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Effect of Melengestrol Acetate on Rate of Excretion of Estrogen in Urine of Dairy Heifers¹

Purdue University, Lafayette, Indiana 47907

Abstract

Twenty yearling Holstein heifers were fed either 0.5 or 1.0 mg of melengestrol acetate (MGA) daily for 14 days, and 62 hr. later 10 heifers, five in each group, were injected intramuscularly with 200 µg of estradiol-17ß. Urine was collected on days 0, 7 and 14 of MGA feeding, at 8-hr, intervals from 24 hr. after MGA withdrawal to ovulation and on days 5 and 9 after ovulation. A similar schedule was used for nine control heifers except collection at 8-hr, intervals started 2 days before predicted estrus. The objective was to evaluate effects of MGA on rate of excretion of estrone, estradiol-17 and estradiol-17ß in urine.

Rate of feeding MGA or stage of the estrous cycle when feeding began had no effect on rate of excretion of estrogens in urine either during or after feeding MGA. Urinary estrone excretion was variable among heifers fed MGA and was not significantly different between collection periods. Excretion of estradiol-17 and estradiol-17ß was significantly different between collection periods. Urinary estradiols (17+17ß) decreased by 32 hr. after last feeding MGA, then increased by 56 hr. and for 2 to 3 days remained near rates observed during feeding of MGA. A majority of the heifers excreted less estradiols at 7 and 14 days of MGA feeding and up to 104 hr. after withdrawal of MGA than controls during proestrus and estrus. However, excretion rates during the period 48 hr. before to 8 hr. after the LH surge were similar for control and MGA-treated heifers. Injection of estradiol-17ß significantly increased urinary estradiol-17 but not urinary estrone or estradiol-17/ß.

¹ Journal Paper No. 4829, Purdue University Agricultural Experiment Station. A contribution from Western Regional Project W-95 by the Departments of Animal Sciences and Veterinary Science.

² Present address: U.S.D.A. Range Livestock Experiment Station, Miles City, Montana 53901.

³ Present address: Department of Dairy Husbandry, University of Missouri, Columbia 65201.

⁴ Present address: Department of Population Dynamics, The Johns Hopkins University, 615 N. Wolfe St., Baltimore, Maryland 21205.