Abstract: In order to explore the relationship between the yield of qualified frozen semen and serum biochemical indexes of bulls at different ages, as well as the effect of aging on the physiological indexes of bulls at different ages. Eventually, the stud bulls were screened more accurately by serum biochemical indicators. Blood samples were collected from 10 Simmental bulls in spring for two consecutive years, some serum biochemical indicators were detected, and the results of frozen semen production were compared and analyzed. The results showed that semen density, qualified frozen sperm yield, serum aspartate aminotransferase and abdominal insulin levels decreased significantly with age (P<0.01). Serum androstenedione, dehydroepiandrosterone, testosterone, and serum γ-glutamyltransferase were significantly increased in both high-yield and low-yield groups, while serum zinc and serum alkaline phosphatase were significantly increased in low-yield group (P<0.01). The serum levels of copper and magnesium in the high and low yield groups increased significantly (P<0.05). The levels of serum total protein and serum uric acid decreased significantly (P<0.05). Therefore, it is necessary to pay attention to the daily health management of breed bulls, timely supply the necessary trace elements, and reasonably arrange the service age of breed bulls to give full play to the breeding potential of breed bulls.

Key words: Simmental, Breed bull, Qualified frozen semen production, Serum biochemistry