Contemporary groups in beef
Understanding about contemporary groups
Animals that have been treated in a similar way – e.g. born over a relatively short period of time on the same farm, and fed and managed similarly – are known as ‘contemporaries’.
Within the analysis animals that have been reared in a similar manner are assigned to ‘Contemporary Groups’ to enable accurate comparisons to be made.
These are created in a flexible manner:
- Contemporary groups are created within each herd rather than across herds
- Animals are assigned to separate contemporary groups for each trait, as some animals will only have been recorded together for a limited range of traits
- Contemporary groups are formed flexibly, so clusters of calves that are born within 92 days of one another end up in the same group
The difference between the last date of each group and the start date of the following one is calculated
- Contemporary groups may be merged where there are too few animals in them. Pairs of groups are considered in order of the smallest difference first:
- Where both groups contain at least five records nothing is done
- Where one or both groups contain fewer than five records the groups are combined as long as all the dates of birth are within a further specified range of 183 days, otherwise nothing is done.
- For 400-day weight, muscle depth and fat depth contemporary groups are then split by management code, as long as each part of the split group contains at least five records, otherwise nothing is done.
As new information is collected between evaluations some cattle may move into a different contemporary group. This can sometimes have a big impact on their EBVs.
High and low variation in a contemporary group or herd
Occasionally, animals in a contemporary group, or even in a herd, have either very low variation or very high variation in their records. For example, they all have very similar 400-Day Weights, or have 400-Day weights that are spread
across a wide range.
If nothing was done, animals from the most variable herds would tend to get the highest EBVs. On the other hand, if the records were adjusted so that all herds have the same range of performance (albeit different averages) there is a risk of downgrading some animals that really are genetically superior.
To address these situations, the Signet analysis reaches a compromise:
- The variation in each herd is scaled towards an overall average range
- The amount of scaling that goes on depends on the size of the contemporary groups which animals belong to.
Small groups with extremely high or low variation get scaled down or up more than big groups because there is a much higher risk that extreme variation in a small group is due to chance or the result of accidental differences in management.