Genetics of dairy production

As we previously see, it is quite impossible to know exactly the true genetic value of a cow. All we have is the estimated breeding value based on phenotypic or genomic information. The accuracy is defined as the correlation between true and estimated breeding value.

The main issue for dairy cattle is that usually one cow has one phenotype in one environment. Even if, we can records several performances for production, reproduction and health traits, we still have very few records per cow. Therefore, the accuracy of estimated breeding value based on the own phenotype of a cow is very low.

However, as siblings share a proportion of the same genes, it is possible to use their information in estimating breeding values.

To summarize, when we estimate breeding value of a cow:

1. The knowledge of records from close relatives increases confidence in accuracy of predicting genetic value

2. In general, as heritability increases, the accuracy of predicting genetic value of a cow also increases. The effect of heritabilities becomes smaller with more information used.

3. The accuracy of parent average depends on the parent EBV accuracy and not on heritability (but note that with low heritability it will be harder for a parent to achieve a certain accuracy).

The table below gives accuracies of predicting value for milk production (h² = 0.3) from a cow's records and records of cow's relatives:

This table shows that a cow's records are more important than records of many relatives. Records of daughters or dam add little accuracy unless the cow has no records. If the cow has no record, the EBV of her sire is an important indication of her genetic value.

The situation is different when evaluating bulls. With the use of artificial insemination, nearly perfect accuracy of predicting a bull's genetic value can be achieved if enough daughters in many herds are analyzed (accuracy of sire's EBV equal to 0.99 with 1000 daughters recorded in different herds). Indeed, having records of daughters in different herds allows a proper estimation of environmental effect and therefore increases accuracy of estimated breeding value of their sire.

Today, genomic test can be done early in an animal life and adds more to the accuracy when there is not much other information available:

We just saw how to estimate breeding value and therefore how to rank cows and bulls for interesting traits with more or less accuracy. To obtain genetic improvment only the best must be selected. The extent to which selection will result in genetic improvement depends on the intensity of selection.