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Influence of Dietary Energy Intake on Bovine Pituitary and Luteal Function¹

U.S. Department of Agriculture, Montana Agricultural Experiment Station, Miles City 59301 and and University of Wyoming, Laramie 82071, Cooperating

Abstract

Three experiments were conducted to measure the effect of dietary energy intake on pituitary and luteal function. Intact cows and heifers and spayed cows were administered injections of gonadotropin releasing hormone (GnRH) following a prolonged period of either dietary energy restriction (low energy) or adequate energy (high energy) intake to determine if energy intake influenced the pituitary responsiveness to GnRH. In addition to measuring luteinizing hormone (LH) release following GnRH, peripheral serum concentrations of progesterone, luteal progesterone content and the pituitary LH content of cows fed low and high energy levels were compared. Low energy increased the LH release after a GnRH injection in intact heifers but not in intact cows. Peripheral serum progesterone concentrations tended to be reduced in low energy heifers and cows. Low energy spayed cows released more LH in response to GnRH than adequate energy spayed cows. Therefore, an increase in the LH release after a GnRH injection was observed despite removal of ovarian steroids. Restricting energy intake increased the pituitary LH content of spayed cows but decreased the pituitary LH content of intact cows. The release of LH following five consecutive injections of GnRH declined nonlinearly after the first injection in both low and high energy fed heifers. Conclusions from these results were that dietary energy restriction may influence the LH release directly at the pituitary level as well as indirectly through effects on ovarian steroid production.

¹ This study was a contribution to Western Regional Research Project W-112, Reproductive Performance in Beef Cattle. Publication has been approved by the Director of the Montana Agricultural Experiment Station, Journal Series No. 720.

² Present address: Department of Animal Science, Cornell University, Ithaca, NY 14850.

³ Livestock and Range Research Station (formerly the U. S. Range Livestock Experiment Station), USDA-ARS, Miles City, MT 59301.

⁴ Division of Animal Science, University of Wyoming, Laramie 82071.

⁵ The authors express appreciation to: The Upjohn Co., Kalamazoo, MI, for PGF₂ and progesterone-TME; G. D. Searle Co., Chicago, IL, for Norgestamet® implants; Dr. G. D. Niswender, Colorado State University for anti-bovine LH serum; Dr. L. E. Reichert, Jr., Emory University for purified bovine LH and NIAMDD for NIH-LH-B8.