Resistance to the cattle tick Boophilus microplus was assessed in heifer herds of various breeds of beef and dairy cattle in spring and summer in Queensland. All cattle had had tick experience before assessment. Resistance levels were determined as the percentage of larval ticks that failed to survive to maturity following infestations with c. 20,000 larvae. Bos indicus Brahman beef cattle were the most resistant (99%), followed by B. indicus × B. taurus (95–97%) and B. taurus British cattle (85%). In the dairy breeds, B. taurus Jersey cattle (98%) were more resistant than Guernsey (93%), Australian Illawarra Shorthorn (89%), and Friesian (85%), but not significantly different from B. indicus × B. taurus Australian Milking Zebu (96%). Cattle were classified as having high (> 98%), moderate (95–98%), low (90–95%), or very low (< 90%) resistance. The frequency distributions of resistance in B. indicus × B. taurus cattle showed that 80% had moderate to high resistance and that culling of 20% of the cattle would about halve the mean tick population. Of B. taurus cattle, 80% had low to very low resistance. Supplementary information on sibling bull herds showed that their resistance levels and frequency distributions of resistance were similar to those of the heifer herds.

AbstracrA pattern of engorgement and dropping of the female cattle tick Boophilus microplus is described. Partl engorged females, which have grown to a length of 4-6 mm (10-30 mg), undergo rapid hal engorgement at night to reach a length of 8-11 mm (150-250 mg) and detach from cattle in the early hours of the morning. The minimum size of females which engorge varies with season. Differences in the engorgement and dropping pattern of ticks on cattle observed in summer and in winter, and on cattle held in open yards and in covered stalls, suggest an influence of the external environment, particularly temperature and light, on tick engorgement and detachment. Counting of ticks 4.5-8 mm in length on one day was shown to provide a reliable estimate of the numbers of engorged ticks dropping the following day and has been adopted for the assessment of tick numbers on cattle.

An Australian Illawarra Shorthorn herd of 24 cows was mated in three consecutive years with an AIS bull. The cows and their progeny were rated for tick resistance at frequent intervals from August 1959 to December 1965 by counting the numbers of semiengorged female ticks on the right side. The mean of log counts for all counts on a particular animal was adopted as the reference value for its degree of susceptibility. The ranking of cattle generally showed a high level of consistency with mean repeatability of counts (r = 0.47, P < 0.01). Discrimination between animals was more reliable (P < 0.01) in summer (r = 0.52) than in winter (r = 0.27). The repeatability of tick counts increased with mean count, from r = 0.27 when the mean count was 3 to r = 0.67 when it was 100. The reliability of counts on the cows decreased with age and with lactation. Supplementary information on a larger herd showed no effect of pregnancy on mean count or on discrimination between susceptible and resistant animals, but showed that there was a partial breakdown of resistance during lactation. In calves infested naturally, no effects of age or sex on tick counts or their repeatability were detected, though male calves yielded significantly larger numbers of ticks than females when infested artificially. The mean yield of mature female ticks on the cows following two artificial infestations with known numbers of larvae ranged from 0.2 to 27.4% of the potential. Natural and artificial assessments of susceptibility were closely correlated. The rank of the bull was similar to that of the more resistant cows. Mean estimates of the heritability of tick resistance based on single counts were 39 % from dam-calf correlations and 49 % from full-sib correlations. Estimates based on summer counts only were 42 and 64% respectively. These results provide strong encouragement for selecting for tick resistance.

Selection and breeding for tick resistance in an Australian Illawarra Shorthorn (AIS) herd was undertaken from 1966 to 1971. Mating of half the AIS herd with a Brahman bull in 1968–69 provided a comparison of tick resistance in selected Bos taurus and crossbred B. taurus x B. indicus cattle. Parent cows and their progeny were ranked for resistance to ticks, infestations of which were acquired as a result of grazing infested pastures, and from periodical artificial infestation with cohorts of c. 20,000 larvae of Boophilus microplus. Ranking and assessment of resistance levels were based on counts of semi-engorged female ticks on the right side. Parent cows selected for high resistance consistently carried fewer ticks than cows selected for low resistance. Mating of cattle selected for high resistance produced progeny that were significantly more resistant than the progeny of cattle selected for low resistance. However, the Brahman bull, which had slightly lower resistance than the AIS bull, produced progeny with higher resistance, even when mated with AIS COWS selected for low resistance. Pregnant cows were significantly less resistant than non-pregnant cows, and lactating cows were very much less resistant than non-lactating cows. Young cows were more resistant than older cows. Calves at foot carried fewer ticks than their dams when grazing infested pastures, but after weaning they were less resistant than their parents when resistance was determined from artificial infestations. Female calves carried fewer ticks than males on almost all occasions, but differences were not significant in winter. Significant seasonal changes in resistance were demonstrated in AIS and Brahman x AIS weaner and yearling cattle. All cattle were less resistant in winter. Changes were greatest in Brahman x AN females, on which the average numbers of female ticks that matured following artificial infestations in January 1970 (summer), July 1970 (winter) and January 1971 (summer) were 151, 1046 and 95 respectively.

Breeding for resistance to the cattle tick Boophilus microplus was undertaken in a herd of Australian Illawarra Shorthorn (AIS) cattle from 1961 to 1978 and in a herd of Braham x AIS cattle from 1970 to 1979. Breeder cows and their progeny were assessed for tick resistance during October to January. Resistance levels were determined as the average percentage mortality of female ticks from two artificial infestations with cohorts of c 20,000 larvae. Resistance increased from 89.2% to 99% in the AIS breeding herd, as a result of the yearly introductions of more resistant individuals and culling of less resistant ones. Concurrently resistance in the AIS progeny increased from 93.7% to 97.7%, thus demonstrating that the selection and breeding of the cows and bulls resulted in genetic improvement in the resistance of the progeny. Milk production tests on heifers from the selected AIS herd during 1975 to 79 indicated that selection for tick resistance did not select against milk production. Resistance of the Brahman x AIS increased from 98.4% to 99.3% in the breeding herd and from 97.6% to 99.6% in the progeny. Female calves of both breeds were more resistant than males.