trait selection

  • Recap of Previous Session

    • When estimating breeding values (EBVs), corrections for various factors are necessary:
    • Environmental Effects: Includes head effects and various conditions affecting animal performance.
    • Age and Sex: Important factors influencing the results.
    • Genetic Linkage: To convey EBVs of groups (herds/flocks), there must be genetic connections between animals, often established using link sires or reference sires.
    • Common Practices:
    • Use of Artificial Insemination (AI) for establishing genetic links between herds.
    • Central project testing stations allow multiple sires to be evaluated under uniform environmental conditions.

  • Multiple Trait Selection

    • Complexity of Breeding Objectives: Most breeding goals involve multiple traits rather than a single one. For instance, the dairy industry considers multiple traits—usually around 10.
    • Selection Objectives vs. Selection Criteria:
    • Selection Objectives: Traits intended for improvement.
    • Selection Criteria: Measurable traits used in practice.
    • Correlations Between Traits:
    • Positive, negative (adverse), or complementary correlations affect selection strategies.
    • Selection for one trait can influence the others due to shared genetic control.
    • Difficult-to-measure Traits:
    • Some traits, like disease resistance, may necessitate the use of indicator traits—traits that are easy to measure and correlate with difficult-to-measure ones.
    • Economic Considerations: Traits that are expensive to measure may not be prioritized unless the benefits outweigh costs.

  • Examples of Indicator Traits:

    • Daily feed conversion efficiency may correlate with live weight and be a feasible measure for selection.
    • Greenhouse gas emissions might also be inferred from related traits like live weight in livestock systems.

  • Selection Methods

    • Tandem Selection:
    • Select one trait at a time until satisfactory progress is achieved before moving to the next.
    • Disadvantages: Time-consuming; risk of undoing previous progress due to correlations.
    • Independent Culling Levels:
    • Set minimum levels for traits. An animal must meet all criteria to be kept.
    • Drawbacks: Prone to losing high-performing animals that fall short in minor traits.
    • Selection Index (Most Effective Method):
    • Aggregate values for multiple traits are computed, creating an index for ranking animals, allowing simultaneous consideration of all traits.
    • Weighting factors are applied to reflect the economic importance of traits.

  • Weighting Factors:

    • Determine how each trait contributes economically to farm profitability.
    • Example: In New Zealand, penalties exist on traits like milk volume due to the economic implications of water content in transported milk.

  • Indices Used in Dairy Industry:

    • Breeding Worth Index:
    • Combines breeding values with relative economic values to generate an effective index.
    • Economic values are updated annually, reflecting changing circumstances and perceptions.
    • Important traits include milk fat, protein, and live weight, each with assigned economic values.

  • Production Worth Index:

    • Focuses on traits related to current production capability rather than genetic potential for offspring.
    • Changes in public perception of traits can shift their economic weighting.

  • Conclusion on Indices:

    • Indices and their calculations are dynamic and can evolve over time as the industry goals and economic landscapes change.
    • A change in trait evaluation or public perception can significantly alter how animals are ranked and selected in breeding programs.

  • Final Notes:

    • Be mindful of the relativity and changing nature of indices concerning farm productivity and breeding objectives.
    • Questions about the course material are encouraged to clarify understanding before practical applications in lab sessions.