Genetic evaluation of dairy cattle using test-day models

Area of Science:

• Animal Genetics
• Dairy Science
• Quantitative Genetics

Background:

• Traditional genetic evaluation in dairy cattle relies on estimated 305-day yields.
• Test-day models (TDM) are gaining interest for genetic evaluations using individual test-day records (TDR).
• TDM offer benefits like accounting for daily environmental effects and modeling lactation curves.

Purpose of the Study:

• To explore the advantages and disadvantages of test-day models (TDM) for dairy cattle genetic evaluation.
• To review various models proposed for lactation curve trajectory in TDM.
• To discuss challenges in estimating dispersion parameters for TDM and suggest future research directions.

Main Methods:

• Comparison of TDM with traditional 305-day lactation models.
• Review of different lactation curve modeling approaches (biological functions, polynomials, character process models).
• Discussion of two-step and one-step procedures for estimating dispersion parameters in TDM using covariance functions, restricted maximum likelihood, or Bayesian methods.

Main Results:

• TDM allow for genetic evaluation of production persistency and avoid issues with short or incomplete lactations.
• Disadvantages of TDM include computational complexity and the need to estimate more parameters.
• No universal agreement exists on the best TDM for routine breeding value prediction, highlighting the need for model comparison methods.

Conclusions:

• Test-day models (TDM) provide a more detailed approach to dairy cattle genetic evaluation than traditional methods.
• Accurate estimation of dispersion parameters and further research into multi-lactation data and variance heterogeneity are crucial for TDM advancement.
• Developing better methods for comparing different TDM is essential for routine implementation.