If I set a threshold of +4.0kg for birth weight and won’t use bulls with EBVs above this, then we will get a set of bulls that reflect this and any other EBV cut offs put in place.
What if a bull is presented that out performs the bulls we have identified across the other traits that we have in our breeding objective, but has a birth weight of +6.0 kg?
No deal, right? ………. hmmm.
Examining the scale, we see that the difference in birth weight EBV between the bull (+6.0) and our threshold (+4.0) is 2.0kg. As the cows in our herd to which the bulls will be joined is a constant, only half of this difference will be observed in the progeny, being 1.0kg.
Therefore, we are potentially compromising greater rates of genetic gain in other areas as a result of, in this example, a 1kg difference in average birth weight. Whilst this may be significant in some circumstances, it is important that we take a dynamic look at the breeding and selection decisions we make.