Modified contemporary comparisons for first and second lactations in the same and different herds.

Area of Science:

• Animal Breeding and Genetics
• Dairy Cattle Evaluation
• Statistical Genetics

Background:

• Accurate sire evaluation is crucial for genetic improvement in dairy cattle populations.
• Modified Contemporary Comparison (MCC) is a standard method for sire evaluation.
• The reliability of MCC for later lactations, considering potential biases, requires investigation.

Purpose of the Study:

• To assess the accuracy of first lactation sire evaluations for predicting second lactation performance in Holstein bulls.
• To identify and address potential biases in Modified Contemporary Comparison for later lactation records.
• To develop and evaluate an adjusted sire evaluation method for second lactations.

Main Methods:

• Calculated separate sire evaluations using Modified Contemporary Comparison for first and second lactations.
• Utilized data from 200 Holstein bulls, including first or second records and combined lactations.
• Employed two independent data subsets to ensure robustness of findings.
• Computed correlations between first and second lactation evaluations and analyzed culling bias.

Main Results:

• First lactation evaluations explained only approximately 75% of the variation in second lactation evaluations.
• Second lactation evaluations showed increased values with higher culling rates, indicating potential bias.
• An empirical adjustment based on first lactation records produced second lactation evaluations with similar means and standard deviations to first lactation evaluations.
• Correlations between first and adjusted second lactation evaluations were high (.87), while correlations with unadjusted second evaluations and culling were lower (.15 vs. .25).

Conclusions:

• First lactation sire evaluations alone are insufficient for accurate prediction of second lactation performance, necessitating later record evaluations.
• Culling bias can affect Modified Contemporary Comparisons for later lactations, highlighting the need for adjustments.
• The proposed adjustment method improves the reliability of second lactation sire evaluations and reduces the impact of selection bias.