Detection of melengestrol acetate residues in plasma and edible tissues ofheifers

The aim of this study was to gain knowledge of residue formation after the use of melengestrol acetate (MGA) as a growth-promoting agent. Two Holstein -Friesian heifers each received a daily dose through the feed of 0, 0.5 mg (2 heifers with and without withdrawal each), 1.5 mg or 5.0 mg MGA for 8 we eks. MGA residues in plasma mere screened by enzyme immune-assay (EIA). Con centrations in kidney, liver, and muscle mere quantified by liquid-chromato graphy-mass spectrometry (LC-MS), and in fat by gas chromatography-mass spe ctrometry (GC-MS). RIGA levels in plasma were 40, 128, and 280 ng/L, respec tively. Residues accumulated in muscle and kidney (5-fold), liver (20-to-40 -fold), and fat (200-fold). After administration of 1.5 mg per day the mean MGA concentration in fat was 29 mu g/kg and thus violated USA regulations which specify a limit of 25 ppb. Therefore the labelled use of MGA (0.5 mg per day) has to be officially controlled.