Libido, hormone concentrations in blood plasma and semen characteristics in Holstein bulls.

Relationships among bull libido, serum hormone concentrations and semen characteristics were studied using 18 Holstein bulls that were 4 to 5 yr old. The hormones studied included testosterone, estradiol (E2), prolactin (PRL), LH and cortisol. Two ejaculates were collected three times per week from each bull during a 5-wk trial. During the last week of the trial, on a day semen was not collected, blood was collected from indwelling catheters every 15 min for 6 h to determine the hormonal profiles of each bull. On the following day, blood was sampled every 10 min before and after the time of semen collection. Libido factors were quantified, and semen volumes and sperm concentrations were recorded. The libido factors included reaction time to first service, latency time between the first and second semen collections, and duration of time the bull mounted the teaser prior to the first (TM1) and second (TM2) semen collection. Average reaction and latency times were correlated (r = .524; P = .026), as were TM1 and TM2 (r = .597; P = .015). Latency times were correlated with average TM2 (r = .669; P = .003). Average PRL concentrations were correlated with average latency times (r = .467; P = .05). Low libido bulls tended to have higher E2:testosterone ratios than did high libido bulls. Both PRL and cortisol concentrations increased at semen collection.