VARIANCE COMPONENTS AND GENETIC PARAMETER ESTIMATES UNDER VARYING DEFINITIONS OF CONTEMPORARY GROUPS

Abstract

A total of 19 385 test day (TD) milk yield records were used to determine the effect of contemporary group (CG) on estimates of variance components and genetic parameters for test day milk yield (TMY) data of Sahiwal cattle in Kenya. Variance components and genetic parameters were estimated based on a derivative free restricted maximum likelihood procedure (DFREML) using multivariate and univariate TD models that defined CG either based on the year-season of calving (YSCV) or on the year-season of TD milk sampling (YSTD). Additive genetic and permanent environmental variances were higher under the YSTD model than YSCV model across the 3 lactations. The estimates of heritability were higher in the YSTD model than in the YSCV model in all lactations under both analyses. The study has shown that fitting YSTD as CG allows for a direct correction for environmental affects specific to the day of recording of a cow's performance.