Are there too many traits in our selection indices?

Area of Science:

• Animal Genetics
• Quantitative Genetics
• Dairy Science

Background:

• Selective breeding historically focused on appearance, shifting towards performance traits like milk yield.
• Breeding indices consolidate multiple trait performances into a single economic value metric.
• The Lifetime Net Merit (NM$) index has evolved from two traits in 1971 to 17 in 2025.

Purpose of the Study:

• To evaluate the effectiveness of USDA selection indices with increasing numbers of traits.
• To analyze correlated response and genetic gain trends over time for dairy cattle selection indices.
• To address concerns about "diluted" selection response in complex breeding indices.

Main Methods:

• Calculated correlated response to selection for 24 traits and composites across 12 USDA selection indices.
• Examined changes in index effectiveness and genetic gain rates over time.
• Assessed heritabilities and rank correlations between different index versions and cohorts.

Main Results:

• Indices with only yield traits showed the highest heritabilities (0.272-0.290).
• Lifetime Net Merit (NM$) index heritability increased from 0.188 (2010) to 0.243 (2025).
• Rank correlations between indices remained high (0.8746-0.9994), indicating consistent sire ranking despite complexity.

Conclusions:

• Properly constructed selection indices achieve desirable genetic gains across multiple traits simultaneously.
• Increased trait inclusion in indices alters individual trait response but not overall economic merit response.
• Selection gains for fat, protein, productive life, and BW composite remain economically significant.