8.5.1: Special case: repeated observations on a single animal

For some traits, in time, more than one record of own performance will be collected per animal. For example, you can have a record on litter size of the first litter, but when the second litter is born this is additional information on the performance of the dam, and so is the birth of the third, and maybe even fourth litter. Because litter size is heritable you would expect that multiple litters within a dam are more alike than litters between various dams. A single record includes genetic and environmental influences. In the second litter the genetics of course is the same, but the environment may be somewhat different. And again, a third litter has the same genetics but maybe somewhat different environment. So the more litters you have, the better you should be able to estimate the genetic potential of the dam. How much better is indicated with a correlation between the subsequent records: the repeatability. The more the subsequent records are alike, the higher the correlation (max = 1).

Availability of more records on an animal allows for a better indication of the phenotypic superiority. You can imagine that the records of an animal in part are influenced by environment that is specific for that record, the so-called temporary environment, but also by environment that is similar across records, the so-called permanent environment. The temporary environment is different every time, so the effect is larger in one records than in the other. By taking the average of the repeated records, the phenotype is corrected for the effect of the temporary environment, and the phenotype based on this average performance thus represents a more accurate representation of the phenotypic superiority as indicated in figure 2.

The better we can express the phenotypic superiority and the higher the repeatability, the better we should be able to estimate the breeding value. Indeed this is the case. Repeated records allow for a better estimate of the regression coefficient. In case of a single record the regression coefficient is h², but if there are multiple records it becomes:

where  n is the number of repeated records, and r is the correlation between subsequent records: the repeatability. If the repeatability is 0.5 and we have 2 records, than the regression coefficient increases from h² to 2h² / 1.5 = 1.33 h². The value of repeated observations depend on the repeatability and on the number of records that are available. The lower the repeatability, the more repeated observations are influenced by different environmental influences, and the more added value it has to collect multiple records and re-estimate the breeding value every time a new own performance record becomes available.