Date of this Version
Adequate sex drive in bulls is essential for natural mating to be successful. Expression of male sexual behavior and mating ability during sexual maturation is dependent upon attaining adequate testicular development and blood levels of luteinizing hormone (LH) and testosterone (T). Several researchers have investigated the relationship between levels of sexual behavior in postpubertal bulls and blood concentrations of LH and T. Some of these researchers reported a positive relationship between T and serving capacity (SC), and others found that individual differences in sexual performance could not be predicted based on circulating levels of T or LH. In those studies, comparisons between sexual behavior of bulls and hormone levels were based on a single blood sample or on infrequent blood sampling. Because no previous studies have utilized a frequent enough sampling regime to determine the episodic release of LH and T, there is a lack of research characterizing the hormonal patterns such as peak frequency, height, and area under the peaks of these hormones in bulls of differing serving capacity. Hormones other than LH and T may influence SC, but little research is available relating these to sexual behavior in bulls. However, thyroid hormones may have some relationship to sexual behavior because earlier research found that removal of the thyroid gland in the bull resulted in disappearance of sex drive and that the feeding of thyroid substance promptly restored sex drive in the hypothyroid bull. Whether this is an effect of a lowered metabolism or a specific endocrine effect has not been established.
The objective of the present study was to provide more definitive information on the relationship between the serving capacity of yearling beef bulls and their profiles of LH and T during sexual rest and their blood levels of thyroid hormones. In addition, the interrelationships between these hormones and parameters of testicular function, such as scrotal circumference, paired testicular volume, and seminal traits, were studied.