Responses of Yield and Conformation to Selection for Milk in a Designed Experiment with a Control Population

A selection experiment was conducted over a 25-yr period to measure long-term responses to selection of AI sires based on milk alone. An unselected control line was maintained as part of the experimental design. The four highest sires available from active AI sire lists for PTA of milk were mated each year to successive generations of a selection line. Selection and control lines were compared for milk and fat yields and conformation traits. Animal models were demonstrated to be particularly preferable to sire models for analysis of data from designed selection experiments. Genetic trend from 1967 to 1988 for the selection line was estimated to be approximately 125 kg/yr. Furthermore, selection cows received higher genetic evaluations than control cows for most conformation traits recorded from 1983 to 1989. Before young bulls are allowed to enter AI progeny-testing programs, their pedigrees are scrutinized heavily for conformation traits. Selection for secondary traits by dairy producers may be justifiable; however, commercial dairy producers may select AI sires that have highest PTA for yield, disregarding other traits, without concern for deterioration of conformation.