This meta-analysis evaluates the impact of reducing crude protein (CP) levels in growing pig diets on manure nitrogen emissions to identify strategies for maximizing environmental benefits. A systematic search of PubMed, ISI Web of Science, and Scopus (2003 to 2024) yielded 56 observations from 10 studies. We employed random-effects models with restricted maximum-likelihood (REML) estimation and used Hedges’ g to calculate standardized mean differences (SMD). Additionally, meta-regression and broken-line regression analyses were conducted to investigate heterogeneity and emission breakpoints. The results indicate that low-protein diets significantly reduce both urinary nitrogen emissions (SMD = -5.09; p < 0.0001) and fecal nitrogen emissions (SMD = -0.79; p < 0.001). Substantial heterogeneity was observed for both fecal ($I^2$ = 81.6%) and urinary ($I^2$ = 81.7%) nitrogen emissions (p < 0.0001), highlighting the influence of varying study conditions. Broken-line regression analysis identified a significant breakpoint for urinary nitrogen emissions at 18.8% CP (p < 0.01), whereas no significant breakpoint was observed for fecal nitrogen emissions. Meta-regression analysis revealed that every 1% decrease in dietary CP was associated with increased supplementation of synthetic amino acids, including DL-methionine (p < 0.05), DL-tryptophan (p < 0.04), and L-threonine (p < 0.02), with L-lysine showing a tendency to increase (p = 0.10). Furthermore, economic analysis demonstrated that a moderate reduction to 15% CP, supplemented with four essential amino acids, yields a net cost saving of approximately $2.80 USD per metric ton; conversely, a drastic reduction to 13% CP incurs a net cost increase of approximately $14.30 USD per metric ton due to the high cost of valine and isoleucine. In conclusion, reducing dietary crude protein levels serves as an effective strategy for mitigating urinary nitrogen emissions, necessitating the precise supplementation of synthetic amino acids to balance environmental benefits with the nutritional requirements of growing pigs.

Abstract
Introduction
Materials and Methods
    1. Data collection
    2. Data analysis
        2.1. Meta-analysis and heterogeneity
        2.2. Meta-regression
        2.3. Economic analysis
Results and Discussion
    1. Nitrogen excretion in manure results
    2. Synthetic AA supplementation in low CP diets results
    3. Overall discussion
Acknowledgement
References

• Jae Cheol Jang(Assistant Professor, Division of Animal Science, Gyeongsang National University, Jinju 52828, Korea)
• Seong Min Koo(Master’s Degree, Gyeongsang National University, Jinju 52828, Korea)
• Sang Hyon Oh(Associate Professor, Division of Animal Science, Gyeongsang National University, Jinju 52828, Korea)
• Yoo Yong Kim(Professor, Department of Agricultural Biotechnology and Research Institute of Agriculture and Life Science, Seoul National University, Seoul 08826, Korea)
• Hee Kyung Oh(Professor, Department of Bio-Animal Health, Jangan University, Hwaseong 18331, Korea) Corresponding author