Genetic analysis of the infestation of Caracu female cattle breed by cattle tick (Boophilus microplus)

The objective of this study was to evaluate environmental aspects that affect the infestation of the Caracu cattle breed by the cattle tick Boophilus microplus (Canestrini, 1887) and to estimate genetic parameters for the level of this infestation. Countings of the parasite were done on female cattle from two herds, during all four seasons of the year, in two consecutive years (from September 1998 through July 2000). The number of ticks (NC) on one side of the animal was counted, and the infestations were also evaluated by scores (EC), according to the estimated number of parasites on the animal. Each animal had from one to eight observations. A total of 4,079 and 3,994 observations were obtained for NC and EC, respectively. The data were analyzed by the least squares method with a model that included the effects of herd (H), color of the animal (C), H x C, animal within H x C as error a, year, season, thickness of the hair, and age of animal as a covariable (linear and quadratic effects). The variance and covariance components were estimated by the restricted maximum likelihood derivative free method, using a model that included the effects of contemporary group (herd - year - season), hair thickness and age of the animal as a covariable (linear and quadratic effects), and the random additive direct and permanent environmental effects. Before the analyses, NC was transformed to log(10) (n + 1) and EC was transformed to (x + 0.5)(1/2), where n is the number of ticks counted on the animal and x is the score for EC (0 to 4). The incidence of ticks was higher during the summer, and the ticker the hair the higher the infestation. The heritability and repeatability estimates were equal to 0.22 and 0.29, and 0.15 and 0.21 for NC and EC, respectively, and the genetic correlation between NC and EC was 1.00. The results indicate that there is enough additive genetic variation on tick numbers to support a selection program to increase resistance.