Optimal Energy Density and a Balance Between Essential and Nonessential Amino Acids in Reduced Protein Diets Can Restore Bird Performance

Feeding meat-chicken with reduced crude protein (CP) diets commonly reduces N release in the environment. However, this is often associated with poor performance and a heightened body fat content due to a high energy-to-protein ratio. Reducing feed CP content affects all amino acids (AA), but the most limiting essential AA (EAA) are commonly considered during feed formulation. It was realised that a chicken body has a constant composition in EAA and non-EAA (NEAA), or EAA-to-true protein (EAA:TP) ratio, whereby TP = EAA + NEAA (Heger, 2003). Therefore, feeding chickens with reduced CP-diets added with some EAA without considering all AA content may cause imbalanced EAA:TP, leading to the N utilisation and growth rate impairment (Pesti, 2009).

The current study examined the impact of AA balance and energy density in reduced-CP diets on nutrient utilisation and performance of Cobb 500 broilers. A 2 × 2 × 2 factorial arrangement of treatments was used, with factors including CP (16% and 18%), net energy (NE, 9.9 and 10.4 MJ/kg) and EAA:TP ratio (0.56 or 0.60). Thus, 8 finisher diet treatments were formulated and tested into 8 replicates, with 16 mixed sex birds per replicate. All diets were formulated to meet Cobb 500 (2018) specifications. Crystalline NEAA supplement was used to lower EAA:TP ratio. The TP contribution of each feed ingredient (other than purified AA) was estimated using a specific N-to-protein conversion factor, also known as KA. The KA value of 6.25 was used for all purified AA used in feed formulation (Pesti, 2009). Birds were fed diets in three phases; starter (d0-8), grower (d9-18) and finisher diets as the treatments (d19-35). Birds and feeds were weighed on d19, 28 and 35. On d35, 4 birds (2 males and 2 females) per pen were sampled to evaluate carcass quality. All data were analysed using JMP software, with gender as a covariate for the performance data.

Energy content or EAA:TP did not significantly (P > 0.05) affect animal performance when CP was 18%. Moreover, applying 10.4 MJ/kg NE and 0.60 EAA/TP ratio in a 16% CP diet removed the difference (P > 0.05) in bird performance (weight gain, feed intake and feed conversion ratio) compared to the 18% CP diets. However, reducing CP content from 18% to 16% increased (P < 0.001) body fat pad for all treatments, regardless of energy and EAA/TP levels. These results show that balancing all amino acids in 16% CP diets, with an energy content of about 10.4 MJ/kg can restore bird performance, but further studies are needed to validate these findings.

ACKNOWLEDGEMENTS: This study was funded by Adisseo France in partnership with AgriFutures Australia and Poultry Hub Australia.