Genetic evaluation of Swedish standard‐bred trotters for racing performance traits and racing status
Authors: Árnason T. 
Published: March 2003

Introduction

The BLUP methodology has been applied to the genetic evaluation of Swedish standard‐bred trotters for well over a decade. Initially a BLUP sire model was applied for evaluating breeding values of stallions on the basis of the racing performance of their offspring (Árnason et al. 1989). In 1992 an animal model (AM‐) BLUP was introduced and official publication of index values were offered as an aid for effective selection of stallions and brood mares (Árnason 1992; Árnason and S
vendsen 1991). The implementation of the BLUP animal model index has apparently caused profound enhancement in the selection intensity for both stallions and mares (Árnason 1997). The AM‐BLUP evaluation has involved the following traits, which were all based on accumulated racing results as 3‐ to 5‐year‐olds: Number of races (starts); % of races ranked first to third at the finish (i.e. placed first to third); earnings per race; total earnings; best racing time per km; and racing (or start) status, which is denoted as one if the horse did race at least once as a 3‐ to 5‐year‐old and zero otherwise. The two traits: number of races and racing status were initially treated as single traits, in the AM‐BLUP analyses, uncorrelated with the other racing performance traits. Those other traits are direct measures of racing performance for horses that have actually competed on the race track. They are highly intercorrelated and have been treated as a multivariate complex in the analyses. The computational load of general multivariate analyses has been considerably reduced by transformation of the records into uncorrelated canonical variates (e.g. Árnason 1982).

In Sweden, about 40% of standard‐bred trotters do not enter a race course and their racing performance variables have previously been treated as missing. Early analysis on the population of standard‐bred trotters in Sweden did not show any correlation between racing status of brood mares and the racing performance of their offspring (M. B endroth, unpublished results). That gave justification for assuming that racing status had mainly environmental causes, and to be practically genetically uncorrelated with performance. The exclusion of nonracers (nonstarters) was therefore not expected to bias genetic evaluations for the performance traits. The expansion of the population during the 1980s and the early 1990s, and increase in the level of prize money, has probably invalidated the implication of these results and partly changed the racing status into a preselection criterion for racing performance. K
lemetsdal (1992) has clearly illustrated that such a culling process can result in substantial bias in estimated breeding values. In 1995 a new enhanced procedure replaced the older version used for routine genetic evaluation of Swedish standard‐bred trotters, with the aim of eliminating, or at least reducing, the selection bias and increasing the accuracy in the genetic evaluations.

The main objective of this paper was to describe the enhanced procedure for genetic evaluation of Swedish standard‐bred trotters where racing status and racing performance traits were treated in a complete multiple trait framework. The task was made computationally feasible by the application of a procedure which allows solution of multiple trait AM‐BLUP with missing data on some traits using multiple step canonical transformation of records and solutions during the iterative solution phase (D ucrocq and B
esbes 1993). The pertinence of the method for establishing a sound selection criterion was verified by stochastic simulations on simplified data structures. The second objective of this study was to evaluate the model and assess the set of genetic parameters used in the BLUP analysis, by linear regressions of the genetic predictions based on the most recent data on genetic predictions obtained in subsamples of the whole data set (method R, R
everter et al. 1994a, b).