Abstract
It has been well demonstrated that the quantity and quality of the semen obtained from bulls improve with age. To understand the relationship between the semen characteristics and hormonal conditions in Holstein bulls, age-related changes in serum levels of inhibin, FSH, LH and testosterone were investigated. One hundred three Holstein bulls were divided into 11 age-groups as 2.5 (n=10), 6.5 (n=10), 12 (n=12), 24 (n=9), 36 (n=10), 48 (n=12), 60 (n=13), 72 (n=8), 84 (n=5), 96 (n=8) and 108 (n=6) months old. Serum levels of inhibin, FSH, LH and testosterone were measured by specific radioimmunoassays. Serum levels of inhibin were highest in bull calves before 12 months of age, and significantly decreased at 12 months old and remained low thereafter. In contrast to inhibin, serum levels of gonadotropins and testosterone were low in young bulls, and then increased with age. In summary, the concentrations of inhibin, gonadotropins and testosterone in serum changed with age in Holstein bulls.
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