Discussion

The aim of this study was to determine whether including the proportions of genes of foreign breeds in the model could improve genetic evaluation for performance in the Hanoverian as indicated by the results of Stewart et al. (2009), with respect to genetic evaluation for dressage of the sport horse population in Great Britain.

Possible optimization of genetic evaluation for performance through inclusion of some breed effect in the model has been previously shown by Vanderick et al. (2009) for a cross-breed dairy cattle population in New Zealand. For the German warmblood horse, especially the Hanoverian, the impact of different proportions of genes of other horse breeds on prediction of breeding values has not been investigated yet.

The breeding aim of the Hanoverian is defined as follows: a rideable, noble, big framed and correct warmblood horse, which, on the basis of its natural abilities, its temperament and character is suitable as a performance horse as well as a pleasure horse. On this basis the HSS strives for the breeding of talented sport horses for the disciplines dressage, jumping, eventing and driving. Genetic evaluation for performance in the main disciplines of riding sport, dressage and jumping is routinely performed using records from competitions and performance tests (Von Velsen-Zerweck, 1998).

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Genetic evaluation for performance

Publication of breeding values allows performance-oriented matings with regard to dressage, jumping or both. Development of MPT scores over time, as shown for the time period 1995 to 2008 in this study, illustrate the breeding progress particularly in the dressage related traits. In this connection, possible sale benefits may have caused that breeders tend to put more weight on dressage than on jumping talent of their foals. At young age, high prices are primarily achieved for foals with good movements.

Results from SBI, MPT and AI have been used for this study. SBI is obligatory for all Hanoverian broad mares, as well as MPT for dams of stallions born after 1990.

For all other mares, MPT is voluntary. Training for and participation in MPT is costly and time consuming, so breeder may only present their best mares. The much lower increase of SBI scores than of MPT scores may therefore also reflect preselection effects.

Unlike MPT, AI gives a more representative cross-section through the Hanoverian horse population, because riding horse auctions are organized by the HSS as a sale platform for their breeders. Horses of any gender and at different training level are presented. AI averages accordingly increased on a lower level than the MPT averages, but also indicated the continuing breeding process. The highly positive additive genetic correlations between analogous performance traits evaluated on the occasion of MPT and AI (Stock and Distl, 2006) justified the combined use of MPT and AI data in this study.

Moderate heritabilities, mostly ranging between 0.15 and 0.58 have been reported for those performance traits evaluated at MPT/AI and SBI in large numbers of horses (e.g. Lührs-Behnke et al., 2006; Stock and Distl, 2006; Stock and Distl, 2007).

Results of this study agree with previous estimates, regardless of which of the two models have been used. In the Hanoverian, breeding use of stallions and mares from other breeds is possible, given the agreement with the breeding directives.

Thoroughbreds and Trakehners are commonly used to make future progeny nobler, the intended use of Holsteiners is to improve jumping performance. Our data support that higher proportions of genes of Thoroughbred and Trakehner had a positively impact performance, particularly rideability. This is in line with the current

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Genetic evaluation for performance

recommendation of the HSS to increase the use of Thoroughbred stallions for breeding. The Holsteiner is especially bred for show jumping (Koenen et al., 2004), so that increased proportions of genes of this breed should be beneficial for free jumping performance in the present analysis.

Accordingly, extension of the model used for routine genetic evaluation by effects correcting for the influence of other breeds, resulted in slightly lower estimates of genetic variances. However, decrease of heritabilities was very small, and the impact on genetic evaluation was negligible. Correlations between corresponding breeding values of >0.98 indicate, that identification of genetically superior individuals will not be relevantly improved by using the extended model. Different results were recently obtained with regard to competition data for dressage performance (Stewart et al., 2009). However, the analyzed horse population was much smaller and more heterogeneous than the Hanoverian population analyzed here. Use of preselected data (scores better than 60% and of internationally competing horses) for a very wide range of horse types with limited pedigree information that performed under different and unknown conditions makes it plausible that stratifications by breed proportions can influence the results of genetic analyses. In the Hanoverian data analyzed here possible breed-related differences may have already been sufficiently accounted for by the relationship matrix. Therefore, genetic evaluation in the linear animal model without explicit consideration of the proportion of genes from Thoroughbred, Trakehner and Holsteiner resulted almost in identical results as the extended model.

These results indicate that analyses of performance data collected under standardized conditions for a large number of horses from the same breed will not relevantly benefit from model extension by breed class effects. Given the positive correlations between young horse performances in performance tests with later success in competitions (Wallin et al., 2003), breeding values estimated on the basis of MPT/AI and SBI information in the standard model should allow reliable identification of individuals for favourable breeding use. Performance-orientated mating will then ensure further breeding progress of the Hanoverian in the main disciplines of riding sport.

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Genetic evaluation for performance

Consideration of the proportions of genes of foreign breeds may be required when using different data sources (e.g., competition data from horses from different breeds) or focussing on identification of genome regions influencing performance.

Stratification of data by the proportion of genes of the most important foreign breeds may then facilitate using molecular genetic tools to speed up the selection response with respect to performance.