To Start or Not to Start-A Matter of Genetics in Swedish Warmblood Horses?

The breeding goal of the Swedish Warmblood horse (SWB) is to produce internationally competitive horses in dressage and show jumping. In the current genetic evaluation, breeding values are estimated in multiple-trait animal models where competition performance is the target trait and results from two different young horse tests serve as indicator traits. However, preselection of horses, both for young horse tests and for competitions, is not considered in the current evaluation. The overall aim of this study was to analyse the all-or-none trait start status, in competition and in young horse tests, for possible use in the genetic evaluation for SWB. All starts in young horse tests have been recorded since long (1973), whereas start status in competition is known from the year 2007 and onwards. Therefore, the studied population was restricted to SWB horses born between 2003 and 2018 that had the possibility to compete during the period from 2007 until 2022. Horses were categorised into four disciplines according to their sire's and grandsire's discipline categories, and only horses in the two major categories, dressage and jumping, were included in this study. In total, 23,125 jumping horses and 14,470 dressage horses were studied separately. Information on discipline-specific start status in show jumping or dressage competitions, young horse test (YHT) and riding horse test (RHT) was available as well as lifetime accumulated competition points, assessed gaits and jumping traits from YHT and RHT. Out of the jumping horses, 31% had participated in YHT, 10% in RHT and 56% in show jumping competition. For dressage horses, the participation rates were 35% for YHT, 11% for RHT and 34% for dressage competition. The genetic analyses were performed with threshold and linear animal models. Horses that had participated in YHT or RHT had competed to a larger extent and had a higher mean of competition points than horses that had not participated in YHT or RHT. The heritability for start status in competition was estimated using a threshold model at 0.48 for show jumping and 0.39 for dressage. Using linear models, the heritability for start status in show jumping was estimated to be 0.30 on the observable 0/1-scale and 0.47 when transformed to the underlying continuous scale. For start status in dressage, the corresponding heritability estimates were 0.20 and 0.34. Genetic correlations, estimated with linear models, were strong between start status in show jumping and jumping traits at YHT and RHT (0.78-0.93) and moderate to strong between start status in dressage competition and gait traits at YHT and RHT (0.46-0.88). The genetic correlations between start status and accumulated lifetime points in competition were strong, 0.93 for show jumping and 0.86 for dressage. Using linear models, heritability estimates for start status in young horse tests ranged from 0.07 to 0.42 on the observable scale and from 0.11 to 0.71 after transformation to the underlying continuous scale. Inclusion of start status in the breeding value estimation of competition performance affected stallion ranking somewhat and increased the accuracies of the stallions' breeding values. We conclude that start status is a heritable trait that would be possible to include in the genetic evaluation of SWB horses.