The effect of the libido of the bulls on pregnancy rates at 30, 60 and 90 days of the breeding season was studied, and the correlations between the serum testosterone levels, seminal quality and libido were estimated. Twelve bulls selected by andrologic evaluation and by the libido test, were randomly allotted to: treatments T1 and T2, three low libido bulls and three high libido bulls, respectively, in the bull:cow proportion of 1:75; and treatments T3 and T4, three low libido bulls and three high libido bulls, respectively, in the proportion of 1:100. No differences among bulls based on the serum testosterone levels were observed. Bulls with high libido (proportion 1:75) provided larger pregnancy rate at 60 and 90 days (90.6 and 94.6%, respectively) of the breeding season, than low libido bulls in the same proportion (80.0 and 86.2%, respectively). The libido of the bulls in the proportion 1:100 had no effect on the pregnancy rates. The effect of bull:cow proportion on the pregnancy rate was not significant and no correlations between libido, seminal quality, scrotal circumference and serum testosterone levels were observed.
cattle; fertility; sexual behavior; testosterone; zebu