The objective of this study was to evaluate environmental factors that affect infestation of Caracu cattle breed by horn fly (Haematobia irritans) and to estimate genetic parameters for level of infestation. Infestation was evaluated on females from two herds in two consecutive years. Total number of flies on animals (FC) was counted, an infestation score (FS: 1 to 5) was recorded according to the estimated number of flies on animals, and number of parasites in photographs (FF) taken when the animals were evaluated was also counted. On each animal from one to eight observations were taken. A total of 3,836, 2,751 and 3,754 records from 718 animals were obtained for FC, FF and FS, respectively. The incidence of flies was lower during winter and higher during summer, and the thicker the hair coat of the animal the greater the infestation. Heritabilities and repeatabilities were equal to 0.10 and 0.10, 0.08 and 0.12, and 0.06 and 0.08, for FC, FF and FS, respectively. Findings indicate that selection for FC will result in low genetic progress and that animals should be evaluated more than once for selection purposes.
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