Seasonal thyroidal activity and reproductive characteristics of Iranian fat-tailed rams

Abstract

An experiment was conducted to study seasonal variations of thyroidal activity, serum testosterone concentration and seminal characteristics of two breeds of Iranian fat-tailed sheep. Eight 3 to 4-year-old rams of Ghezel and Mehraban breeds (4 rams/breed) were randomly selected from a flock of fertile rams. Semen was collected by using an artificial vagina, and blood samples were obtained via jugular vein. Seminal characteristics, scrotal length, width, and circumference, and blood parameters were measured at 3-week intervals. Time of the year significantly ( P < 0.05) affected the volume of semen, sperm concentration, percent live sperm, percent normal sperm, total sperm number, seminal pH, seminal lactate dehydrogenase (LDH) concentration, scrotal circumference, scrotal width, and serum levels of testosterone, cholesterol, triiodothyronine (T 3), tetraiodothyronine (T 4), free T 4 index (FT 4I), and TSH. Scrotal length was not affected by the time of sampling ( P > 0.05). The effect of breed on the serum concentrations of hormones and cholesterol was not significant. Volume of semen, percent normal sperm, and total number of normal sperm were significantly ( P < 0.05) higher in Mehraban but scrotal circumference, scrotal width and scrotal length were greater in Ghezel rams. Breed × time interaction effect was significant for T 4, FT 4I, sperm concentration, and total sperm number. The largest values for TSH, T 4, FT 4I, testosterone, total sperm number, percent normal sperm, percent live sperm, sperm concentration, volume of semen and scrotal circumference were found from early summer to winter and the lowest values were recorded at the end of spring and in early summer. In both breeds, high and negative correlations ( r = −0.7 to −0.8) were found between LDH levels in semen and percent live sperm, percent normal sperm and total sperm number ( P < 0.01). It is unlikely that such variations in seminal quality would affect the fertility under natural mating.