1. Heterosis: Defined and Research Experience
Scott P. Greiner, Ph.D.
Extension Animal Scientist, Beef
Department of Animal & Poultry Sciences
Virginia Tech

2. Can We Have It All??? Reproductively efficient cow herd
Cows that are low-cost, adaptable to feed and environmental resources
Superior growth/feed efficiency
End product merit

3. Crossbreeding Fundamentals
Heterosis (Hybrid Vigor)
Individual heterosis
Maternal heterosis
Paternal heterosis
Breed complementarity

4. Heterosis Defined -Superiority of crossbred animal relative to average of its straightbred parents
Breed A Weaning Wt. = 530 lb.
Breed B Weaning Wt. = 470 lb.
A x B Crossbred Calf
Expected performance 500 lb. (average of A and B)
Actual performance = 520 lb.
20 lb. (4%) increase = heterosis

5. Genetic Basis of Heterosis
Additive vs. non-additive gene effects
Additive: favorable effect on performance results from increase in homozygosity (heterozygote intermediate to homozygotes)
Non-additive: favorable effects realized through increase in heterozygosity
Dominance and epistasis

6. Dominance

7. Economically Important Traits
Reproductive efficiency
Calving ease
Calf survival
Weaning Wt.
Post-weaning growth
Feed efficiency
Mature size
Red meat yield
Palatability

8. Individual Heterosis Advantage of the Crossbred Calf
Cundiff and Gregory, 1999

9. Carcass Traits
Long, 1980

10. Maternal Heterosis
Cundiff and Gregory, 1999

11. Impact of Heterosis Heritability Low Moderate High Heterosis High Low
Reproduction
Growth
Carcass Merit

12. Maternal Heterosis Advantage of the Crossbred Cow
Advantage of crossbred cow vs. straightbred
Reproductive efficiency
Maternal ability
Longevity
Increased lifetime productivity
Maternal heterosis accounts for largest portion of total heterosis advantage (60%)

13. Estimating Heterosis Expected Performance = General purebred mean
+ ½ sire breed direct value
+ ½ dam breed direct value
+ dam breed maternal value
+ individual heterosis
+ maternal heterosis

14. Weaning Weight Example
General mean = 600
Individual heterosis = 4.0%
Maternal heterosis = 4.0%

15. Sire C x crossbred A-B dam
Calculated performance with direct and maternal breed values
(40)+ 0.25(20) (-10) + 0.5(10) + 0.5(30) = 642.5
Add individual heterosis
(642.5) = 668.2
Add maternal heterosis
(668.2) = 694.9

16. Paternal Heterosis Advantage of the Crossbred Sire
Advantage in reproductive traits
Realized primarily when single sires mated to high numbers of cows (> 40)
Difficult to measure due to large influence of female in total herd reproductive efficiency

17. Breed Differences Sire breed of calf Gestation length, d Unassisted
calvings, %
Birth
weight,
lb.
Survival
to wean.,
200-d
wean. wt.,
Hereford
284
95.6
90.4
96.2
524
Angus
282
99.6
84.0
96.7
533
Red Angus
99.1
84.5
526
Simmental
285
97.7
92.2
553
Gelbvieh
97.8
88.7
97.1
534
Limousin
286
97.6
89.5
96.9
519
Charolais
283
92.8
93.7
540
source: Cundiff et al., 2001, Germplasm Evaluation Program Progress Report No. 21

18. Effect of breed type on level of heterosis
Cundiff and Gregory, 1999

19. Heterosis: Bottom Line
Heterosis offers best genetic solution for improvement of lowly heritable reproductive traits
Majority of heterosis advantages realized through crossbred dams

20. Considerations Breed contributions Optimizing heterosis
Heterosis retention
Interaction with environment
Economics of crossbreeding
Sustainable systems to capture heterosis

21. Heterosis: Defined and Research Experience
Scott P. Greiner, Ph.D.
Extension Animal Scientist, Beef
Department of Animal & Poultry Sciences
Virginia Tech