Feed Efficiency: A Composite Trait of Dairy Cattle

A pleiotropic pathway of correlated response of feed efficiency from selection for milk yield has been inferred from a large genetic correlation coefficient. However, in studies to estimate the genetic correlation, cows were fed according to yield (i.e., not ad libitum), and there was no accounting for body tissue balance (i.e., catabolism or repletion status). Research evidence indicates that the apparent feed efficiency of dairy cattle is a composite trait of physiological processes that does not include genetic covariance between yield and rate (efficiency) of converting dietary nutrients directly into milk. Absence of pleiotropy in direct nutrient utilization makes feed efficiency an artifact of the selection response in milk, which depends on the genotypes and within-animal environments for milk yield, tissue balance, and appetite. Several reports point to a major connection between genetic ability in milk and tissue balance.