Low crude protein diets for broiler chickens aged 8 to 21 days should have a 50% essential-to-total nitrogen ratio
Introduction
Protein has been described as the most expensive nutrient in the diet of nonruminants, and therefore, much research has been conducted to reduce crude protein (CP) levels and the associated feeding costs. Protein reduction is also associated with decreased nitrogen excretion and environmental pollution (Kerr and Easter, 1995; Rotz, 2004). Excess protein can lead to higher levels of amino acids (AAs) in the gut and at the sites of protein synthesis, and once AA requirements are met, the excess will be catabolized, requiring substantial energy for their excretion.
The large-scale use of AAs in industrial diets enables a substantial reduction in protein levels. Once it has been established that animals can absorb AAs, it is acceptable to compare high protein diets with low protein plus crystalline AAs to meet requirements, and theoretically they should exhibit the same performance; however, this does not occur in practice (Summer et al., 1991). This discrepancy occurs because although animals require the same essential amino acids (EAAs) for growth, they do not require the same nonessential amino acids (NEAAs); consequently, they require different AA ratios.
Currently, there is considerable disparity in the literature regarding the estimated EAA to NEAA ratio, probably due to the different classifications of EAAs and NEAAs and different methodological approaches (Heger, 2003). AA ratios can be expressed on the basis of protein, AAs, or nitrogen. Because there is a range of NEAA content in different nitrogen sources, diets can have the same proportion of total NEAA but different nitrogen concentrations (Sosulski and Imafidon, 1990). Because the primary role of NEAAs is as nonspecific nitrogen, AAs in the diet are best expressed in terms of the molecular weight of nitrogen.
To express nitrogen ratios, it is possible to relate essential nitrogen (eN) to nonessential nitrogen (NeN) or eN to total nitrogen (tN). The second approach is more practical because if NeN tends to zero, the eN-to-NeN ratio approaches the infinite (Heger, 2003).
The definition of eN can be given as the sum of nitrogen from all essential amino acids (EAAs) in the diet or the animal requirements. Using requirements to express eN better reflects biological reality because any excess EAA will be catabolized and employed as a source of nitrogen for the animals.
In practical diets, high CP levels lead to low eN-to-tN ratios, and as CP decreases, the tN in the diet also decreases, leading to a high eN-to-tN ratio. It is vital to understand the optimum ratio between eN and tN to know to what extent to diminish protein without diminishing performance.
Based on this conceptual framework, experiments were conducted to evaluate the effects of eN-to-tN ratios in low-protein diets supplemented with NEAAs on the performance, nitrogen retention and blood parameters of broiler chickens from 8 days to 21 days posthatch.
Section snippets
Ethical matters
The Animal Care and Use Committee of the Universidade Federal de Viçosa, Brazil, approved all animal handling procedures (protocol number 106/2014), and the experiment was conducted according to the experimental protocol for the use of live birds of the Brazilian College of Animal Experimentation (Marques et al., 2009).
Birds, experimental design, and diets
The experiment was performed on a poultry farm at the Universidade Federal de Viçosa located in the state of Minas Gerais, Brazil. A total of 300 male Cobb 500® broilers were
Results
Birds fed diets with a 56 % eN-to-tN ratio exhibited a lower FBW and FCR (P < 0.05) at 21 day than birds fed diets with a 47 % eN-to-tN ratio from intact protein (Table 2). There was no difference in the performance of broilers fed diets with eN-to-tN ratios of 50 %, 47 % and 44 %, whereas birds fed a diet with a 53 % eN-to-tN ratio had a lower FCR than birds fed the control diet. The eN-to-tN ratio did not have a quadratic or QBL effect on the performance parameters, whereas LRP effects were
Discussion
Dietary CP levels are inversely proportional to the eN-to-tN ratio. It was clear that when CP levels were reduced, the broilers exhibited decreased performance even with the supplementation of EAAs. This finding could be related to the lack of nonspecific nitrogen needed to synthesize NEAAs, leading the animal to use nitrogen from EAAs for NEAA synthesis. Was observed that when the mixture of glutamic acid, glycine and alanine was added to the diets, performance recovered until it plateaued.
Conclusions
The use of low crude protein diets can lead to a nitrogen imbalance in broilers, leading to the use of essential nitrogen for nonessential nitrogen synthesis; thus, when the ideal protein concept is applied, the essential nitrogen-to-total nitrogen ratio should be considered. To maintain broiler performance and efficient nitrogen metabolism, low crude protein diets should possess an essential nitrogen-to-total nitrogen ratio of 50 % or less.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors wish to thank Universidade Federal de Viçosa (Viçosa, Brazil) and Ajinomoto Animal Nutrition (São Paulo, Brazil)for the analysis of raw materials and diets and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq; Brasília, Brazil) for financial support.
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