title: Melengestrol acetate blocks the preovulatory surge of luteinizing hormone, the expression of behavioral estrus, and ovulation in beef heifers.
abstract: We tested the hypothesis that melengestrol acetate (MGA), an orally active progestin, blocks estrus and the preovulatory surge of luteinizing hormone (LH) in beef heifers. Cycling yearling Angus heifers were divided randomly into two groups: MGA-treated (n = 6) and control (n = 5). All heifers received injections of prostaglandin F2alpha (PGF) on d -25, -11, and 0 to synchronize estrus. Following the last PGF injection on d 0, heifers were fed either 0.5 mg MGA in a carrier or the MGA carrier each day for 8 d. At 4-h intervals on d 1 through 6, all heifers were observed for expression of estrous behavior, and blood samples were collected and assayed for LH. Daily blood samples were collected at 0800 on d 1 through 10 and assayed for circulating progesterone concentrations. All control heifers exhibited estrus and a preovulatory surge of LH. In each case, this was followed by increases in circulating concentrations of progesterone indicative of ovulation and normal luteal function. In contrast, none of the MGA-treated heifers exhibited estrus, LH surges, or evidence of ovulation. The results of this experiment show that MGA prevents ovulation in cattle by inhibiting the preovulatory surge of LH.
authors.affiliation: Department of Animal Sciences, University of Kentucky, Lexington 40546-0215, USA.
authors.forename: D B
authors.initials: DB
authors.lastname: Imwalle
authors.forename: D L
authors.initials: DL
authors.lastname: Fernandez
authors.forename: K K
authors.initials: KK
authors.lastname: Schillo
mesh_terms: D000818:Animals; D002417:Cattle / Q000097:blood / Q000502:physiology*; D015237:Dinoprost / Q000494:pharmacology; D004971:Estrus / Q000187:drug effects; D004973:Estrus Synchronization / Q000187:drug effects*; D005260:Female; D007986:Luteinizing Hormone / Q000097:blood*; D008552:Melengestrol Acetate / Q000494:pharmacology*; D010060:Ovulation / Q000187:drug effects*; D011374:Progesterone / Q000097:blood; D045167:Progesterone Congeners / Q000494:pharmacology*; D011897:Random Allocation
chemical_list: D045167:Progesterone Congeners; D011374:Progesterone; D008552:Melengestrol Acetate; D007986:Luteinizing Hormone; D015237:Dinoprost