Genetic Contemporary Groups
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This page covers the concept of contemporary groups, which are cohorts of cows managed under identical conditions, essential for precise animal evaluations.
Further reading
Contemporary groups
A contemporary group is a cohort of cows that have had an equal opportunity to perform in their herd.
Animals in a contemporary group are managed with the same feed resources and environmental conditions.
The definition of contemporary groups for NZAEL’s animal evaluations vary slightly by the nature of the trait, but generally a contemporary group reflects animals that are:
- in the same herd (e.g. same farm location and herd number)
- tested/inspected/weighed on the same day (e.g. herd-tested or TOP inspected)
- belonging to the same age group (e.g. 2 year old, 3 year old or 4 year old)
- with the same season of calving (e.g. spring or autumn)
For production traits, sampling regime (e.g. twice-a-day or once-a-day) is also considered, which is important given the different herd testing options now available to farmers.
Animal evaluation is not driven by the actual performance of cow, it is driven by the differences in performance between cows that are managed similarly. In other words, how an individual cows performance differs from the average of her contemporary group.
When estimating breeding values, we partition an animal’s performance into differences due to management, climatic or environmental effects in order to obtain reliable estimates of genetic differences (i.e. breeding values) between animals. Using well-defined contemporary groups helps to identify animals that will produce more productive offspring.
If this was not the case, animals who were fed to higher levels, were tested on a day with more favourable weather conditions, under a different calving regime or were at a higher stage of production would seem better candidates to use as parents than they actually are. For example, 6-year-old cows will produce more on average than 2-year-olds, so for the purposes of animal evaluation, it is important to compare all the 6-year-olds from one herd with each other, and to compare all the 2-year-olds from one herd with each other, and not to directly compare the 2-year-olds with the older cows.
If the contemporary groups used in animal evaluation are not a true reflection of cohorts of animals, the evaluation will be suboptimal. If two different cohorts of animals are not properly distinguished into two contemporary groups, the evaluations on those animals, and their ancestors and descendants, will be biased. The evaluations of some animals will be biased upwards and others will be biased downwards.
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