Physiology underlying the variation in feed conversion efficiency in meat sheep.

Research output: ThesisDoctoral Thesis


The research reported in this thesis looked at some of the physiological mechanisms, which are thought to underpin variation in feed conversion efficiency (FCE), and thus efficiency of energy use in meat sheep. Following evaluation of the models typically used to calculate residual feed intake (RFI), there were a number of broad hypotheses in relation to explaining variation in feed conversion ratio (FCR) and RFI which postulated that variation in both FCR and RFI could be explained to varying degrees by differences in digestibility, body composition, age, stress responsiveness, oxygen consumption and metabolite and hormone profiles.

Providing feed to animals is one of the greatest costs in livestock production systems, and as such livestock production research often looks at means to decrease inputs or increase outputs for the same level of inputs. Previous research has shown that efficiency of feed use in ruminants, as measured by RFI, is heritable, yet a large amount of the variation in RFI, is currently not explained sufficiently by physiological processes. There are a large number of physiological processes which can impact on energy utilisation within an animal and thus increasing our knowledge of the relationships between these processes and variation in RFI may enable the minimisation of any detrimental effects that selection for RFI may have, as well as indicating more suitable methods of identifying superior animals.

In this research, notable advances were made in identifying physiological parameters, which were linked to FCR and RFI in meat sheep. Of particular significance was the repeated demonstration of a significant relationship between RFI and an animal’s serum cortisol response to a known stressor (adrenocorticotropin hormone, ACTH). This observed relationship clearly highlighted the importance of the hypothalamic-pituitary-adrenal axis, and when combined with differences in epinephrine-stimulated metabolism, indicated that an animal’s response following exposure to a stressor has a significant impact on efficiency of energy use.

Other physiological parameters were found to be of less importance, or in contrast to previous research. With respect to differences in body composition, the relationship with RFI was variable, yet for FCR, differences in body composition could account for over 60% of the variation. Differences in oxygen consumption were observed before and after feeding at maintenance, following a 72-hour fast, and before and after a series of thyroxine injections, yet not related to either FCR or RFI, possibly due to the novel methodology used. The relationships observed between dry matter digestibility and RFI and FCR are in contrast to what has been previously observed, and what would be expected to occur, indicating that further clarification is required.

Initially, strong negative relationships were observed between both RFI or FCR and glucose and insulin concentrations before and after feeding, suggesting differences in insulin sensitivity, however these did not transpire in later experiments. Although glucose, insulin, cortisol and epinephrine are all intrinsically linked, the results suggest that given the elucidated relationships between cortisol, epinephrine and RFI, closer attention needs to be given to changes in glucose and insulin metabolism.

It is clear from the results obtained from the various physiological processes, that the mechanisms underlying variation in FCE are multiple, complex and interlinked in nature, with not one physiological process likely to be the key regulator. Within this thesis, significant gains have been made in contributing to the knowledge base of the physiological processes underpinning variation in RFI. Of particular importance, is the identification of the HPA axis, and thus the stress response as a major contributor to variation in efficiency of energy use. However, variation in RFI is still not fully defined and thus it remains necessary to continue exploring the impact of various physiological mechanisms.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • University of Melbourne
Publication statusPublished - 2006


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