Early genetic selection of Spanish Trotter Horses by new precocity criteria

National trotting races held in Spain between 1990–2022 were used to estimate genetic para-meters for 5 precocity criteria: age at first ranked race (AFR), age when the animal reached 50% of ranked races (A50%), age when the animal reached the first 10 ranked races (A10), total number of races in which the animal has competed before the first ranked race (NRC) and percentage of first placings/year of participation (PFP), in order to evaluate their usefulness for genetic evaluation and their possible inclusion in the Spanish Trotter Horse official genetic program. A total of 176,137 records belonging to 4947 animals were used. A Bayesian multivariate BLUP animal model showed heritability values between 0.08 (NRC) and 0.44 (A10). Medium-high genetic co-rrelations were obtained between precocity traits, ranging between −0.78 (A10-PFP) and 0.96 (A50%-A10). NRC and PFP exhibited negative correlations, with the lowest range for NRC. Additionally, a Random Regression Model (RRM) was applied to PFP to evaluate changes in performance during the animals’ sporting life. Heritability values decreased slightly up to 4 years of age (0.26–0.17), with a slight increase after the age of 5 (0.22). Genetic and permanent environmental correlations between age groups were in the high range between the nearest age groups. Variations of the expected breeding values (EBV) over the trajectory of all the age groups allowed us to differentiate robust and plastic animals (animals capable of maintaining sporting potential throughout their lives or not). Correlations between the EBV estimated with both methodologies evidenced the benefits of PFP for the early selection of performance traits in STH. Therefore, the application of an RRM for the PFP trait is recommended as it has shown acceptable heritability and adequate correlations with the rest of the precocity criteria studied. The incorporation of this trait in the breed improvement program will allow breeders to have early genetic information on animal precocity potential and ensure the inclusion of more robust individuals in their mating design.