To get an impression of the realized genetic response in the longer term, it is possible to evaluate genetic trends across generations. A genetic trend is a compilation of average EBV per generation, and indicates the direction of change across generations. It is often visualized in a graph and is useful to check whether there are unexpected deviations from linearity, e.g. due to a selection limit. The figure below shows the genetic trend for milk production in Dutch black-and-white dairy cattle from 1995 to 2022, expressed per birth year, rather than per generation. Reason for this is that in dairy cattle generations are not discrete , but overlap because some cows get older than othersbecause the age of the sires and dams of dairy cows can vary considerably. By expressing a genetic trend per birth year this overlap in generations is overcome. It also provides more insight in what has happened during a defined time span. The fact that there is an increase in EBV visible in the right figure below indicates a positive genetic trend. Animals are genetically improved across birth years, so selection has been successful. In the same figure you also see the phenotypic trend in milk production (the green line that starts just below the genetic trend in 1995). The fact that both trends have approximately the same slope indicates that environmental factors such as management, feeding, housing have been supporting, and not limiting the expression of the genetic improvement.
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