Estimation of genetic parameters for the annual earnings at different race distances in young and adult Trotter Horses using a Random Regression Model

From a total of 231,609 records of individual earnings, registered from the racing performances of 4715 Spanish Trotter Horses which took place from 1991 to 2007, the annual earning (AE) was estimated for each horse. This produced a total of 99,419 records. The AE at different stages-both young horses (2 to 4 years old) and adult horses (over 5 years old) -and over different distances (from 1600 to 2700 m) were considered as different traits and a Random Regression Model (RRM) was applied to estimate the (co)variance components throughout the trajectory of the age groups and distances. The following factors were considered as fixed effects: the combinations of hippodrome/year of the race; the sex of the animal; the type of start; its racing experience; the effect of the number of races recorded for each horse; and the fixed regression of Legendre polynomials of distance on the AE. The number of races in the year was included as a covariate on this model. The animal (out of a total of 10,089 animals in the pedigree) and the permanent environment effect of individual (4715 animals with data) were considered as random and were modeled using a bi-variate RRM with a first order Legendre polynomial. The heritability estimated by distance ranged from 0.08 to 0.10 for young horses and from 0.10 to 0.14 in adult animals. Within the same age, the genetic correlations (rg) between different distances ranged from 0.69 to 0.99. For the same distances but across the age ranges, the rg ranged from 0.30 to 0.59, which means that the AE results in young and adult horses do not represent exactly the same traits. The changes of the relative accumulated predicted earnings in different stages of the horses' sporting lives were analyzed. Differences were detected in age and distance. Therefore, the use of RRM is highly recommended as a very useful tool, because it allows us to estimate the animal's breeding value through all the stages of its trajectory in racing competitions. This contrasts with the classic procedures used to estimate breeding value, where all the results of each animal are considered as the same traits.