The Suitability of test-day models for genetic evaluation of dairy cattle in South Africa

Abstract

In this study the possibility to change to test-day models for genetic evaluation of production traits and somatic cell score of South African dairy breeds (i.e. Ayrshires, Guernseys, Holsteins and Jerseys) was investigated. Fixed Regression BLUP Animal Models were therefore developed, using test-day records of the first three lactations as repeated measures of the same trait. Milk, butterfat and protein yields were included in multitrait evaluations. A permanent environmental effect was fitted across lactations. Heritabilities estimated were comparable with other yield and somatic cell score estimates obtained from test-day models. Breeding values of qualifying sires were presented to INTERBULL for participation in the March 2005 test-runs. Genetic correlations between South Africa and other participating countries compared well with those amongst other countries, participating in these international evaluations. Trend validation tests were successful for all traits and breeds except for somatic cell score of the Guernsey breed, due to insufficient data for this trait. South Africa is now participating in routine INTERBULL evaluations in order to obtain MACE (multiple across country evaluation) breeding values, using this methodology. Further refinement of the model was tested, i.e. inclusion of a fixed calving year effect in the model and pre-adjusting records for heterogeneous variances due to days in milk and parity. This was investigated for the Jersey breed and recommended for implementation in the other South African breeds. South Africa’s methodology is now more comparable to that of the leading dairy producing countries of the world.