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The Effect of Dietary Monensin on the Luteinizing Hormone Response of Prepuberal Heifers given a Multiple Gonadotropin-Releasing Hormone Challenge^{1, 2 ,3,}

Texas A & M University Agricultural Research and Extension Center, Overton 75684

Abstract

Ten prepuberal Simmental x Brahman-Hereford heifers (average weight 208 ± 4 kg) were randomly assigned to receive either 2.7 kg/head/day of ground milo containing 0 mg monensin sodium (C) or 2.7 kg/head/day of ground milo containing 200 mg monensin sodium (M). Both groups of animals (n = 5) received Coastal bermudagrass hay ad libitum throughout the trial. On day 21 of the feeding period all heifers were fitted with jugular cannulas. Immediately after cannulation, the heifers were injected IM with 100 µg of gonadotropin-releaing hormone (GnRH) and blood was collected every 10 min for 4 hours. Four hours after the first GnRH challenge, a second 100-µg GnRH injection was administered, and blood samples were collected at 10-min intervals for an additional 5 hours. Serum was stored at –20 C until radioimmunoassayed for luteinizing hormone (LH). The amount of LH released after each GnRH injection was greater in the heifers fed M than in the controls (P<.05). Peak LH after the first GnRH challenge was greater (P<.05) in heifers fed M than in controls. The area under the first GnRH induced LH curve tended (P<.20) to be greater for the M group than for the controls. Peak LH concentration was greater in heifers fed M than in control heifers, as was the duration (P<.05) and area under the second GnRH-induced LH curve. In prepuberal heifers, dietary monensin appears to increase hypophyseal capability of releasing LH after a first and second GnRH challenge.

¹ Journal paper TA 15664, Texas Agr. Exp. Sta.

² This study was a contribution to the Western Regional Research Project, W-112, Reproductive Performance in Beef Cattle.

³ The authors gratefully acknowledge Eli Lilly and Co. for monensin and funding; Mr. D. Neuendorff, Mrs. J. Schmidt and Mr. B. Skrivanek for skilled technical help, and Mrs. C. Walters for manuscript preparation.

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