Monthly variation of plasma concentrations of testosterone and thyroid hormones and reproductive characteristics in three breeds of Iranian fat-tailed rams throughout one year.

Nine sexually mature rams, three Shall, three Afshari and three Zandi were used in this study. Semen of rams was collected twice per month during the four seasons of the year and blood samples were obtained via jugular vein. Seminal characteristics, scrotal dimension and blood parameters were measured at 2 week intervals. Time of the year significantly (p < 0.05) affected the volume of semen, sperm concentration, percentage live sperm, percentage abnormal sperm, sperm mass motility, total sperm number, scrotal circumference, relative testis volume and serum levels of testosterone, triiodothyronine and thyroxine. All of semen characteristics, except sperm abnormality, were high in summer months than at other months of the year (p < 0.05). Sperm abnormality was higher in autumn months (p < 0.05). Relative testis volume and testicular circumference were higher in autumn and summer months than at other mouth of the year, respectively (p < 0.05). Scrotal circumference were not significant between Shall and Afshari breeds but comparison with Zandi breed, Scrotal circumference was bigger (p < 0.05). The largest values for thyroid hormones synchronized with low reproductive performance of rams, but testosterone was highest levels at this time (p < 0.05). TimexBreed interaction effect was significant for sperm concentration, total sperm number, semen volume, testis circumference, relative testis volume, thyroid hormones and testosterone (p < 0.05).