In the mid-1970s, ionophores were approved by the Food and Drug Administration for addition to the rations of ruminants in the United States. Monensin has been used most extensively, but others, including lasalocid, tetronasin, lysocellin, narasin, and laidlomycin, either have been investigated or are used commercially. lonophores generally have improved the efficiency of feed utilization; with...