The Effect of Crude Protein Levels in Growing Pig Diets on Manure Nitrogen Emissions: A Meta-Analysis

Abstract

This meta-analysis evaluates the impact of reducing crude protein (CP) levels in growing pig diets on manure nitrogen emissionsto identify strategies for maximizing environmental benefits. A systematic search of PubMed, ISI Web of Science, and Scopus (2003to 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 regressionanalyses were conducted to investigate heterogeneity and emission breakpoints. The results indicate that low-protein diets significantlyreduce both urinary nitrogen emissions (SMD = -5.09; p < 0.0001) and fecal nitrogen emissions (SMD = -0.79; p < 0.001). Substantialheterogeneity was observed for both fecal ($I^2$ = 81.6%) and urinary ($I^2$ = 81.7%) nitrogen emissions (p < 0.0001), highlightingthe influence of varying study conditions. Broken-line regression analysis identified a significant breakpoint for urinary nitrogen emissionsat 18.8% CP (p < 0.01), whereas no significant breakpoint was observed for fecal nitrogen emissions. Meta-regression analysis revealedthat 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 costsaving 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 servesas an effective strategy for mitigating urinary nitrogen emissions, necessitating the precise supplementation of synthetic amino acidsto balance environmental benefits with the nutritional requirements of growing pigs.