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U. S. Department of Agriculture,3, Clay Center, Nebraska 68933
Abstract
A procedure for assessing sexual activity (libido) in individual rams is described. Mating activity measured by this procedure and serum levels of luteinizing hormone (LH) and testosterone in Finnish Landrace and Suffolk rams was determined at 8-week intervals from October, 1974 through October, 1975. Seasonal changes were observed for serum LH, testosterone and libido index scores. Mating activity in rams was highest for both breeds during the peak breeding season (October) and declined 50% by late spring and summer before it increased the next October.
Serum LH concentrations were lowest in May (.54 ng/ml) and increased abruptly in July (>2 ng/ml) when daily photoperiod began to decrease. Serum testosterone concentrations (>6 ng/ml) and mating activity were highest during the October evaluations. Testosterone decreased gradually through the winter months and reached its lowest levels in late March (2.06 ng/ml in Finn rams and 1.01 ng/ml in Suffolk rams). Thereafter, concentrations gradually increased to levels observed the previous fall. A positive correlation (r = .59) between mean testosterone and mating scores collected across months suggested that seasonal fluctuations in serum testosterone influence the sexual behavior of rams. A temporal relationship was not detected between circulating testosterone and sexual behavior.
1 The authors gratefully acknowledge Dr. J. J. Ford2 for assaying serum LH and Dr. G. D. Niswender for generously supplying the LH antiserum. Cooperation of the Nebraska Agricultural Experiment Station, University of Nebraska, Lincoln, is acknowledged.
2 U.S. Meat Animal Research Center, Agricultural Research Service.
3 Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may be suitable.
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 H. Tucker and R. Ringer Controlled photoperiodic environments for food animals Science, June 25, 1982; 216(4553): 1381 - 1386. [PDF]
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