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Effect of Dietary Monensin on Ovarian Response following Gonadotropin Treatment in Prepuberal Heifers^{1 , 2, 3,}

Texas A&M University, Agricultural Research and Extension Center, Overton 75684 and and Eli Lilly and Company, Dallas 75234

Abstract

Twenty prepuberal Charolais x Brahman-Hereford heifers were randomly assigned to be fed a concentrate containing either 0 mg (C) or 200 mg (M) monensin sodium/head/day. Coastal bermudagrass hay was fed ad libitum. Average daily gain was similar for the two groups. Each heifer received 1 mg of porcine follicle stimulating hormone (FSH-P) (Armour⁶) at 0800 and 2000 hr on days 22 through 26 (10 mg total) and 2,500 IU human chorionic gonadotropin (HCG) on day 27. Flank laparotomy was performed on day 30, for examination of ovaries, and ovariectomy was performed on day 37. The average ovarian size ± standard error at day 15 was 3,730 ± 66 mm³ and 1,848 ± 55 mm³ for groups M and C, respectively (P<.025), as measured by rectal palpation. Numbers of ovulation sites measured on day 30 were 9.1 ± 2.2 and 4.9 ± 1.8 per heifer for groups M and C, respectively (P<.01). After ovariectomy on day 37, heifers fed M were found to have greater ovarian weight (P<.05), more corpora lutea (CL) (P<.05), greater total luteal weight (P<.05), more follicles (P<.01) and greater weight of follicular fluid (P<.05) and stroma (P<.025) than controls. CL were analyzed for progesterone content by spectrophotometric procedures. Heifers fed M had slightly larger CL (P<.10) with progesterone concentrations similar to those in CL from controls. This resulted in more luteal progesterone per CL and more luteal progesterone per heifer in the M heifers than in the controls. Prepuberal heifers fed M, which caused the expected shifts in rumen fermentation and volatile fatty acid production, exhibited an enhanced ovarian response to gonadotropins compared to that exhibited by controls.

¹ Journal paper TA 15577, Texas Agr. Exp. Sta

² The authors gratefully acknowledge Eli Lilly and Co. for financial support.

³ This study was a contribution to Western Regional Project W-112, Reproductive Performance in Beef Cattle.

⁴ Texas A&M University Agr. Res. and Ext. Center, Overton 75684.

⁵ Eli Lilly and Co., Dallas, TX 75234.

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