In-Depth Notes on Breeding Values and Livestock Evaluation

Breeding Values (BV)

  • Definition: A prediction of an animal's likelihood to possess and pass on favorable traits.

  • Purpose: Measures how useful an individual is for breeding future generations with desirable characteristics, in comparison to the population mean.

  • Example: A breeding value of +5 kg for weaning weight indicates that the bull is likely to transmit favorable genetics regarding weight.

  • Contextual Relevance: BVs are particularly relevant to industries like dairy and sheep, relative to a base year.

Estimating Breeding Values (EBVs)

  • EBV Definition: Estimated Breeding Value, which estimates an animal's true breeding value based on its performance (phenotype).

  • Estimation Formula: ext{EBV} = h^2(P - ar{P}) where:

    • h^2 = heritability

    • P = performance of the animal

    • ar{P} = mean performance of the herd/flock/population

  • Example Calculation:

    • Population mean for milk yield = 24 litres

    • Milk yield for Cow A = 27 litres

    • Heritability for milk yield = 0.4

    • Thus, ext{EBV} = 0.4(27 - 24) = 1.2 ext{ litres}

Properties of EBVs

  1. Units: EBVs have specific units such as kg for weaning weight or mm of fat.

  2. Relative Values: They can be positive, zero, or negative compared to others in the population.

  3. Reliability/Accuracy: EBVs have associated reliability values depending on the amount of data used in their estimation.

Reliability and Accuracy of EBVs

  • Importance: Reliability indicates how trustworthy an EBV is in predicting true genetic merit.

  • Factors Influencing Reliability:

    • Amount of information: more data enhances reliability.

    • Types of data sources:

    • Records of individual animals

    • Records from relatives (siblings, progeny)

    • Effective for traits that are costly or difficult to measure.

  • Accuracy Values: For example, a 30% accuracy indicates a lower confidence in prediction compared to a 90% accuracy.

Genomic Breeding Values (GBVs)

  • Definition: Utilize a large database to estimate the breeding values of unproven animals based on genetic information.

  • Advantages:

    • Reduced generation intervals

    • Access to more genetic information.

  • Disadvantages: Not 100% reliable; GBVs are considered until progeny demonstrate proven performance.

Factors Affecting EBV Accuracy

  • Record Quantity: More records, especially from progeny or full siblings lead to higher accuracy.

  • Age of Information: Record quality may vary with the age of the animal.

  • Environmental Corrections: Adjust for environmental factors like dam age or specific growth periods (e.g., 200 days vs. 400 days weight).

  • Heritability Levels: Changes in heritability significantly affect the reliability of breeding values, influencing accuracy based on breeding or genetic estimates.

Conclusion

  • When estimating breeding values, it's crucial to consider all relevant environmental and genetic factors to improve reliability and accuracy. This includes correcting for age, environmental conditions, and utilizing records from various relatives to maximize data.