By Dr. John Comerford
Purpose of Crossbreeding
- To take advantage of heterosis
- To use average breed effects
- To design a cow herd
- To target markets
- Create breeding plan for a herd
Heterosis is defined as the difference in the value of a trait compared to the average value of the parents for the trait. For example, if the average value for weaning weight of Breed 1 is 500 lbs. and the average value for Breed 3 is 600 lbs. and the resulting calf crop after mating these two breeds averages 580 lbs., then heterosis for weaning weight is 30 lbs. or 5.5%. This extra weaning weight is free because you did nothing more than use a different breed.
Tables 1 and 2 show that heterosis is not consistent from one breed to another. Breeds that are more genetically different (Brahman and Hereford) will exhibit more heterosis than breeds that are more genetically alike (Simmental and Limousin). The greater the difference in breeds, the greater the effect.
Table 1. Heterosis and Cattle Breed: Weaning Weights (%)
| Breeds | Hereford | Limousin | Brahman |
|---|
Comerford, 1987. |
| Simmental | 5.3 | 0.4 | 8.9 |
|---|
| Hereford | | 4.2 | 9.9 |
|---|
| Limousin |
|---|
A second genetic effect in crossbreeding is the average effect of a breed in crosses. It can be shown breeds can make a specific difference in crossbred progeny, such a marbling in Angus cattle, ribeye size in Limousin cattle, and milk production in Simmental cattle. Unfortunately, this feature is often over-emphasized in a crossbreeding program. The additive effect of a breed in crosses will generally have less effect on the calf than the direct genetics for the trait passed from the parent.
A word of caution
Just because you mate cattle of different breeds does not mean there will be a large heterotic or average breed effect. Selection of the parents for their potential genetic contribution to a trait (called the additive genetic effect) will be more important than heterosis or breed effects. The use of EPDs and other selection tools within a breed should not be discarded for the sake of heterosis. Heterosis will not improve poor cattle.
Adapted from Cundiff et al., 1994
Table 2. Average heterosis in the economically important beef traits when crossing divergent breed types of cattle.
Trait:
Individual Heterosis
(Calf Performance) | Bos taurus X Bos taurus % | Bos indicus X Bos taurus % |
|---|
| Adapted from Cundiff et al., 1994 |
| Birth Weight | 2.4 | 11.1 |
| Weaning weight | 3.9 | 12.6 |
| Post-weaning gain | 2.6 | 16.2 |
Trait:
Maternal Heterosis
(Cow Performance) | Bos taurus X Bos taurus % | Bos indicus X Bos taurus % |
|---|
| Calving rate | 3.7 | 13.4 |
| Calf survival | 1.5 | 5.1 |
| Birth weight | 1.8 | 5.8 |
| Weaning weight | 3.9 | 16.0 |
Crossbreeding improves more lowly heritable traits
Heritability describes the proportion of the variation in a trait due to genetics as compared to the environment (nutrition, health, etc,). More lowly-heritable traits-such as milk production, longevity, reproductive fitness-will result in more heterosis than highly heritable traits such as carcass traits. Table 3 shows how important traits vary in heritability.
Table 3. Heritability and Heterosis
| Traits | Heritability | Heterosis |
|---|
| Reproduction | low | high |
| Growth | medium | medium |
| Carcass | high | low
|
An important feature of crossbreeding is maternal heterosis, which can be described as the advantage of the crossbred cow in the mating system. A review by Reuter (2001) of several crossbreeding experiments showed that crossbred cows had a 9 percent advantage in calving rate and an 8 percent advantage in calf weaning weight over their straight-bred counterparts.
Crossbreeding adds consistency to a breeding program
A crossbreeding system must be a planned process that takes advantage of breed effects and heterosis or it becomes chaos. To effectively design a crossbreeding system, use these standards:
- Design a cow herd that fits the environment
- Use breeds for the cow herd that are similar
- Use a terminal sire breed that fits the market
- Use a system that is manageable over many generations
To design an effective crossbreeding system, consider how many breeding groups can be maintained on the farm, how bulls can be managed before and after the breeding season, how replacement females will be secured, what the standards are in the market (such as coat color), and if a singular trait (weaning weight for calves sold off the farm or marbling for calves retained through finishing, etc.) must be heavily considered.
Mating systems that can be effectively used in small cow herds are:
The Two-Breed Rotation
A single-breed cow herd is mated to sires of a second breed:
- Simple
- Cow herd is a single breed
- Only one breeding group
- Maximizes breed influence
- 15% increase in weight/cow exposed
- No source of replacements
Two-Breed Backcross
The crossbred progeny of two breeds are mated back to one of the parental breeds:
- Good use of breed effects
- Two breeding groups
- Replacements produced
- Maintains good level of consistency in calf crop
- Some inconsistency in the cow herd
Table 4. Inconsistency of the cow herd in a 2-breed backcross system
Generation 1: 50% breed A: 50% Breed B
Generation 2: 25% breed A: 75% breed B
Generation 3: 62.5% breed A: 37.5% breed B
Generation 4: 81.25 breed A: 18.75% breed B
Generation 5: 40.6% breed A: 59.4% breed B
Three-breed terminal rotation
Crossbred cows of two breeds are mated to a third breed of sire:
20% increase in weight/cow exposed
Complements the environment for the cow herd and the market for the calf crop
No replacements are produced
Very consistent cow herd and calf crop
One sire group
Crossbreeding can be used to develop a composite breed
The value of a composite breed (mating crossbred parents with the same breed composition or mating specified crossbred female breed composition to specified crossbred sire breed composition) is to capture additive breed effects and heterosis that complement both the environment and the market. Composite progeny can be very phenotypically consistent, which is an advantage in the marketplace. There need only be one breeding group, and replacements are produced in each generation in inter se matings. Composite breeds are most often used to address specific environments, and this can be shown in the significant number of composite breeds that use Brahman in the cross (Brangus, Santa Gertrudis, Beefmaster) to capture adaptability to hot environments while adding other breeds to capture weight or carcass traits.
A word of caution
Composite breeds are not for everybody. There should be a good reason to specify breeds in the composite, and the breeding program will be a very long-term commitment. They are only effective when maintained generation after generation. Adding an outside breed to the program will diminish the results and create more variability in the progeny.
Crossbreeding is an important part of the beef industry because of the variation in environments and markets available in the US. It should be accomplished with specific goals in mind and with a long-term commitment.