13.2: Genetic trend (New)

To get an impression of the realised 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 others. 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 EBV are estimated relative to the average EBV in 2009. In technical terms: 2009 was the reference year. Animals with higher genetic potential than the average in 2009 have EBV larger than 0, and animals with genetic potential lower than the average of 2009 have negative EBV. In the figure are the annual average EBV’s. The fact that there is an increase in EBV 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.

 On the left is the phenotypic trend in milk production of Dutch black-and-white dairy cattle in the period 1945 – 2022. On the right is the phenotypic trend (red) compared to the estimated genetic trend (green) for milk production in Dutch black-and-white dairy cattle in the period 1995 – 2022. Ebv = estimated breeding value (source: CRV, the Netherlands).