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Endogenous Hormone Response and Fertility in Dairy Heifers Treated with Norgestomet and Estradiol Valerate^1,2,

Clemson University⁵, Clemson, SC 29631

⁴ Reprint requests.

Abstract

Seventy postpuberal dairy heifers were assigned to remain untreated (n = 26) or to receive the Syncro-Mate-B (SMB) treatment (n = 44). Untreated heifers were inseminated at 12 hr after detection of estrus, while SMB-treated heifers were inseminated either at 48 hr after implant removal (group 1) or at 12 hr after detection of estrus (group 2). Jugular blood was collected at regular intervals from four untreated heifers and from 20 SMB heifers. In untreated heifers, estradiol-17β (E) was unchanged throughout the estrous cycle. Progesterone (P) reached maximum concentrations during the luteal phase and then declined to less than 2 ng/ml before estrus. Follicle stimulating hormone (FSH) ranged between 50.1 and 100.2 ng/ml during the estrous cycle, then reached a peak (P<.05) concentration of 516.2 ng/ml coincident with peak luteinizing hormone (LH) concentration at 12 hr after the detection of estrus. In SMB-treated heifers, plasma E increased by 12 hr after injection of estradiol valerate and then declined throughout the rest of the sampling period. P declined to less than 2 ng/ml by 24 hr after administration of the SMB treatment and remained low throughout the sampling period. FSH declined in response to SMB treatment, then reached a peak (P<.05) concentration coincident with peak LH 40 hr after implant removal. First service pregnancy rates among the untreated, group 1 and group 2 heifers were 65, 58 and 38%, respectively, and did not differ (P>.05). SMB treatment was effective in inducing luteolysis and in suppressing estrus and peak gonadotropin release. Intervals from implant removal until estrus and from implant removal until peak gonadotropin release were highly variable among SMB-treated animals.

¹ Published with the approval of the Director of the South Carolina Agr. Exp. Sta. as Technical Contribution No. 1823.

² The authors are indebted to K. W. Cheng, Univ. of Manitoba, Winnipeg, Manitoba, for providing purified bovine follicle stimulating hormone (FSH) and specific bovine FSH antiserum; to L. E. Reichert, for providing purified bovine luteinizing hormone (LH;LER-1072); to G. D. Niswender, Colorado State, Ft. Collins, for providing specific bovine LH antiserum (B-225); to the Endocrine Study Section, NIH, Bethesda, MD, for providing pituitary hormones; to Dell Collins, Emory University, Atlanta, GA, for providing specific E2-17(3 antiserum, and to G. D. Searle and Co., for providing Syncro-Mate-B used in this experiment.

³ Dept. of Dairy Sci. VPI & SU, Blacksburg, VA 24061

⁵ Dept. of Dairy Sci.