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Genetic evaluation for somatic cell score with a test day model for multiple lactations

R Reents1, J C Dekkers, L R Schaeffer

  • 1Department of Animal and Poultry Science, University of Guelph, ON, Canada.

Journal of Dairy Science
|December 1, 1995
PubMed
Summary

A new multiple lactation model accurately predicts genetic merit for somatic cell scores in Canadian Holsteins using test day data. This practical approach improves genetic evaluations for udder health in dairy cattle.

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Area of Science:

  • Animal Genetics
  • Dairy Science
  • Quantitative Genetics

Background:

  • Somatic cell score (SCS) is a key indicator of udder health and milk quality in dairy cattle.
  • Accurate genetic evaluation for SCS is crucial for breeding programs aimed at improving herd health and reducing economic losses.
  • Previous genetic evaluation models for SCS often relied on lactation averages, potentially overlooking variations within lactations.

Purpose of the Study:

  • To develop and implement a practical multiple lactation test day model for national genetic evaluation of somatic cell scores in Canadian Holsteins.
  • To assess the efficiency and accuracy of the test day model compared to traditional single-trait and repeatability models.
  • To investigate the genetic associations between somatic cell score and other important production and conformation traits.

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Main Methods:

  • A multiple lactation model incorporating fixed effects (herd test date, lactation stage) and random effects (permanent environment, animal genetics, residual) was applied to test day SCS records.
  • Data from the first three lactations were treated as separate traits within the model.
  • The test day model's performance was compared with single-trait models using lactation average SCS, evaluating computing efficiency and animal ranking.

Main Results:

  • The multiple lactation test day model proved practical for national genetic evaluation of SCS in Canadian Holsteins.
  • Using starting values from previous evaluations expedited model convergence.
  • Differences in estimated breeding values (EBVs) between the test day model and repeatability models were minimal for bulls with extensive progeny, but significant differences emerged when compared to single-trait models based on first lactation averages.
  • Desirable genetic associations were observed between SCS EBVs and udder conformation traits, while undesirable associations were found with milk and protein yield EBVs.

Conclusions:

  • The multiple lactation test day model offers a practical and effective method for genetic evaluation of somatic cell scores in Canadian Holsteins.
  • This model provides more nuanced genetic insights compared to traditional lactation average-based models, particularly for differentiating genetic merit.
  • Genetic correlations suggest potential trade-offs between improving udder health (lowering SCS) and increasing milk production, highlighting the complexity of selection goals.