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Effects of Natural Mating Stimuli on Serum Luteinizing Hormone, Testosterone and Estradiol-17ß in Yearling Beef Bulls¹

U.S. Department of Agriculture, Clay Center, NE 68933 and Kansas State University, Manhattan 66506

Abstract

The objective of this study was to determine the effect of natural mating stimuli on serum concentrations of LH, testosterone (T) and estradiol-17ß (E2) in beef bulls. Twenty sexually experienced, yearling beef bulls were bled every 15 min during a 9-h period, 4 h before and 5 h after exposure to estrual females. For exposure, each bull was placed individually in an isolated pen with two restrained estrual heifers for 10 min or until one service was achieved. Timing and number of all behavioral events, including flehmen responses, abortive mounts and services, were recorded for each bull by two independent observers. Of the 20 bulls, 9 bulls mounted and were removed immediately after achieving a service, 8 bulls mounted widiout achieving a service and 3 bulls exhibited no interest during exposure. Twelve bulls achieved fewer than three and eight bulls achieved three or more flehmen responses during exposure. Postexposure responses in LH, T and E2 were not consistently correlated with number of mounts or presence or absence of a service. However, postexposure LH and T, but not E2, responses were highly correlated with number of flehmen responses achieved (r = .40 to .66; P = .08 to .001). In bulls that achieved three or more flehmen responses, serum LH increased within 30 min after exposure (P = .02) and serum T was increased dramatically within 1 h after exposure (P < .01), compared with preexposure hormone concentrations, regardless of the number of mounts and regardless of the presence or absence of a service. Natural mating stimuli had no effect on serum E2, and mounting activity alone and mounting that culminated in a service did not necessarily result in increased LH or T in beef bulls. However, number of flehmen responses achieved during exposure to females dramatically influenced postexposure serum LH and T concentrations in beef bulls.

¹ The authors gratefully acknowledge Alan Kruger, George McMillan, B. D. Schanbacher, Edward Mohney, Steven Furman and Thomas Garvin for their assistance during blood sampling, handling of animals and radioimmunoassays. William Trout provided invaluable assistance for analysis of hormone profiles using the Goodman and Karsch algorithm within a microcomputerized program.

² Reprod. Res. Unit, R. L. Hruska U. S. Meat Anim. Res. Center, ARS.

³ Address requests for reprints to D. D. Lunstra, R.L.H. U. S. Meat Anim. Res. Center, USDA-ARS, P. O. Box 166, Clay Center, NE 68933-0166.

⁴ Dept. of Anim. Sci. and Ind., Kansas State Univ.

⁵ Present address: Dept. of Anim. Sci., Colorado State Univ., Fort Collins 80523.