U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


Document Type


Date of this Version



Published in Journal of Animal Science (1976) 43:3, 644-650


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.