If the influence of milk production on culling is not taken into account when analyzing stayability, one is measuring the ability of the cow to stay in the herd given management's criteria for culling based on production. If milk production is included in the analysis, that is, an attempt is made to adjust the stayability measure for a particular animal for its level of production, then one is attempting to measure the biological stayability of an animal or its ability to delay involuntary culling. Before discussing genetic evaluation of bulls for stayability, we must first consider whether or not stayability in itself is an important trait. Much work has been done on lifetime profitability of dairy cattle. An important variable in lifetime productivity and profitability is longevity.
Many producers agree with the importance of herd life to profitability.
Some have chosen methods of selection to improve or attempt to improve herd life. It is often argued that selection for type characteristics is important based on its relationship to stayability. The concept of functional type also has come about as it relates to improvement in herd life of cattle. Whether or not measures of phenotypic appearance relate to stayability, the fact does exist that emphasis is placed on these components in genetic programs and, hence, indirectly on trying to improve or insure satisfactory levels of stayability. Although longevity is a desirable quality in dairy cattle, many studies have shown that the genetic variability associated with longevity is low.