Management or Contemporary Groups are the “building blocks” of the Breedplan analysis. These groups of animals are the only ones that are directly compared, thus grouping and comparing only animals which had an equal opportunity to perform. This concept allows the analysis to eliminate non-genetic effects.
“If these groups are not formed correctly, the EBVs calculated will be
less accurate and possibly misleading.” (Breedplan Manual, Chapter 11)
To demonstrate what this means, have a look at this group of calves and their weights:
(all the weights have been adjusted to 200 days of age, some forward, some backwards; this is a standard practice in the analysis).
From this example it is clear that ABC1810 deviates from the group average more than any of the other animals.
The basic formula for calculating an EBV is:
(weight of the animal – average weight of the group) x heritability of the trait
If applied to this the resulting EBVs will be (heritability in this case will be 0.3):
The example shows that the basic EBV calculation reflects the ranking of the animals in the previous table, leaving ABC1810 with the lowest EBV.
However, if the difference in weight resulted from illness, injury or any physical reason, then the animal’s weight should be allocated a management group code, implying that the animal did not have the same opportunity to perform as its contemporaries. This will allow the analysis to remove it from the group. If removed, the group average will change, and the EBVs will now have different values, because the relative distance from the group average is different for each animal.
Important: the decision to include or exclude ABC1810 in the group, will result in two different sets of EBVs. The questions now is – which is the correct set?
That depends entirely on the management of the animals.
If the weight of ABC1810 was the result of an “environmental” effect, e.g. illness, injury, etc., then it should be excluded, and the EBVs in figure 3 would be correct. If it was business as usual, with no difference in handling, health or food, then its weight should remain included in the group, and the EBVs in figure 2 would be correct.
Remember – an EBV is not static, nor is it supposed to be. It is a very sophisticated comparison or ranking system, becoming more accurate as information is added and moving in the correct direction until the animal’s breeding ability can be proven by its own progeny.