Genetic analysis of ‘breeding field test status’ in Icelandic horses

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Abstract
Genetic evaluation of Icelandic horses is currently based on results from breeding field tests where riding ability and conformation of the horses are evaluated over the course of 1-2 days. Only a small part of registered horses attend these field tests, and it can be assumed that these are not a random sample of the population. In this study, the trait test status was introduced, describing whether a horse was assessed in a breeding field test. This trait was analysed to find out whether it has a genetic variation and how it correlates genetically to other traits in the breeding goal. Breeding field test data included 39,443 mares born in Iceland in 1990-2001, of which 7431 were assessed in the period 1994-2007. The trait was defined in relation to age, gender and stud of horses. Variance and covariance components were estimated using the Markov Chain Monte Carlo method by applying the Gibbs sampler procedure in the DMU program. Three multivariate analyses were performed where the test status trait was analysed with breeding field test traits. Animal models and sire models were applied. Based on estimated heritabilities (0.51-0.67) and genetic correlations (0.00-0.87), the test status trait showed significant genetic variation and was strongly correlated to some traits. The test status trait reflects preselection in the breeding field test traits and should be included in the genetic evaluation to enhance the procedure, reduce selection bias and increase accuracy of the estimation.