Better hoof, better horse - genetic correlations between ability to race barefoot and performance in trotting horses

Harness racing is popular in several parts of the world, and the ability to race barefoot is an important trait for success because of its effect on racing time. Barefoot racing is also discussed in terms of animal welfare because not all horses have hooves that tolerate racing without shoes. In Swedish Standardbred trotters (SB) and Swedish-Norwegian Coldblooded trotters (CB), the proportion of barefoot races and the probability to race barefoot in any given race (barefoot status) have low-to-moderate heritability (h² = 0.07-0.28). However, the genetic correlation between the ability to race barefoot and performance in harness racing has not previously been studied. Therefore, this study aimed to estimate the genetic correlations between the barefoot traits and performance traits. Three- to ten-year-old horses born from 2002 to 2018 (SB) or from 2002 to 2017 (CB) were included for the two barefoot traits. The horses were required to have raced at least ten (SB) or five times (CB). For the proportion of barefoot races, 24 928 SB with 724 232 observations and 4 050 CB with 97 682 observations were included. For barefoot status, 25 973 SB with 875 056 observations and 3 384 CB with 93 376 observations were included. Data for racing performance consisted of summarised records for 115 185 SB aged 2-5 years born 1976-2019, and 16 360 CB aged 3-6 years born 1978-2017. The performance traits included best racing time per km, summarised earnings, and placings. For SB, also earnings per start and number of starts were included. Genetic correlations and breeding values were estimated with bivariate mixed linear animal models for each breed. The absolute value of the genetic correlation between the proportion of barefoot races and performance was 0.42-0.50 in SB and 0.41-0.62 in CB. For barefoot status, the corresponding genetic correlation for SB was 0.01-0.63 and for CB 0.26-0.59. Mostly, for performance traits, a stronger genetic correlation was found with the proportion of barefoot races than with barefoot status. The annual genetic improvement was larger for the proportion of barefoot races than for barefoot status for both breeds. In conclusion, the proportion of barefoot races and barefoot status was favourably correlated with performance. These novel traits seem to add information beyond performance and could be of interest for inclusion in the genetic evaluation of SB and CB, and possibly also for other trotter populations.