A total of 32 Horro (Bos indicus) bulls, with an average initial body weight and age of approximately 211 kg and 6 years were evaluated. The bulls were divided into two treatment groups. One group was given a supplementary concentrate at a rate of 1.5 kg/day and the second group served as the control and received no supplementation. The observation period lasted for 50 weeks. Semen was collected every two weeks by mean of electrical stimulation with the aid of an electro-ejaculator. The percentage of abnormal sperm (total abnormalities, head, midpiece and tail) remained lower in the supplemented group than in the non-supplemented group. The percentage of live sperm tended to be higher in the supplemented group throughout the 50-week trial period. Until week 17 of the trial there were no significant differences; however, starting at week 18 there were significant differences between the two treatment groups except for week 26. Thereafter, significant differences were recorded until week 38; from week 40 no significant differences were observed between the two groups in respect of percentage of live sperm (except for week 48). Season (week) during which the semen samples were collected had no significant effect on the percentage of abnormal sperm in both treatment groups (supplemented and non-supplemented). At the start of the trial the nutritionally supplemented group had a higher occurrence of sperm abnormalities (11.4% +/- 1.1%) compared to 8.3% +/- 1.3% in the control group. Thereafter, sperm abnormalities declined rapidly in the supplemented group (except for week 50). The sperm abnormalities for the non-supplemented group showed a tendency to increase during the entire observation period.