Adjustment of heterogenous variances and a calving year effect in test-day models for national genetic evaluation of dairy cattle in South Africa

  • BE Mostert ARC-LBD, Private Bag X2, Irene 0062, South Africa
  • HE Theron ARC-LBD, Private Bag X2, Irene 0062, South Africa
  • FHJ Kanfer Department of Statistics, University of Pretoria, Pretoria 0002, South Africa
  • E Van Marle-Köster Department of Animal and Wildlife Sciences, University of Pretoria, Pretoria 0002, South Africa
Keywords: Fixed regression, Jersey, ranking, repeatability model


South Africa implemented test-day models for genetic evaluations of production traits, using a Fixed Regression Test-Day Model (FRTDM), which assumes equal variances of the response variable at different days in milk, the explanatory variable. Data at the beginning and at the end of lactation period, have higher variances than tests in the middle of the lactation. Furthermore, first lactations have lower mean and variances compared to second and third lactations. This is a deviation from the basic assumptions required for the application of repeatability models. A modification was therefore implemented to reduce the effect of deviating from this assumption. Test-day milk, butterfat and protein yield records of Jersey cows, participating in the South African Milk Recording Scheme, were therefore pre-adjusted such that the variances are on the same scale. Variance components estimated using the adjusted records were higher than using unadjusted records. Convergence of breeding value estimation is reached significantly faster when using adjusted data (± 4000 iterations) compared to unadjusted records (± 15 000 iterations). Although cow and bull rankings were not influenced much, significant changes in breeding values for individual animals and genetic trends of especially young animals, were found.

South African Journal of Animal Science Vol. 36(3) 2006: 165-174

Journal Identifiers

eISSN: 2221-4062
print ISSN: 0375-1589