Selection of cattle for tick resistance, and the effect of herds of different susceptibility on Boophilus populations

Abstract

Weekly counts of Boophilus microplus (Canestrini) on 30 Australian Illawarra Shorthorn heifers enabled the cattle to be ranked in order of tick infestation, with highly significant correlations between counts of two observers and between counts of one observer on different occasions. In May 1960, when the heifers were 1½–2 years old, 12 were selected as relatively tick-resistant and 12 as relatively tick-susceptible. Each of these groups was divided at random into herds of six, and the four herds were then allotted randomly to separate paddocks, each onequarter of the area previously grazed. A herd was sprayed with 0.5% DDT emulsion when its average count of ticks (adult females over 5 mm in length) on one side of the animals exceeded 40. During the ensuing tick season, from October 5, 1960, to June 7, 1961, the sums of average weekly tick counts, and the numbers of sprayings (in parenthesis) were: susceptible herds 4853 (5) and 5962 (6): resistant herds 718 (0) and 1073 (1). Counts of tick larvae on defined body areas showed that, in the summer after segregation, resistant herds carried fewer larvae than the susceptible herds, apparently because fewer mature ticks fell from the resistant cattle in the preceding spring and winter. As a consequence of this, counts of adult ticks were comparatively lower after than before segregation. There was little or no 'spring rise' of tick infestation on the resistant herds. There was no significant correlation between tick resistance and coat score, sweat gland dimensions, or total skin thickness, but a correlation of -0.53 with follicle depth was significant at the 1% level. There was no evidence of adaptation of cattle ticks to the resistant animals, either in the field experiment or in observations on stalled cattle. The experiment draws attention to the appreciable proportion of tick-resistant animals within the Australian Illawarra Shorthorn breed, which has largely been overlooked in past discussions on tick-resistant breeds of cattle. It also suggests a technique for estimating the improvement in tick control that may be obtained by a given degree of selection within any breed, for any given environment.