The use of test day models with small size of contemporary groups

Abstract

IntroductionFor many years genetic evaluation of dairy cattle has been based on the analysis of 305‐day lactation milk production. Thus, to account for environmental variance, records are usually grouped in herd–year–season (HYS) classes. The 305‐day milk yield is computed on the basis of single test day records. An alternative approach to genetic evaluation is an analysis of individual test day records (P tak and S chaeffer 1993). A complete set of records contains more information than 305‐day milk yield, and, in consequence, by including the single herd–test date (HTD) effect, this approach leads to an improvement in the accuracy of genetic evaluation. Modeling the time–herd effect in this way reduces residual variance. In populations with a small herd size a number of records with no contemporaries in HTD classes could appear. It should be noted that accurate estimation and prediction in mixed linear models requires both model adequacy and respective data structure. Unfortunately, inclusion of a HTD effect when herds are small (typical situation for European cattle breeding) causes a decomposition of data structure (more unbalanced experimental design). Hence, a modification to the grouping of the records was recommended (S walve 1995; S trabel 1997).When a lactation model is used various modifications to HYS definition could be implemented to improve its adjustment to environmental effects (S chmitz et al. 1991; S trandberg and G randinson 1997; C rump et al. 1997). The definition of herd–test day class is so precise that it does not allow for a different assignment of records to classes. What could be done is to combine HTD classes within the herd by joining the consecutive records (S walve 1995) or across herds looking for records from ‘similar’ herds taken at ‘similar’ periods of time (S trabel 1997). HTD effect can also be set as random to avoid the loss of information especially when the number of single HTD classes is high (R eents et al. 1995; P& ouml; s& ouml; et al. 1996). Finally, the HTD effect can be replaced by HYS, typical for the traditional model (P tak and Żarnecki 1998).The objective of the present study was to compare animal model evaluations using test day (TD) models with different definitions of contemporary groups (CG). Evaluations based on the 305‐day lactation model were also carried out.