Document Type : Research Paper
Authors
1 Faculty member, Department of Animal Sciences, Isfahan University of Technology
2 Department of Animal Sciences, Faculty of Agriculture, Isfahan University of Technology, Isfahan, Iran
3 Department of Animal Science, Faculty of Agriculture, Isfahan University of Technology, Isfahan, Iran
Abstract
Objective: In recent years, the poultry industry has shown increasing interest in low-protein diets due to their economic and environmental benefits, including reduced feed costs and lower nitrogen excretion. However, reducing dietary protein levels may lead to deficiencies in essential amino acids. Therefore, extensive research over recent decades has focused on evaluating the relationships between dietary nutrients, production performance, and nutritional requirements of broiler chickens, particularly metabolizable energy, protein, and essential amino acids. The primary objective of these studies has been to provide more accurate estimations of nutritional requirements to maximize the genetic potential for growth and performance in poultry.
Method: To investigate the effects of dietary metabolizable energy, protein, methionine +cysteine, and lysine levels on performance and physiological responses of Arian broiler chickens, a total of 2,160 one-day-old chicks (mixed sex) were used in a 4 × 4 × 4 × 4 factorial experiment with 16 treatments, 5 replicates per treatment, and 27 birds per replicate. Nutrient levels were formulated at 97, 100, 103, and 106% of the recommended requirements according to the Arian catalog. Performance parameters, including body weight gain, feed intake, and feed conversion ratio, were recorded. At 35 days of age, blood samples were collected from one bird per replicate to evaluate serum biochemical parameters (cholesterol, triglycerides, HDL, LDL, albumin, globulin, ALP, AST, and ALT). Antibody titers against sheep red blood cells (SRBC) were measured at 28 and 35 days of age. At the end of the experimental period, five birds per treatment were slaughtered to determine carcass, liver, and lymphoid organ weights. Additionally, intestinal tissue samples were collected from one bird per replicate to evaluate micromorphological traits, including villus height and width, crypt depth, villus height to crypt depth ratio, and muscular layer thickness.
Results: Statistical analysis revealed that the optimal nutrient levels for maximizing daily weight gain over the entire rearing period were 103% of energy and lysine requirements, 106% of protein requirements, and the standard level of methionine +cysteine. Standard levels of energy and protein, 97% of methionine +cysteine requirements, and 103% of lysine requirements were optimal for feed intake. Increasing dietary energy and protein levels improved the feed conversion ratio and the European production efficiency index. Serum cholesterol and triglyceride concentrations increased with increasing dietary energy levels. Higher dietary energy combined with lower protein levels led to increased HDL concentrations. Elevated energy and protein levels increased serum total protein, globulin, and the activities of AST and ALT enzymes. Moreover, villus height and surface area increased with increasing dietary lysine levels.
Conclusion: Overall, the results of the present study suggest that increasing dietary metabolizable energy beyond 103% of the recommended requirement may negatively affect performance and feed intake. Increasing dietary protein up to 106% of the requirement improved performance; however, considering the similar performance observed at 103 and 106% protein levels, a protein level of 103% is economically recommended. Additionally, the standard level of methionine +cysteine and 103% of lysine requirements are recommended for optimal performance in Arian broiler chickens.
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