Resistance to Boophilus microplus (Canestrini) in genetically different types of calves in early life.

Tick resistance and blood composition were studied in British (1/2 Shorthorn x 1/2 Hereford) and zebu (1/2 Brahman x 1/2 British) calves from birth to 33 days of age in a tropical grazing area in which B. microplus is endemic. Calves of the 2 breeds were either naturally infested or were, in addition, artificially infested with 5,000 larvae at 2 and 9 days of age. Total numbers of mature female ticks carried from either type of infestation were significantly lower (P less than 0.01) on zebu than on British calves. In the artificially infested calves of both breeds, the total number of ticks maturing between 20 to 26 days of age was significantly higher (P less than 0.01) than the number maturing between 27 to 33 days of age emphasizing that a major component of resistance is acquired. However, in the naturally infested calves, breed differences in the numbers of ticks maturing during these 2 periods suggested the presence of genetic differences in innate resistance. Within breeds, total tick numbers carried during the study were negatively correlated with calf weight gain and with the concentrations of serum albumin, total protein, and cholesterol.