Quantification of the effect of reducing dietary crude protein in broiler chickens on nitrogen flows and litter characteristics by meta-analysis

In broiler chickens, reducing dietary CP content is an effective strategy to improve the efficiency of dietary nitrogen ( N ) utilisation by broilers while reducing N losses through volatilisation. The purpose of this study was to quantify the effect of lowering dietary CP on N flows (intake, retention, excretion, manure accumulation, and volatilisation). The database included studies that measured N volatilisation using a mass balance approach. A total of nine papers describing 16 trials and 46 observations were found. The effect of CP content on N flow variables was assessed using a linear mixed-effects model with the trial as a random effect. Broilers of the control treatments ingested an average of 4.2 g of N per day and retained an average of 55% of this N. Nitrogen intake that was not retained was excreted (1.9 g/d), and 33% of this excreted N was lost through volatilisation. Reducing dietary CP by 1% point (%-point) decreased N intake by 0.21 g/d ( P < 0.001) without any effect on N retention. Nitrogen excretion and the volatility of excreted N decreased by 0.20 g/d and 4.22%-points, respectively, for each 1%-point reduction in CP content ( P < 0.001). The synergy between the reduced excreted N and its volatility decreased the amount of volatilised N by 0.12 g/d (−23%) for every 1%-point reduction in CP content ( P < 0.001). Reducing CP content also decreased litter mass ( P < 0.05) and increased its DM ( P < 0.01). The data presented show that increasing litter DM content ( P < 0.05) and lowering pH ( P < 0.001) reduces the volatility of excreted nitrogen. The results of this meta-analysis highlight the benefits of this low-CP diet for reducing N losses, and the equations created can be used in future evaluations of the effects of reducing dietary CP content.