Amino Acid Supplementation in Reduced Protein Diets and the Impacts on Meat-Chicken Performance, Metabolism, and Physiology

Five industry-relevant animal experiments were completed to investigate amino acid supplementation in reduced protein diets and the impacts on meat-chicken performance, metabolism, and physiology. Specifically, the role of the non-essential amino acid glycine in reduced protein wheat-based diets was investigated. This work was developed in response of increasing expenses and environmental impacts associated with current dietary crude protein levels in broiler diets, however, reducing the crude protein and supplementing all assumed essential nutrients typically results in impaired performance. The work completed in this thesis contributes to our understanding of broiler amino acid requirements in reduced protein wheatbased diets. The first experiment investigated the role of the non-essential amino acid, glycine, in broiler performance when fed reduced protein wheat-based diets. An industry-standard protein diet and three reduced protein diets with and without glycine supplementation were fed to Ross 308 cockerels. Glycine was supplemented to equal that in the standard protein diet at 0.712 and 0.648% in grower and finisher diets respectively. Performance, water intake, and amino acid digestibility were measured. Reducing crude protein reduced body weight gain by up to 18% and reduced water intake by 28%. However, supplementing glycine in reduced protein diets restored the impaired performance and increased water intake. Supplementing crystalline amino acids in reduced protein diets increased their respective apparent ileal digestibility, with the greatest difference in threonine digestibility increasing by up to 8.3 percentage points. When feeding reduced protein wheat-based diets, supplementing crystalline amino acids, including glycine, can maintain performance like that observed in standard protein diets. A second experiment was completed to determine if supplementing glycine precursors have the same effect on performance as glycine in reduced protein diets. Ross 308 cockerels were fed one of two protein levels; an industry-standard protein diet with meat and bone meal or a plant-based diet approximately 3% lower in crude protein. In the reduced protein diet, glycine, serine, and threonine were assigned a glycine equivalence and supplemented to match the glycine in the standard protein diet at 1.558 and 1.390% in grower and finisher diets respectively. Reducing the dietary protein by 3% reduced feed efficiency by 10-points, however, the supplementation of glycine or serine did not restore performance. Threonine supplementation further reduced body weight gain in reduced protein diets by 6.8%. Therefore, glycine did not appear to be limiting in the reduced protein diet investigated, however, using threonine to supplement glycine requirements exacerbated the performance reducing effects of reduced protein diets. A third experiment explored the effects of different crude protein diets and the supplementation of glycine precursors on the in vivo synthesis of glycine. To investigate this, a second glycine equivalence level was added to the reduced protein treatments described above at 1.8% giving a total of eight treatments. Using these treatments, the effects of reduced protein diets on glycine and uric acid metabolism was investigated. The in vivo conversion of serine and threonine to glycine was evident in the blood plasma, however, reducing dietary protein reduced blood serum uric acid. The expression of hepatic genes for enzymes associated with threonine degradation to glycine, glycine degradation, and uric acid-synthesis were downregulated in reduced protein diets. No effect of crude protein or supplementing different glycine precursors at differing levels was observed on the expression of the enzyme associated with the interconversion of glycine and serine. The supplementation of excess essential amino acids and non-specific nitrogen may not fulfil the non-essential amino acid requirements for efficient growth as the in vivo synthesis of non-essential amino acids is altered in reduced protein diets. A fourth experiment investigated the differences in performance when increasing both the essential and non-essential amino acid densities by 15% in reduced protein diets. Ross 308 cockerels were fed one of three protein levels; standard, reduced, or low protein, with one of three amino acid profiles using AMINOChick®2.0 (Evonik Animal Nutrition, 2016) software recommendations; 100% amino acids (100% AA), 115% essential amino acids (115% EAA), or 115% amino acids (115% AA). In this experiment, glycine was considered an essential amino acid and formulated to 1.6% glycine equivalence. Performance, nitrogen digestibility, and blood parameters were measured. No significant difference was observed between feeding 100% AA and 115% EAA treatments at each protein level in body weight gain. However, feeding 115% AA increased body weight gain in standard and low protein diets by up to 18.7%. Blood parameters indicated reduced uric acid synthesis in low crude protein diets, despite glycine supplementation. Feeding 115% AA in the standard and low protein treatments increased nitrogen digestibility compared to the other amino acid profiles. The results from this study indicate that non-essential amino acids play key roles in growth and development that cannot be overcome with extra essential amino acid supplementation. Finally, a fifth experiment assessed the efficacy of low protein diets and increasing amino acid densities by 15% on mitigating the effects of sub-clinical necrotic enteritis. To investigate this, the standard and low protein treatments and the 100% AA, 115% EAA, and 115% AA profiles of the previous experiment were used in birds either challenged or not with sub-clinical necrotic enteritis. Ross 308 cockerels were fed one of six dietary treatments across 12 replicates. Six replicates per treatment were challenged with sub-clinical necrotic enteritis using Eimeria spp. and Clostridium perfringens. Differences in bird performance between those fed the standard and low protein diets were not apparent in challenged birds. Feeding diets with 115% AA increased body weight gain after the challenge by 10.5%. As evident from the finisher body weight gain, regardless of amino acid profile, low protein diets delayed recovery. However, in standard protein diets, feeding 115% AA increased body weight gain by 16.7%. These results suggest reducing crude protein may not mitigate the effects of sub-clinical necrotic enteritis, however, increasing both essential and non-essential amino acids will assist with recovery. The work completed has been highly industry-relevant and has highlighted the complex relationships between amino acids and the effects they have on meat-chicken performance, metabolism, and physiology. The Australian chicken meat industry now has a dataset of Australian diets for assessment to consider implementing reduced protein diets to improve industry sustainability and access potential diet cost savings