Sire × stud interaction for body measurement traits in Spanish purebred horses.

The mating policy for Spanish Purebred horses encourages the same or closely related stallions to be used for reproduction across studs differing in environments. Stallions may perform differently across studs, and if so, adjustment for the sire x stud (ss) interaction is recommended. However, there are no studies available that analyzed the influence of ss interaction on estimation of genetic parameters in horses. This research is aimed at assessing the implications of including ss interaction in models fitted for the estimation of genetic parameters for traits under selection using 8 body measurements in Spanish Purebred (Andalusian) horses: height at withers, height at chest, leg length, body length, width of chest, heart girth circumference, knee perimeter, and cannon bone circumference. Body measurements were available for 11,894 horses. Genetic parameters and EBV were estimated using animal models by REML. The inclusion of the ss interaction gives reduced estimates of heritability. Generally, the genetic correlations estimated using the models including the ss interaction were greater than those obtained using the simpler model. Estimates for the ss variance as a proportion of the total phenotypic variance ranged from 0.02 (height at withers) to 0.11 (width of chest). Estimates between the ss interaction correlations associated with each body measurement were positive, significant, and from moderate to low. The range of EBV estimated using the model including the ss interaction decreased with respect to those estimated using the simpler model, and the EBV ranking order was affected. The models including ss interaction fitted significantly better. Therefore, the genetic parameters estimated using the ss interaction may be more realistic, and the obtained (co)variances may be assumed to be closer to the true population parameters. This interaction may result from selective decisions or preferential management practices causing greater similarity between the progeny of a sire in the same stud. It is therefore advisable to include it in genetic models used in horse breeding schemes.