Using other traits for indirect within breed selection

• Direct genetic control refers to traits (such as tick resistance) that are heritable.
• Indirect genetic control refers to traits that associate with other heritable traits, they are inherited together.
• Cattle tick resistance can be selected for indirectly by finding other traits that associate with it such as low rectal temperature under heat stress, or low worm counts.

To improve traits through breeding, the traits being selected must be under direct or indirect genetic control. Direct genetic control is assessed by estimating the heritability of traits. Indirect genetic control relies on the heritability of the traits as well as the favourable or unfavourable associations (genetic correlations or co-heritabilities) between different traits. The ‘Within breed selection’ and ‘How and when to score’ sections outline the processes needed to genetically improve tick resistance using direct genetic control methods, based on the moderate to high heritabilities estimated for cattle resistance to ticks.

Selecting cattle directly for their resistance to ticks is generally the easiest and preferred method of achieving genetic improvement, but might be enhanced by indirect selection. This section describes the possibilities of using indirect selection for resistance to ticks in cattle.

In tropical composite beef cattle grazed at pasture in the tropics, genetic correlations (associations) have been found between tick and worm counts, as well as tick count and rectal temperature (Figure 1). Lower rectal temperature has also been favourably associated with production measures such as weight gains, birth weight, pregnancy rate and days to calving. A study that selected for higher growth rates in cattle resulted in improved resistance to ticks in the tick-susceptible cattle but not the tick-resistant cattle.

Figure 1. Genetic association between rectal temperature and tick resistance. Calves of a tick-susceptible line of cattle selected over an 8 year period for low rectal temperature under heat stress (blue left axis) also displayed lower tick numbers (orange right axis). Image courtesy of Heather Burrow.

These correlated reductions in tick counts as a result of selecting animals with lower EBVs for rectal temperatures suggest that an alternative approach to genetically improving tick resistance might be to select cattle for reduced rectal temperatures under conditions of heat stress. It can be concluded that selection to improve resistance to any one stressor of tropical environments will improve resistance to other stressors. This is particularly true for resistance to ticks, worms and heat stress, where genetic correlations have been consistently moderately positive, suggesting the same or closely-linked genes affect all three traits.

With the exception of heat stress, resistance to ticks and most environmental stressors appears to be largely independent of productive traits such as growth, reproduction and product quality.