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Plasma LH, Progestins and Corticoids in Heifers Injected with Estradiol Valerate and FSH during Estrous Synchronization with MGA^{1, 2,}

South Dakota State University, Brookngs 57006 and Purdue University, Lafayette 47906

Abstract

Two replicates of 12 yearling Hereford heifers were used in this study. Estrous cycles were synchronized in all heifers by feeding 1 mg melengesterol acetate (MGA) per head per day for 14 days. The heifers were injected with 0 or 5 mg estradiol valerate (EV) on day 2 and with 7 mg follicle stimulating hormone (FSH) on day 10, 12 or 14 of MGA feeding. Each heifer was bled by jugular puncture on day 8 and 10 through 20 (day 1 was first day of MGA feeding). Plasma was analyzed for LH, total progestins and total corticoids. Estrus was checked twice daily with a vasectomized bull and ovulation determined by laparotomy.

Heifers receiving an EV injection on day 2 of MGA feeding had a longer interval from last MGA to onset of estrus than heifers not receiving EV (6.4±0.39 vs. 4.7±0.31, respectively).

LH levels ranged from 0.8 to 1.3 ng/ml plasma during and after MGA feeding until the preovulatory surge of LH during the day of estrus. Total progestins decreased during the early days of MGA feeding and this decline was most rapid in heifers receiving 5 mg EV. Subsequent to MGA feeding, plasma progestin levels were generally less than 1 ng/ml. Total corticoids fluctuated from day to day and did not appear to be related to MGA feeding.

There were no significant differences in plasma LH, total progestin or total corticoid levels between non-, single- and multiple-ovulating heifers subsequent to FSH injection.

¹ Published wth approval of the Director of the South Dakota Agricultural Experiment Station as Publication No. 1049 of the Journal Series.

² We are indebted to Dr. L. E. Reichert, Jr., Emory University, Atlanta, Georgia for providing purified bovine LH, to Dr. G. D. Niswender, University of Michigan, Ann Arbor for providing antibovine LH antisera and to the Upjohn Company, Kalamazoo, Michigan for supplying the melengestrol acetate.

³ Present address: U.S.D.A. Ranae Livestock Experiment Station, Miles City, Montana 593O1.