Phenotypic associations between linearly scored traits and sport performance in the Swedish Warmblood horse population

Abstract

The goal for most warmblood studbooks is to produce horses that are internationally competitive in sports like show jumping or dressage. The linear scoring system, describing the horse between two biological extremes, is commonly used for a more objective assessment of young horses in many studbooks. However, few studies have examined the phenotypic association between traits linearly scored at a young age and sport performance, and whether there might be an intermediate optimum on the linear scale. This study investigated the phenotypic association between linearly scored traits and competition performance in show jumping or dressage, using the results of linear scoring from young horse performance tests between 2013 and 2021 and competition data between 2014 and 2021 for Swedish Warmblood horses. Sport performance was defined as lifetime accumulated points achieved in show jumping or dressage competitions. Horses were classified as jumping (J) or dressage (D) horses according to their sires’ and grandsires’ classification. In total, 48 linearly scored traits, assessed on a biological scale from A to I, were analyzed. The phenotypic association between the linear score for each trait and sport performance was studied using linear models for sport performance including fixed effects of sex, birth year and linear and quadratic regression on adjusted linearly scored trait values. Significant differences in LS means between J and D horses were found for all linearly scored traits except for length of body and five traits referring to leg conformation. For J horses, 25 linearly scored traits (eight conformation traits, three gait traits, 13 jumping traits and one behavior trait) were found to be significantly associated (p < 0.05) with show jumping performance. A majority of these traits (21 out of 25 traits) showed a linear association with performance, indicating that stronger expression (towards A on the assessment scale) was favorable for performance in show jumping in J horses. For D horses, 21 linearly scored traits (eleven conformation traits and ten gait traits) were significantly associated with dressage performance. Most of these traits (15 out of 21 traits) showed a linear association with performance, while six traits showed an association with optimal scores, indicating that breeding for more extreme expression of the specific trait is not associated with better sport performance. These results underline the importance of linearly scored traits as indicator traits of later sport success.