Abstract
The use of test day model (TDM) for genetic evaluation results in predicted transmitting abilities (PTAs) for any stage of lactation. However, in the United Kingdom (UK), publication and selection of parents are still based on PTAs computed on a 305-day basis from the random regression coefficients of the TDM. It is expected from selection index theory, that selection on 305-day PTAs would place different weights on PTAs at different days in milk (DIM). This could result in changes in the lactation curve of animals over time as a result of genetic progress. This study evaluates genetic trends in terms of changes in PTAs at different DIM for production traits and somatic cell counts (SCC) in the Holstein breed. In addition, the inclusion of SCC in the national profitable life index (£PLI) is based on its relationship with milk computed on a 305-day basis. This study also examines what impact such selection has on the relationship between PTAs for milk and SCC at different DIM.
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