Dose-dependent effects of melengestrol acetate (MGA) on plasma levels of estradiol, progesterone and luteinizing hormone in cycling heifers and influences on oestrogen residues in edible tissues

Melengestrol acetat (MGA) is widely used as a growth promoting feed additiv e in cattle breeding in the USA and several other non-European countries. T o explore the physiological effects of MGA four heifers were fed during 8 w eeks with 0.5 mg MGA daily as registered in the USA and two heifers each re ceived 0, 1.5 or 5 mg/day, respectively. Plasma samples were collected twic e a week and concentrations of MGA, progesterone (P-4) and estradiol-17 bet a (E-2-17 beta) were quantified. The pulsatile secretion of luteinizing hor mone (LH) was investigated in 6-hour profiles before and during treatment. After slaughter the reproductive organs were examined and oestrogen residue s in edible tissues were measured. Four days after the beginning of MGA fee ding MGA concentrations in plasma reached levels of 30 and 100-400 pg/mL de pending on the dose received. Three weeks after the beginning of MGA feedin g P-4 plasma concentrations had dropped to base levels below 0.3 ng/mL in a ll three treatment groups. Mean plasma E-2-17 beta levels increased in phys iological range from I to 5 pg/mL during 0.5 mg MGA/day feeding with many a cyclic peaks. Overdosed MGA decreased E-2 levels and suppressed cyclic peak s. Number and size of ovarian follicles were not altered by any treatment. Mean LH levels and pulse frequencies increased significantly during labelle d treatment (0.5 mg/day), while higher doses had reducing effects. The deve lopment of corpus luteum was suppressed. E-2-17 beta residues in fat increa sed about 300% following labelled MGA treatment.