A comparison of three methods of assessing sex-drive in yearling beef bulls and relationships with testosterone and LH levels.

Three methods of assessing sex-drive were compared in 113 yearling beef bulls. These were the serving capacity score (SC), the libido score (L) and reaction time to first service (R). Ovariectomized heifers restrained in service crates were the stimulus for all tests. Bulls were assessed twice by each method. For the first libido and serving capacity tests (L1 and SC1), the heifers were induced to show estrus. For the second tests (L2 and SC2), the heifers were not induced to show estrus. On a non test day, single blood samples were taken from all bulls and assayed for LH and testosterone. Reaction times to first service (R1 and R2) in the two serving capacity tests were not significantly correlated. Although the numbers of services (SC1 and SC2) in both serving capacity tests, were significantly correlated (r = .67), 57% of the bulls did not achieve a service in both the tests with heifers in heat and with heifers not in heat. Libido scores between the test with heifers in heat and heifers not in heat were significantly correlated (r = .67). The libido score method had the advantage that more bulls received a positive score and the test duration was shorter than in the serving capacity test. Of the three scoring procedures compared, libido score appeared to have most advantage in assessing sex-drive in yearling beef bulls. The total number of services achieved in the first serving capacity tests (using 'estrus' heifers) did not differ from that achieved in the second tests (using non-estrus heifers). The total services achieved in 10-min compartments of the 30-min serving capacity tests showed no difference either within or between tests. It is concluded that a 10-min test provides as much comparative information on the sex-drive of yearling beef bulls as longer tests. Further, the use of females in estrus appears unnecessary to satisfactorily assess bull sex-drive, provided proper restraint and presentation of stimulus females is employed. LH and testosterone values were not significantly correlated or were poorly correlated with sex-drive measurements.