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 acetate (MGA) is widely used as a growth promoting feed additi ve in cattle breeding in the USA and several other non-European countries. To explore the physiological effects of MGA four heifers were fed during 8 weeks with 0.5 mg MGA daily as registered in the USA and two heifers each r eceived 0, 1.5 or 5 mg/day, respectively. Plasma samples were collected twi ce a week and concentrations of MGA, progesterone (P-4) and estradiol-17 be ta (E-2-17 beta) were quantified. The pulsatile secretion of luteinizing ho rmone (LH) was investigated in 6-hour profiles before and during treatment. After slaughter the reproductive organs were examined and oestrogen residu es in edible tissues were measured. Four days after the beginning of MGA fe eding MGA concentrations in plasma reached levels of 30 and 100-400 pg/mL d epending on the dose received. Three weeks after the beginning of MGA feedi ng P-4 plasma concentrations had dropped to base levels below 0.3 ng/mL in all three treatment groups. Mean plasma E-2-17 beta levels increased in phy siological range from 1 to 5 pg/mL during 0.5 mg MGA/day feeding with many acyclic peaks. Overdosed MGA decreased E-2 levels and suppressed cyclic pea ks. Number and size of ovarian follicles were not altered by any treatment. Mean LH levels and pulse frequencies increased significantly during labell ed treatment (0.5 mg/day), while higher doses had reducing effects. The dev elopment of corpus luteum was suppressed. E-2-17 beta residues in fat incre ased about 300% following labelled MGA treatment.