The objective of this study was to investigate whether reducing dietary CP concentration decreases faecal VFA, manure ammonia (NH3) emission and odour, and urinary phenolic metabolites. Six barrows were allotted to one of six dietary treatments in a Latin square design. Treatments consisted of four corn-soybean meal based diets containing 15, 12, 9, and 6% CP, a casein-based diet containing 15% CP, and a protein-free diet (0% protein). Crystalline AA were included in the 12, 9, and 6% CP diets. The casein-based and protein-free diets were used to determine basal endogenous contribution of VFA, phenolics, NH3, and manure odour. Pigs were housed individually in metabolism cages to allow total collection of faeces and urine. Faeces and urine were collected and pooled within pig and period. Faeces and urine were analysed for VFA and phenolic metabolite concentrations, respectively. Faeces and urine were then mixed, stored, and fermented at room temperature for 30 d. For NH3 determination, headspace air was sampled from manure slurries at 24, 48, and 72 h after fermentation. Slurry samples were placed into vials, capped, and randomized before odour panel evaluation. Odour offensiveness was classified on severity: 1=non-offensive; 2=mildly offensive; 3=moderately offensive; 4=strongly offensive; and 5=extremely offensive. Reducing dietary CP increased (P<0.05) faecal VFA concentrations but did not affect phenolic concentrations in urine. Manure NH3 emission was reduced (P<0.05) as dietary CP concentration decreased from 15 to 0%. The 15% diet had the least offensive manure slurry with odour qualitative ranking of 2.58 (i.e., mild-moderately offensive). Compared with the 15% CP diet, manure from the 9 and 6% CP diets was found to be more offensive (P<0.05), with qualitative rankings of 2.92 and 3.10, respectively. Odour qualitative rank for the 12% CP, protein-free diet, and casein-based diet did not differ from that of the 15% CP diet. These results indicate that reduction in dietary CP concentrations decreases manure NH3 emission, but it does not diminish manure odour offensiveness and faecal VFA concentrations.