This investigation was carried out due the lack of information about the possible effects of the blood-feeding horn fly, Haematobia irritans (L.), on Nellore cattle. Data were recorded during four rainy seasons (October to April) from 1991 to 1995. In each year, 80 animals were utilized according to the following treatments: twenty-eight 1-year old bulls divided into four groups; twenty 2-year old cattle divided into four groups and thirty-two 3-year old cattle divided into eight groups. Treatments against H. irritans were conduced at about 28-day intervals on half part of each age groups and the remainders groups maintained without treatment as control. The flies were counted at 14-day intervals and the animals weight registered at each 28-day intervals. The values for the mean flies number on the animals of the control groups in the first, second, third and fourth years of this study were: five, five, four and five, in the 1-year old group; 15, 11, 13 and 27, in the 2-year old group, and 55, 31, 40 and 51, in that 3-year old group, respectively. It was observed that higher number of flies (P<0.05) attack adult animals than younger ones. Despite of the low number of flies per cattle, there was a significant weight gain (P<0.05) in favour of treated groups than the controls, which gained 8.6, 16.0 and 10.4% over the period, for groups one, two and three years old, respectively. The weight gain per year was 9, 5, 7 and 8 kg for the 1-year old groups; 26, 10, 23 kg and 12 in 2-year old groups, and 16, 8, 9 and 11 kg in 3-year old groups.
cattle; Nellore; horn fly; economic damage; Brazil
