a single, ordered categorical trait reflecting a continuum of reproductive soundness. Phenotypic records for cryptorchidism, testicular hypoplasia, monorchidism, and testicular asymmetry were obtained from the DeltaGen ® breeding program (1999–2023), comprising 50,205 phenotyped animals and 24,562 genotyped individuals. Due to the low incidence of each defect (0.38–2.80%), all abnormalities were combined into a single ordered categorical trait. Variance components were estimated using a single-step threshold animal model under a Bayesian framework implemented in GIBBSF90+. On the liability scale, the additive genetic variance (σ²ₐ) was 0.54 and the residual variance (σ²ₑ) was 1.11, yielding a heritability estimate of 0.33. When converted to the observed scale, heritability decreased to 0.07 due to unequal class frequencies. These results indicate that congenital testicular defects have a moderate genetic component but are strongly influenced by environmental and developmental factors. Although their low incidence limits immediate selection gains, consistent recording and genotyping of affected animals can enhance the precision of genetic evaluations and support long-term strategies for improving reproductive soundness in Nellore cattle.