Breeding cattle that are better adapted to parasites can reduce the reliance on chemicals and other management interventions, and improve productivity and animal welfare.
To genetically improve any economically important trait through breeding, the trait(s) being selected for need to be under some degree of genetic control i.e. they must be heritable.
Heritability (H) of a trait is defined on a scale of 0 – 1 where:
H = 0 the trait is not heritable, but based on environment.
H < 0.1 there is low heritability of the trait.
0.1 < H <0.4 the trait is moderately heritable (Resistance to cattle tick falls here for beef and dairy cattle)
H > 0.4 the trait is highly heritable.
Resistance of cattle to ticks is moderately to highly heritable so it is possible to directly improve resistance of cattle to ticks through crossbreeding and within-breed selection. Preliminary evidence suggests that resistance to cattle tick may also provide resistance to other ticks.
To breed for any economically important trait, breeders should record all animals within a contemporary group (i.e. those that have been managed together, ideally since birth or weaning) for the trait of interest (in the case of tick resistance, undertake a tick count or a tick score - see sections ’How to count cattle tick’ and ‘How and when to tick score’). Animals can then be ranked on the basis of lowest (most resistant) to highest (most susceptible) tick count, see Figure 1.
For genetic improvement purposes, this ranking is best done in conjunction with Estimated Breeding Values (EBVs) as calculation of EBVs also accounts for genetic linkages or relationships and other effects impacting on different animals in the contemporary group and therefore provides a more accurate assessment of the ranking of those animals. Although more research is needed, resistance to most environmental stressors appears to be largely independent of productive traits such as growth, reproduction and product quality.
The process to assess resistance of cattle to ticks is the same for all cattle breed types (beef and dairy cattle and across Bos taurus and Bos indicus breeds). Cattle need to be exposed to ticks and allowed time to acquire their level of resistance before assessments of tick resistance are undertaken. Cattle which are reared on pasture in tick-endemic areas can be assumed to have acquired their resistance to ticks by the time they are weaned at around 6-9 months of age. Weaning tends to occur around the start of the tropical ‘dry season’, to ensure cows are not lactating during periods of nutritional stress. The start of the ‘dry season’ also tends to coincide with the start of cooler months when tick infestations are lower and less reliable. Hence it is recommended that assessments of cattle tick resistance be undertaken from the commencement of warmer periods in post-weaning cattle (e.g. in the southern hemisphere that would equate to calf weaning from May to July and tick resistance assessments undertaken from August or September, when the calves are 9-12 months of age).
As increasing genomic information becomes available for cattle it has become clear that resistance to cattle tick is not controlled by a simple presence or absence of a single genetic trait. Instead 100’s to 1,000’s of genes appear to be involved. Breeding values for cattle tick resistance have not yet been achieved. The resistance status of a very large number of cattle populations need to be determined first. These populations are currently being set up for study in Australia and several other countries.
Assuming the measurement of host resistance becomes more feasible, future animal breeding approaches using genomic information should be able to improve host genetic resistance to cattle ticks.