Genetic and Phenotypic Correlations Among Growth, Disease Occurrence, and Reproductive Performance From Calf to Heifer in Girolando Dairy Cattle.

The Girolando breed, a cross between Holstein (Bos taurus) and Gir (Bos indicus), is the primary choice for dairy production in Brazilian tropical regions, as it combines high productive potential with adaptation to the tropical environment. Since the pre-weaning period is crucial for development, this study aimed to evaluate genetic and phenotypic associations among growth, health, and reproductive traits in Girolando cattle. The dataset included 15,311 heifers born between 2017 and 2024. Traits evaluated included birth weight (BW), weaning weight (W70), and average daily gain (ADG); health traits: diarrhoea (CD), pneumonia (BP), umbilical infection (UI), and tick fever (TF); and heifer reproduction: age at first calving (AFC). (Co)variance components were estimated using an animal mixed model under Bayesian inference. Single-trait analyses estimated heritabilities, while three-trait analyses estimated correlations among traits. Heritabilities ranged from low to moderate, with higher values for BW (0.27) and W70 (0.27), and lower for ADG (0.17), AFC (0.07), and diseases (0.04 to 0.13). Favourable genetic correlations were observed between growth and reproductive precocity, specifically ADG × AFC (-0.75), indicating that animals with higher growth performance tend to have reduced AFC. However, UI showed unfavourable genetic correlations with growth traits (BW = 0.75; W70 = 0.69; ADG = 0.50). Given that umbilical infections typically occur in the first weeks of life prior to the main growth period, these associations should be interpreted with caution, as they represent underlying genetic relationships rather than direct linear causality. Phenotypic correlations followed similar patterns but with a lower magnitude. These results confirm that growth, calf health, and reproduction are genetically correlated, reinforcing the importance of integrated selection programs to promote productive efficiency and calf health in tropical dairy systems.