Variations between and within breeds of cattle in resistance to field infestations of the cattle tick ( Boophilus microplus )

Maturing female ticks were counted in herds of Brahman x British, Africander x British, and British breed (Shorthorn x Hereford) cattle run together. About 3000 counts were made on over 1000 cattle. The Zebu crossbreds on average carried two-fifths the number of ticks carried by the British cattle, though in some herds the difference was greater, and it tended to be greater in females than in males, in summer than in winter, and in F2 than in F3 animals. Brahman cross and Africander cross cattle did not differ significantly in resistance. Male cattle carried significantly more ticks than females, and the resistance of lactating British breed cows, but not Zebu crossbreds, was lower than that of dry cows. Repeatability of counts was higher in females than in males, and higher in Zebu crossbreds than in British cattle. Expression of inherent resistance in the British cattle was variable according to circumstances which are only partly defined. Heritability estimates were up to 48%, but in some cases much lower. In the Zebu crossbreds there was little heritable variation in F, cattle, but in subsequent generations heritability was estimated as 82%. The needs for selection for resistance, and the control of error and the prospects of success, are discussed.