1. Genetic evaluation of an index of birth weight and yearling weight
Michael MacNeil
USDA Agricultural Research Service
Miles City, Montana

2. I = yearling weight - 3.2(birth weight)
Proposed by Dickerson et al. (1974)

3. Objectives
Estimate genetic parameters for birth, weaning, yearling, and mature weights of CGC
Evaluate genetic responses resulting from selection based on the index

4. Selection Lines Established in 1989 Generation intervals 3 generations
Control line (n = 912)
Index line (n = 950)
Generation intervals
Control line 3.90  0.08 yr
Index line 3.16  0.04 yr
3 generations
212 kg selection differential

5. CGC population
Stabilized composite of ½ Red Angus, ¼ Charolais, and ¼ Tarentaise

6. Phenotypes Birth weight (n = 5,083) 200-d weight (n = 4,902)
365-d weight & index (n = 4,626)
Cow weight (n = 1,433; 4,375 obs)

7. Derivative-free multiple-trait REML
Calf traits
fixed contemporary groups
random direct & maternal additive effects
uncorrelated random maternal permanent environmental effects
Cow weight
fixed contemporary group effects,
random direct additive effects,
uncorrelated random permanent environmental effects

8. Three Sets of Analyses a single-trait analysis of the index
four 2-trait analyses of the index with birth weight, 200-d weight, 365-d weight, and cow weight
three 2-trait analyses of 365-d weight with birth weight, 200-d weight, and cow weight.

9. Heritability Estimates
Trait
Direct effects
Maternal effects
Index
0.32  0.04
0.05  0.02
Birth Wt.
0.49  0.05
0.11  0.03
200-d Wt.
0.30  0.04
0.19  0.04
365-d Wt.
0.04  0.02
Cow Wt.
0.70  0.04

10. Genetic Trends for the Index

11. Genetic Trends for Birth Weight

12. Genetic Trends for 365-d Weight

13. Genetic Trends for 200-d Weight

14. Genetic Trends for Cow Weight

15. Relationship of Index to Weight Traits & Response Versus to Selection for 365-d wt
Genetic
Correlation
With
Index
365-d wt
Relative response
Birth wt
0.11
0.71
13%
200-d wt
0.70
0.85
67%
0.90 -
73%
Cow wt
0.77
68%

16. Implications
Despite a genetic antagonism that compromises selection response for reduced birth weight and increased postnatal growth, favorable genetic responses can be achieved.
Selection for the index favorably affected the shape of the growth curve, restricting response in birth weight and mature weight of cows.
Selection intensity in experiment was reduced relative to that which would be available across a breed using national cattle evaluation and AI.