Summary

The Australian chicken-meat industry is extremely interested in low-protein diets which, axiomatically, contain high inclusions of an increasing range of synthetic amino acids. A fundamental issue to the development of low-protein diets is how efficiently broiler chickens utilise synthetic amino acids. Ted Batterham demonstrated that synthetic lysine utilisation in pigs was influenced by feed access. Pigs fed on a continuous basis utilised synthetic lysine far more effectively than restricted-fed pigs. This was attributed to an imbalance of synthetic lysine versus protein-bound amino acids at sites of protein synthesis because the absorption of synthetic amino acids is more rapid than the prior digestion and absorption of protein-bound amino acids. Continuous access to feed dampens this potential imbalance. This outcome suggests that the utilisation of synthetic amino acids would be compromised in poultry by lighting regimes with relatively long ‘lights-off’ phases. However, David Baker failed to replicate Batterham’s findings in poultry and suggested that free lysine may be conserved in tissue pools rather than undergoing deamination. Moreover, early work with rats on lysine free diets found that lysine was retained in tissue pools (plasma, liver, muscle) to substantially greater extents than threonine. Other relevant issues include synthetic amino acid requirements in low-protein diets and catabolism of amino acids in the gut mucosa and their subsequent post-enteral availability. Thus the core of this project is to determine the requirements for (synthetic) amino acids, impacts of feed access on their utilisation, their post-enteral availability and retention in tissue pools.

Program

Chicken Meat

Research Organisation

The University of Sydney

Objective Summary

One objective, which is essential, is to assess requirements for 12 synthetic amino acids in the context of low-protein diets using the Plackett-Burman design in the first study. A second objective is to determine the impact of feed access duration (lighting regimen) on synthetic amino acid utilisation. Therefore, the second study will almost repeat the Batterham (1974) study but include methionine and threonine plus lysine to determine if feed access (16 versus 24 hours) influences retention in tissue pools and utilisation of these critical synthetic amino acids. Either glucose or amino acids (predominantly glutamate) are catabolised in the gut mucosa for energy provision (Watford et al. 1979; Reeds et al., 2000). Synthetic amino acids may be less prone to catabolism than protein-bound amino acids because they are more rapidly absorbed, if so their post-enteral availability is enhanced. Therefore, the third objective is to investigate the post-enteral availability of synthetic amino acids in the final study. Both Baker and Izquierdo (1985) and Benevenga and Blemings (2007) sugggested that lysine is retained in tissue pools in rather than deaminated. Yamashita and Ashida (1969) found that lysine was retained in plasma by 37.5% (0.072 versus 0.192 mole/ml), in liver by 84.0% (0.168 versus 0.200 mole/ml) and in muscle by 76.5% (0.364 versus 0.476 mole/ml) in rats. In contrast, corresponding retentions of threonine were only 5.6%, 10.8% and 15.3%, respectively. Therefore, in the two final studies retention of amino acids in tissue pools at intervals following feed withdrawal will be determined.