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Effects of 17ß-Estradiol and Diets Varying in Energy on Secretion of Luteinizing Hormone in Beef Heifers¹

University of Nebraska^,4, Lincoln 68583

Abstract

An experiment was conducted to test the hypothesis that 17ß-estradiol (E₂) would not suppress secretion of luteinizing hormone (LH) in heifers fed a diet limited in energy during the period before the onset of nutritionally induced anestrus. Sixteen of 20 heifers that had been exhibiting normal estrous cycles (20 mo of age, 409 ± 6 kg body weight) were ovariectomized, and half of them were assigned at random to receive an E₂ implant. The ovariectomized heifers were assigned at random to receive diets that contained low (L; 5.8 Mcal·animal^–l ·d^–1, n = 8) or high levels of energy (H; 20.0 Mcal·animal^–1 ·d^–1, n = 8) for 100 d. The other four heifers remained intact and were fed the L-diet. The intact heifers were utilized to determine the status of reproductive function in animals fed the L-diet. Heifers lost body weight rapidly after initiation of feeding the L-diet. Heifers fed the L-diet then stabilized at a lighter weight until the latter part of the experiment. One of the four intact heifers fed the L-diet became anetrus near the end of the study. Mean concentrations of LH in blood serum increased linearly (P<.05) in ovariectomized heifers fed the L- and H-diet. Mean concentration of LH in heifers fed the H-diet that were implanted with E₂ was similar to ovariectomized hiefers fed the H-diet that received no E₂. Mean LH in serum of ovariectomized heifers implanted with E₂ fed the L-diet was suppressed and remained low throughout the study. Frequency of pulses of LH in ovariectomized heifers fed the L-diet was less (P<.01) than that in ovariectomized heifers fed the H-diet. Estradiol decreased the number of pulses of LH in heifers fed the L-diet. We conclude that dietary energy restriction in beef heifers has a direct action on the hypothalamo-pituitary axis to lower the number of pulses of LH in the absence of ovarian steroids. However, ovarian E₂ appears to suppress further secretion of LH in heifers fed limited levels of dietary energy before the onset of nutritional anestrus occurs, therefore, our working hypothesis is rejected.

¹ Published as Paper No. 8143, Journal Ser. Nebraska Agr. Res. Div. The authors thank J. A. Ossenkop and C. C. Rieck for assistance in preparation of the manuscript, K. V. Moline for assistance in data collection, D. R. Brink for assistance in preparation of diets, K. A. Pearson for technical assistance in analysis of data. We also thank Dr. Jerry Reeves, Washington State Univ., Pullman for the LH antisera; Dr. N. Mason, Dow-Corning Lab., Midland, MI for the E₂ antisera; and Dr. Leo Reichert, Albany Med. College, Albany, NY for the purified LH used for iodination.

² Present address: Dept. of Anim. Sci., Univ. of Missouri, Columbia.

³ Reprint requests to: Dr. James E. Kinder, 218 Marvel Baker Hall.

⁴ Dept. of Anim. Sci.