Melengestrol acetate (MGA)—17α-hydroxy-6-methyl-16-methylene-pregna-4, 6-diene-3,20-dione acetate (Fig. 1)-is a synthetic oral progestational steroidal hormone that is added to the feed of heifers to suppress estrus (heat), thereby leading to improved feed efficiency and rate of weight gain. In the United States, the Food and Drugs Administration (FDA) has set the tolerance level for residues of MGA in edible tissues at 25 ppb based on evidence that residues at or below this concentration do not elicit a hormonal response (1), whereas in the European Union (EU) the residue limit for this steroid in animal products is 0 ppb (2).