Impact of Reactive Temperament on Bull Fertility in Natural Service

The study aimed to evaluate the influence of temperament on semen quality and fertility in Nelore bulls. Two groups were selected based on ethological analysis, consisting of animals with either calm temperament (CA, n = 8) or reactive temperament (RE, n = 8). All bulls underwent two semen collections for sperm evaluation (pre- and post-thawing), as well as serial blood collections for testosterone and cortisol measurements. Ejaculates recovered from the three most reactive and the three least reactive bulls were pooled to form two semen batches, which were subsequently cryopreserved and used for fixed-time artificial insemination (FTAI) to evaluate the fertility of semen from bulls with the two different temperament profiles. Additionally, each bull was housed in individual paddocks with cyclic cows (average of 20 cows per bull) for 90 days for a natural service capacity test. No differences were observed in sperm kinetic parameters, morphology and plasma membrane integrity for fresh or post-thaw semen when comparing the two experimental groups. Likewise, no effect of reactivity was observed on the fertility of calm and reactive bull semen in FTAI (p > 0.05). CA bulls showed lower average cortisol concentrations compared to the RE group during the second experimental routine, but there were no differences in testosterone values. CA bulls achieved higher conception rates in natural service (84.56%) compared to reactive animals (76.41%; p < 0.0001). In conclusion, no significant effects of temperament on fresh or post-thaw semen quality parameters were detected in Nelore bulls under the conditions of this study. However, bulls with a calmer temperament exhibited higher reproductive performance in natural service programs, supporting the hypothesis that behavioural mating efficiency rather than intrinsic gametic failure influences pasture breeding success.