Selection procedure has been examined in Friesian and Ayrshire populations of dairy cattle in an attempt to determine the extent to which the relatively simple selection process in the first lactation was modified by the effects of age, by the incomplete repeatability of milk yield, and by the availability of production information from more than one lactation.

As a result of a greater intensity of selection for milk yield in the lower yielding herds, effective selection for yield was maintained at the later cullings despite increased wastage for reasons other than yield in the ageing population.

The high yielding animals that were selected by farmers in early life continued to outlive and to outyield their lower yielding first lactation contemporaries, notwithstanding a small amount of evidence that the heaviest yielding group were severely culled in the second lactation, mostly for reasons beyond the farmers' control. There was also some evidence that a high first lactation yield predisposed the animal to disposal for reasons other than yield in later life.

Survival at any particular culling was primarily determined by the milk yield in the previous lactation. In this respect, the culling after each lactation appeared to be independent of previous selection decisions, and the individual animal had to demonstrate its merit repeatedly to survive—earlier merit did not compensate for a subsequent failure.