Sexual behavior and pregnancy rate of Bos indicus bulls.

To study sexual behavior and pregnancy rate of Zebu bulls, 9 Brahman and 6 Nelore bulls were first examined for libido and serving capacity, semen characteristics and scrotal circumference, a week before being placed, individually, into a group of 16 estrus-syncronized heifers to observe mating activity. Scores for libido (0 to 10) and for serving capacity (based on the number of services per period) were assigned for each bull over a 15-min period. Estrus was synchronized with Norgestomet (3 mg, s.c. implant and 3 mg, i.m.) and estradiol valerate (5 mg, i.m.). The implants remained in place for 10 d, then 40 h after their removal a bull was placed with the heifers. Four Brahman and 6 Nelore bulls were monitored for 2 h while the remaining 5 Brahman bulls were observed for 30 h. Number of services and mounts per bull, the hour at which each event took place, and the identification of the heifers serviced were recorded. Pregnancy was diagnosed by transrectal palpation 50 to 60 d after mating. Mean libido score (+/- SEM) of the 15 bulls was 6.4 +/- 0.5, and mean serving capacity was 0.53 +/- 0.19; both of these measures were correlated with age (r = 0.78, P < 0.01, and r = 0.56, P < 0.05, respectively). In the 2-h trials, the number of mounts was correlated with the libido score (r = 0.73, P < 0.05), and the number of services was correlated with age (r = 0.76, P < 0.05) and with libido and serving capacity score (r = 0.91 and 0.92, P < 0.01). In the 30-h trials, the mean (+/- SEM) number of mounts and of services per bull was 74.4 +/- 21.1 and 14.6 +/- 0.5, respectively. The mount-to-service ratio and percentage of serving efficiency were 5.1 +/- 1.4, and 19.8 +/- 4%, respectively. Libido score was correlated with the mount-to-service ratio (r = -0.86, P < 0.10) and with serving efficiency (r = 0.84, P < 0.10). The percentage of heifers serviced, of those serviced 2 or more times and of those pregnant was 55 +/- 5.3; 47.1 +/- 6.4, and 59.2 +/- 5.8, respectively. Pregnancy rate was not correlated with any trait (P > 0.10). It was concluded that libido and the serving capacity test seem to indicate the mating potential of Zebu bulls over a relatively short period of time (e.g. 30 hours), in which resultant pregnancy rates are comparable to those of European bulls.