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Education - Base Changes

What is a Genetic Base?

The genetic base is the group of animals which is used to form the zero point against which all the proofs are expressed. It effectively forms a fixed point to allow the comparison of animals. For production proofs, this fixed point is determined by the average PTAs of cows born in one year. For PTA2000’s, the base or fixed point is the average PTA of cows born in 1995. This means that the PTA is now viewed as being better or worse than the PTA of the average cow born in 1995.
As the average cow in the UK was born before 1995, and the UK Holstein population has been improving quickly for at least the last decade, this means that the average cow in the UK will no longer have a positive PTA. In fact only around 40% of milking cows will be positive for PIN.

How often does the base change?

The base for production is updated every five years. A five-yearly base change was agreed by most Interbull member countries, including the UK and others such as the Netherlands and the USA.
As genetic progress is being made and the national herd is improving over time, the base is changed to keep the indexes relevant and meaningful. One of the advantages of updating the base is that it allows breeders to realign their breeding criteria. For example, if you decide to set minimum sire selection criteria of +800 kg milk, the relative amount of improvement you will see at the beginning of the new base period will be greater than at the end of the period. As the dairy cattle population as a whole makes progress, without realising it, over the five-year period you are lowering the selection pressure you are placing when selecting bulls.
If the base had been fixed at one point in history (e.g. in the 1960s), almost all breeding age animals in the UK would now have PTAs for milk of greater than 1,500 kg. In other words, the PTA of the animal would no longer be meaningful.