Seasonal variation in semen quality of bulls and correlations with metabolic and endocrine parameters.

Sperm output and semen quality of 17 bulls sampled over 12 months showed minimal output in mid-winter and late summer and minimum quality in late summer. Monthly measurements of luteinising hormone and testosterone concentration in plasma and testosterone concentration in semen were made over 12 months. Serum non-esterified fatty acids (NEFA), albumin and total protein were also measured for the final seven months of this period. Plasma testosterone showed a strong negative correlation with sperm numbers two months hence but not in the current month. Plasma testosterone by bull and semen testosterone by month was also correlated with sperm output. Plasma luteinising hormone three months and serum total protein two months prior was positively correlated with sperm numbers ejaculated and the normality of sperm morphology, possibly by affecting the luteinising hormone dependent A0 to A1 spermatogonial division. NEFA was correlated with initial and post freezing/thawing motility in the current month, possibly by affecting membrane stability. The value of examining the bull in diagnosing infertility of cows where nutritional stress may have occurred is suggested, as is the use of albumin/total protein and NEFA measurements as a prognostic aid for time to return to normality of function of such bulls.