Relevance of test information in horse breeding

The aims of this study were 1) to determine the role of test results of young horses in selection for sport performance, 2) to assess the genetic diversity of a closed horse breed and 3) the consequences of inbreeding for male reproduction. The study was performed using existing databases containing records collected on young horses during inspections, which were linked to databases containing records on sport performance. Multivariate animal models were applied in estimation of genetic parameters. Heritability estimates of movement and free-jumping traits collected at Studbook Entry and at First Stallion Inspection were moderate to high. Free-jumping traits collected at both inspections showed high to very high positive genetic correlations to show-jumping in competition. Movement traits collected at both inspections showed favourably genetic correlations to dressage in competition. Subsequently, the effect of limb and foot conformation, in particular the trait uneven feet, as assessed at Studbook Entry Inspection on performance and length of sport career have been analysed. Limb and foot conformation had only weak to moderate genetic correlation to sport performance. Some foot conformation traits could be identified as risk factors for early retirement from sports; e.g. occurrence of uneven feet shortened the competitive life at elite level of jumping. The development of genetic diversity in a closed breed have been studied using the pedigree structure of the Friesian horse breed. Considerable loss of genetic diversity have taken place during the history of the studbook, corresponding to an average inbreeding rate of 1.3% per generation. Loss of genetic diversity was mainly due to drift from small effective population size during several generations. In a subsequent analysis the relation of inbreeding with semen quality of young Friesian stallions was investigated. It was concluded that low semen quality in Friesian stallions could not be attributed to inbreeding. Heritability estimates for semen quality traits were moderate to high and had substantial variation. Selection can be used to improve semen quality. Finally, opportunities for improvement of the breeding program have been discussed. Selection potentials are calculated to gain insight in the relative importance of each of the young horse tests to the breeding program. Additional opportunities to increase selection response have been discussed.