Effects of different levels of dietary crude protein on growth performance, blood profiles, nutrient digestibility, pork quality and odor emission in growing-finishing pigs

Purpose: This experiment investigated the effects of varying dietary crude protein (CP) levels on growth performance, blood profiles, nutrient digestibility, pork quality, and odor emission in growing-finishing pigs. Methods: A total of 210 growing ([Yorkshire × Landrace] × Duroc) pigs (39.93 ± 0.080 kg body weight [BW]) were assigned to 1 of 6 treatments with 5 replicates of 7 pigs per pen. Diets with different CP levels (%) for early growing, late growing, early finishing, and late finishing phases, respectively, were as follows: CP1411: 14, 13, 12, 11; CP1512: 15, 14, 13, 12; CP1613: 16, 15, 14, 13; CP1714: 17, 16, 15, 14; CP1815: 18, 17, 16, 15; CP1916: 19, 18, 17, 16. Results: Overall average daily gain (ADG) decreased with increased dietary CP (linear, P < 0.01), while average daily feed intake (ADFI) increased during the late finishing period (linear, P < 0.01). Final BW increased as dietary CP decreased (linear, P < 0.01). Total protein concentration increased with higher CP levels at the 7^th, 11^th, and 13^th weeks (linear, P = 0.02; P < 0.01; P < 0.01; respectively). Lower CP levels decreased creatinine concentration at the 4^th and 13^th weeks (linear, P = 0.03; P < 0.01; respectively). Blood urea nitrogen (BUN) and urea concentrations decreased with lower CP (linear, P < 0.01). Emissions of ammonia, amine, mercaptan, and hydrogen sulfide decreased with lower CP (linear, P < 0.01; respectively). Excreted nitrogen in urine increased with higher CP (linear, P < 0.01). No significant differences were observed in carcass characteristics, pH, or pork color among treatments. Conclusion: Reducing CP levels in the diet did not negatively impact growth performance and improved protein metabolism, reducing odor emissions from feces and urine in growing-finishing pigs.