It is possible to simultaneously select for productive and adaptive traits in tropically-adapted cattle, without compromising genetic progress in either, recent Beef CRC research has shown.
Cattle in Northern Australia experience numerous environmental stressors that reduce their growth and reproductive performance and decrease their beef quality. These often include parasites (tick, buffalo fly, worms), seasonally poor nutrition, high heat and humidity and diseases, often transmitted by parasites.
The impact of each stressor on production and animal welfare is often multiplicative rather than additive, particularly when animals are already undergoing physiological stress such as lactation.
Hence under highly stressful conditions, cattle deaths can occur due to the stressors. Under extensive production systems common in the tropics it is generally not possible to control the stressors through management strategies alone.
Therefore the best method of reducing their impact to improve productivity and animal welfare is to breed cattle that are more naturally resistant to productivity decline when challenged, without the need for management intervention.
The impacts of productive and adaptive traits
In every production environment, factors limit beef production, meaning no one breed is best in all environments. Comparative rankings of different cattle breed types for different characteristics in tropical environments are shown in the table below.
Any cattle breeding program designed for the tropics and sub-tropics must consider the impacts of both productive and adaptive traits, even though the adaptive traits (and some productive traits) are very difficult and/or expensive to measure. However the differing impacts of environmental stressors across the breed types indicates that genetic parameters and economic weightings for use in selection indexes must be specific for each breed type and environment.
Comparative rankings of breed types for productive and adaptive traits in tropical environments (more stars = higher value for that trait)
For traits to be included in effective breeding programs, they must be under direct or indirect genetic control. Direct genetic control is assessed by estimating the heritability of traits. Indirect control is achieved through favourable or unfavourable associations (genetic correlations) between different traits.
Beef CRC and earlier research primarily from northern Australia indicates that all the key productive and adaptive traits are at least moderately heritable in tropically-adapted cattle reared in tropical environments, meaning they will respond to genetic improvement through crossbreeding and within-breed selection programs.
In addition, no major antagonistic relationships have been found that would preclude simultaneous genetic improvement of all the traits in tropical beef breeding objectives.
Studies at Belmont Research Station near Rockhampton showed that resistance to parasites and productive attributes such as growth and reproduction are largely genetically (though not phenotypically) independent, meaning selection for parasite resistance will not genetically change productive attributes, or vice versa.
However resistance to heat stress and productive attributes are favourably correlated, particularly in breeds that are not as well-adapted as the Brahman, meaning that selection for growth or reproduction will improve resistance to heat stress and vice versa.
Strategies to optimise adaptation and production
Based on extensive reviews of the scientific literature, the Beef CRC has developed a number of ‘rules of thumb’ to optimally match cattle ‘genotypes’ (breeds or sire lines) to their production and marketing environments. These ‘rules of thumb’ as they apply to crossbreeding systems include:
- Depending on the severity of the environment and the level of stressor challenge, 25pc to 75pc ‘adapted genes’ are required for optimal production. Only exceptionally stressful environments (rare in Australia) require 100pc ‘adapted genes’.
- Adapted genes can be derived from Bos indicus and their derivatives, as well as the tropically adapted Taurus breeds, providing an opportunity to use heterosis from crossbreeding and to maximise productivity without reducing resistance to environmental stressors below levels acceptable for the production environment
- For most tropical environments, optimal levels of productivity and adaptation will be achieved using a combination of multiple breed types (e.g. Bos indicus, tropically adapted Taurus (like Senepol, Belmont Red), British and European).
- The harsher and wetter the environment, the greater the need for Indicus content to ensure sufficient adaptation to parasites (mainly ticks and worms).
In breeds that are well-adapted to their production environment, there are no major antagonistic relationships to preclude simultaneous genetic improvement of both productive and adaptive traits through selection to maximise herd profitability, CRC research shows.
The major constraint to such genetic improvement is the difficulty and expense of measuring the complete range of economically important productive and adaptive traits required to achieve a balanced breeding objective. This same constraint also applies to genomic selection using DNA-based technologies, which offer new opportunities for tropical beef producers.
Until phenotypes for these traits become available, northern producers can confidently select to improve productive attributes in their cattle, knowing they are unlikely to compromise adaptation of their herds.
Contact: Anne Marie Huey, Department of Agriculture and Food WA, Broome. Email: firstname.lastname@example.org