Shuttle: refers to the use of two or more products during the grow-out of a flock. The reason for use of a shuttle is that results are better than if either drug was used by itself. The principle of shuttles is that the best drug is used for each phase of the grow-out i.e. most suitable drug is used for starter, while another drug is used for grower and finisher. Drug withdrawal period is the most important consideration for drugs that will be used in finisher feeds.;