Reproductive characteristics and thyroidal function in relation with season in Khuzestan buffalo (Bubalus bubalis) bulls

High ambient temperature is the major constraint on Buffalo productivity. The aim of this study was to evaluate the reproductive performance and thyroid gland function in winter and summer seasons in Khuzestan buffalo bulls. Six male indigenous buffaloes of Khuzestan with nearly the same age (2-3 years old) and weight were used. Semen and blood samples through jugular vein were collected, every two weeks throughout the summer and winter seasons. The thyroid hormones and thyrotropin stimulating hormone (TSH) concentration in blood serum were measured by radioimmunoassay method. Semen quality was determined, using computer assisted sperm analyzer (CASA) and routine methods. The concentration of thyroxin (T4) was lower in winter than summer (p ≤ 0.05). The level of T3 uptake was higher in cold season than that of in hot season (p ≤ 0.05). The differences of tri-iodotyronine (T3) and TSH concentrations, as well as free thyroxin index were not significant between seasons. The semen volume and spermatozoa parameters including concentration, progressive motility, linear velocity, mean velocity, beat cross frequency, linear coefficient and straightness coefficient were higher in winter than summer (p ≤ 0.05). Semen pH and amplitude of lateral head displacement of spermatozoa were higher in summer than winter (p ≤ 0.05). In winter, there was positive correlation between spermatozoa concentration and T3 value of blood serum (p ≤ 0.05). There were positive correlations between values of semen volume and T4, progressive spermatozoa motility percent and TSH, as well as, total motility of spermatozoa and TSH in summer (p ≤ 0.05). In general, thyroid function and semen quality of Khuzestan buffaloes may be affected by seasons.