Just as you would not automatically assume that a bull weighing 750kg will produce heavier calves than a bull weighing 800kg, you should not automatically assume that a bull with a sean of 110 sq em will produce more heavily muscled calves than a bull with a scan of 100 sq cm.
You need to adjust the “raw” measurement for between-animal differences in age and nutrition before you can compare the bulls on a level playing field.
The logic of genetic improvement is that selection decisions are based on differences – in reality, it is not how different bulls appear to be, but how different their progeny will be that is important. How you measure the differences dictates how much genetic improvement you can make, for example:
- Visual assessment: Tells you what you think is there.
- Scans: Tells you what is there, but not how it got there.
- EBVs: Tells you how much of what’s there will be passed onto the progeny.
The actual EMA observed, visual or scan, results from the interaction between:
The animal’s genetic blueprint sets the limits for the expression of muscling at a given age and within a given environment. This genetic potential is inherited from the animal’s parents and passed onto its progeny.
The animal’s environment controls the degree of expression, hence the expressed level of muscling will vary under differing environments, for example, the muscling of a bull in drought condition is less than when in show condition – same bull, the same genetic potential for muscle, same EBY, different expression because of environment.
Environmental factors that affect the expression of muscling include:
- Nutrition and management
- The administration of anabolic steroids and other muscle-enhancing chemicals
When comparing EMA scans, if we don’t account for variation influenced by environmental factors, then we are most likely comparing environments, not genetics, and selection decisions will be compromised.
While it might be argued that a well-fed bull expresses his full potential for muscling, this assumption is only relevant if we are comparing bulls of the same age and who have enjoyed the same level of nutrition and management. Then a comparison is valid, but such comparison is only commercially useful if the environmental conditions under which the bull was reared match the environmental conditions under which his progeny will be required to perform.
This explains why the commercially breed progeny of some show bulls fail to reflect their sire’s perceived excellence for growth and muscling.
Summary: Unadjusted (raw) scans have limited usefulness for comparing bulls as they don’t account for differences in expressed muscling that result from non-genetic differences in age, nutrition, or chemical enhancement.
EBVs estimate how much of the expressed muscling (scan) will be inherited by their progeny – the bull’s breeding value for the trait. Thus, EBVs don’t necessarily agree with what you see or what the scan measures because they take into account how the difference was achieved By adjusting the (raw) scan for known differences in age, nutrition, and management.
EBV removes disparate environmental influences from the equation, thus allowing comparison to be based on differences in genetic potential rather than differences in environment.
The adjusted scan is then multiplied by the known heritability of the trait and compared against the
breed benchmark to estimate the animal’s estimated Breeding Value for the trait.
The EBV is reported in units of “sq cm” relative (+/-) to the breed benchmark. To make the EBV most relevant, it is reported at a 300g, HSCW steer carcass equivalent, better reflecting the true commercial breeding value of a bull.
EBVs are calculated from scans that have been adjusted for disparate differences in age, nutrition, and management. EBVs are comparative, not categorical. That is they compare bulls for muscling on a
“level playing field”, benchmarked to a breed average and to a commercial endpoint.
Examples of GXE interactions demonstrate why raw scans can be misleading:
1. Take two identical twin calves. They have exactly the same genes for EMA and if reared exactly similarly,
their EMA scans would be similar.
However, if one calf was fed higher energy, higher protein diet than the other, the calf having preferential treatment would have a higher EMA scan for no reason other than he was better fed.
Providing the management groups were correctly recorded, the two bulls would have similar EBVs for
EMA – more accurately estimating their breeding value for EMA than the raw scan.
2. Take two embryos resulting from a split – they have identical genes, Hatch one out weeks before the other. Even if they are then reared similarly, At yearling time the older calf wil have a larger EMA for no other reason than being older. Again, the difference is due to age, not genetics. The difference measured will not be passed onto the progeny – again the bulls would have similar EBV which more accurately estimates their true breeding value.
3. Take a bull in drought condition (scan of 80 sq cm, with a muscle score D) compared to the same bull in show condition (muscle sore B, scan 100 sq cm). Even though visual muscle score and EMA scan are quite different for the same bull under the drought vs show comparison, the EBV remains the same, as does the bull’s genetic merit for muscling, regardless of the environmental influence.
In the above examples, if differences in muscling resulting from differences in the age or nutrition are not
adjusted for, comparison between the pairs based on visual assessment and/or scan information may be