Effects of Dietary Protein Levels, Net Energy Levels, and Essential Amino Acid-to-True Protein Ratios on Broiler Performance

Supplementing essential amino acids (EAAs) without considering non-EAA (NEAA) and energy contents in reduced-crude protein (CP) diets may alter EAA-to-true protein (E:T) and energy-to-protein ratios, potentially compromising growth. This study aimed to evaluate the effects of CP, E:T, and net energy (NE) on broiler performance. The treatments were as follows: T1—reduced CP (16%, RP), low NE (9.9 MJ/kg, LNE), low E:T (0.56, LE:T); T2—RP, LNE, high E:T (0.60, HE:T), with imbalanced EAA (excess Met and deficient Thr); T3—RP, high NE (10.4 MJ/kg, HNE), LE:T; T4—RP, HNE, HE:T; T5—normal CP (18%, NP), LNE, LE:T; T6—NP, LNE, HE:T; T7—NP, HNE, LE:T; and T8—NP, HNE, HE:T. The study employed as-hatched Cobb 500 broilers in two experiments. Exp.1 studied the broiler performance from d19 to 35, with eight replicates per treatment and 16 birds per replicate (n = 1024). Birds were randomly assigned to different treatments, and at the end of their lives, all were dissected to determine their sex and account for any gender-related effects. Exp.2 measured NE values in respiration chambers from d25 to 28, with six replicates per treatment, and two birds (a male and a female) per replicate (n = 96). The bird gender was determined through high-resolution melting curve analysis of feather DNA. The measured NE values were used to calculate NE intake (NEi) in Exp.1. The results showed that T4 improved (p < 0.001) weight gain (WG), feed conversion ratio (FCR), and NEi relative to T1, T2, and T3, and protein efficiency (WG/CP intake) relative to all treatments. The live performance (feed intake, WG, FCR) and NEi of birds fed T4 reached a level equal to those of birds fed NP diets (T5 to T8). These results suggest that a dietary E:T ratio of 0.60 is necessary to maximize nutrient utilization and to restore growth rate in broilers fed RP diets.