Summary
Feed costs are the major cost in most animal production systems. Profitability and productivity are a function of both the inputs and the outputs, and typically ways to decrease the cost of feed or the cost of gain have focussed on genetic improvements aimed at improving the outputs eg. carcass traits, fertility and liveweight. However, it is also possible to look at ways of reducing the inputs into a livestock production system. The efficiency with which an animal utilises feed for both maintenance and gain is one such trait that can reduce the inputs into a system. Improved feed conversion efficiency (FCE) could potentially contribute to the profitability of the sheep industry by reducing the amount of feed required for lambs to reach slaughter weight or to maintain adult flocks. However, the high cost involved in setting up facilities for screening for FCE in large numbers of sheep is unlikely to be commercially applicable. Therefore it is necessary to understand the underlying differences in physiology that lead to some animals displaying greater levels of efficiency compared to others. From this knowledge, it may then be possible to identify suitable indicators (either genetic or physiological markers) associated with FCE or residual feed intake (RFI) which can then be used for selection in breeding programs at a reasonable cost.
Currently, there is very little information on why feed conversion efficiency varies between animals, and whether the variation is influenced by maturity, gender, nutrition and other environmental influences. Earlier work, conducted outside this project, but within the PhD program of the principal investigator, has shown that there is a significant relationship between RFI and an animal’s response to a known stressor (adrenocorticotropin hormone, ACTH) as well as animals of known efficiency showing differences in body composition, metabolism and respiration. Following on from this earlier work, the objectives of the current work were to identify animals, which were differing in serum cortisol response following administration of a known stressor. The animals identified as extremes (High Cortisol, HC or Low Cortisol, LC) were then used in more intensive studies. The objectives of the more intensive studies were to measure feed intake and weight gain to enable feed efficiency (RFI) to be calculated; explore the relationship between stress response, feed efficiency, and respiration following the administration of thyroxine to stimulate oxygen consumption; and finally to increase our understanding of the differences that exist between animals of known feed conversion efficiency and both basal and hormone or metabolite stimulated metabolism through the use of insulin, adrenaline, ACTH and glucose infusions. As part of the final objective, tissue samples were taken for gene expression work, however this has not been undertaken.
The key achievements for this work are:
• Successfully identifying rams within an unselected population of animals that are significantly different in their serum cortisol response following the administration of a known stressor. The rams selected as extremes had significantly different basal serum cortisol levels and post-ACTH serum cortisol levels. The test used was a simple challenge, involving the use of two blood samples, and administration of a specific dose of adrenocorticotropin hormone.
• Confirmation of results found in earlier work, in that there is a significant relationship between an animal’s stress response to a known stressor and its feed conversion efficiency or residual feed intake. These results show that less efficient animals (more positive RFI) are more responsive to the application of a known stressor.
• Demonstrating that there are differences in oxygen consumption, and therefore heat production between animals of different stress susceptibility (either HC or LC) and animals of different feed conversion efficiency, both basally and after administration of thyroxine.
• Demonstrating that rams of different feed conversion efficiency display differences in both basal and hormone or metabolite stimulated metabolism. This could have important implications for the development of future work in this area.
These key achievements indicate that there are a number of physiological mechanisms which are possibly driving the differences between animals in terms of how well an animal utilises the energy it consumes. Increasing our understanding of the physiological mechanisms behind why some animals are more efficient than others has clearly shown that there is a strong relationship between stress susceptibility and feed conversion efficiency. Other mechanisms such as respiration (oxygen consumption/heat production) and response to various hormones and metabolites have also been shown to be of importance, although the reason for this is currently unknown and more analyses are required of the data.
These results have important implications for the meat and livestock industry in determining alternative methods of identifying animals, which are more or less efficient than others in a particular population group. Although the practical impact on the meat and livestock industry in the immediate future is not large, due to the commercial development required to extend the results of this research, in five years time it is hoped that improvements in feed efficiency in Australia’s sheep flock (both meat and fibre producing animals) are starting to have a significant economic and productive impact, through enabling producers to identify those animals which are more efficient, using these animals in selective breeding programs and thereby increasing their productivity.
It is important to understand that selection for feed conversion efficiency is likely to impact on all the mechanisms (body composition, metabolism, stress plus others not studied in this work) behind this economically significant trait. Based on this relationship, the key recommendation is that further work needs to be undertaken on both the physiological basis behind feed conversion efficiency plus expression of genes, which may be important in determining why some animals are more efficient than others in the utilisation of energy. In particular, feed efficiency and the physiological basis for this trait needs to be explored in greater numbers of sheep, and comparisons need to be made between different genotypes.
