Effects of Feeding Low Protein Diets with Different Energy-to-Protein Ratios on Performance, Carcass Characteristics, and Nitrogen Excretion of Broilers

**Background:** Broiler production contributes to environmental ammonia pollution through nitrogen excretion from undigested protein and uric acid. Feeding low-protein (LP) diets balanced with crystalline amino acids can reduce nitrogen excretion and ammonia emissions. However, the ideal energy-to-protein ratio for LP diets that maintains performance and carcass quality is not well established. This study aimed to evaluate the effects of LP diets with different apparent metabolizable energy corrected by nitrogen (AMEn)-to-crude protein (CP) ratios on production traits, carcass composition, meat quality, nitrogen retention, and excreta composition in broilers.

**Methods:** A total of 576 one-day-old male Ross 308 broilers were randomly assigned to 24 floor pens (24 birds/pen) and fed one of four dietary treatments over three phases (starter day 0–10, grower day 11–24, finisher day 25–41). The control diet (C) followed breeder recommendations. Three LP diets (LP1, LP2, LP3) contained 1.5% less CP than diet C in each phase (relative reductions of 6.5%, 7.0%, and 8.0% in starter, grower, and finisher, respectively). LP1 was isocaloric with C; LP2 and LP3 had 1.5% and 3.0% lower AMEn content, respectively. All LP diets were supplemented with six crystalline essential amino acids (Lys, Met, Val, Thr, Arg, Ile) to meet standardized ileal digestible amino acid requirements. Body weight, feed intake, and feed conversion ratio were recorded. At day 35, two broilers per pen were selected for nitrogen balance and excreta collection using titanium dioxide as an indigestible marker. At day 41, two broilers per pen were slaughtered for carcass composition and breast meat quality analysis. Data were analyzed by one-way ANOVA with Tukey HSD test (p < 0.05).

**Key Results:** The LP1 diet did not significantly affect final body weight (BW) or body weight gain (BWG) compared to the control. LP2 and LP3 diets resulted in significantly lower final BW at day 41 compared to C (p < 0.05). No significant differences in feed intake were observed among treatments. Feed conversion ratio (FCR) was significantly higher in LP3 compared to C only in the starter phase (p < 0.05). A quadratic relationship was found between dietary starch:CP ratio and mean FCR in the starter phase (R² = 0.9998; p = 0.014). Relative breast meat yield was significantly higher in the LP1 group compared to C. Drip loss of breast meat was significantly lower in LP1 and LP2 groups compared to C (p < 0.05). No significant differences were found in carcass weight, thigh weight, abdominal fat, or meat pH. Nitrogen retention was significantly higher in LP1 and LP2 groups compared to C (p < 0.05), with LP2 achieving the highest retention. The LP1 diet resulted in a 23% decrease in total-N and a 26% decrease in uric acid-N excretion compared to C. The LP3 diet did not significantly improve nitrogen retention or reduce nitrogen excretion compared to C.

**Clinical Implications:** This study demonstrates that reducing dietary crude protein by 1.5% while maintaining energy content (isocaloric LP diet) can sustain broiler growth performance, improve nitrogen retention, increase breast meat yield, reduce drip loss, and significantly lower nitrogen excretion—particularly uric acid-N, which is a major precursor for ammonia emissions. These findings support the use of carefully balanced LP diets as an effective nutritional strategy to reduce the environmental impact of broiler production without compromising productivity. However, reducing both protein and energy content simultaneously may impair growth performance, indicating that the energy-to-protein ratio must be carefully managed. The results also highlight the importance of supplementing with multiple crystalline amino acids (beyond the first four limiting ones) and using phase feeding with pelleted diets to achieve optimal outcomes.