Influence of selective treatment of bulls on the infestation of Haematobia irritans on untreated cows.

The distribution of Hematobia irritans is not homogeneous in cattle populations. Generally, between 15% and 30% of bovines carry more than 50% of the flies, and higher infestations occur in bulls. In this study, the effect of treating the bulls with ear tags containing 40% diazinon on the infestation of flies on the herd during the breeding season was evaluated. Four groups of cattle were made, each containing one bull and 35 Braford cows. Groups 1 and 2 were on pastures in highlands. Groups 3 and 4 were on pastures in lowlands. At the beginning of the trial, each bull from Groups 1 and 3 was treated with an ear tag. Bulls from Groups 2 and 4 remained untreated. Weekly, for 10 weeks, horn flies were counted from each bull and 15 cows from each group. A repeated measures linear mixed model was used with cows as random effects. The number of horn flies in the treated groups was lower than that in the untreated group, suggesting that selective treatment of bulls could be an effective strategy to control horn flies on the herd. The number of flies was higher in the lowland than that in the highland groups, suggesting an effect of the environment on horn fly infestations. In addition, we estimated the cow level predictions from the model to estimate the cow effect on fly counts, and we compared those values with the observed weekly quartiles for each group. Fly-susceptible cows were defined as those for which the infestation of flies appeared in the upper quartile more than 50% of the weeks and in the lower quartile less than 20% of the weeks. In contrast, a fly-resistant cow was defined as one for which the fly counts appeared in the lower quartile more than 50% of the weeks and in the upper quartile less than 20% of the weeks. In all, 8.3% of the cows were susceptible and 15% were resistant. These results may be used to selectively treat bulls or to treat the bulls and the most susceptible cows during the breeding season.