Seasonal changes in breeding activity, testicular size, testosterone concentration and seminal characteristics in rams with long or short breeding season.

Breeding activity (libido), testicular size, testosterone levels and seminal characteristics of 10 DLS adult rams (a population of 1/2 Dorset, 1/4 Leicester and 1/4 Suffolk selected for extended breeding season) were compared with those of 10 adult Suffolk rams. The study lasted 18 mo during which the data were recorded over 30 collection periods. All characteristics varied significantly with the season of the year, being lowest during the summer and highest during the fall. Suffolk rams had higher libido but lower testosterone levels than the DLS rams throughout the study. Testosterone increased earlier and decreased later in the breeding season in DLS compared with Suffolk rams, the interaction breed X period being significant. Testosterone concentration was highly associated with testicular size and libido, accounting for 65 and 28% of the variation in the Suffolk and 37 and 18% in the DLS rams, respectively. Seminal output and quality (motility and percentage of live spermatozoa) were highest in October and November and lowest in April and May. Rams within breeds differed significantly in the seminal characteristics studied. Results of the study showed that selecting the DLS ewes for extended breeding season may have altered the reproductive performance of the rams of that breed.