Effect of lowering dietary crude protein with different protein sources on growth performance, litter characteristics, and nitrogen balance in broiler chickens

Reducing the CP content in broiler chicken diets can decrease nitrogen (N) losses through volatilization and improve protein self-sufficiency in regions that import their protein-rich feed ingredients, generally soybean meal (SBM). In most studies focusing on CP reduction, the effects of CP and SBM content of feed are confounded. Therefore, the objective of this study was to investigate the effect of reducing CP at different levels of SBM inclusion on broiler growth performance, body composition and N volatilization. A total of 2 961 one-day-old Ross 308 males were randomly allocated to 63 pens (nine pens per treatment). After receiving the same starter feed from day 1 to day 10, seven experimental diets were tested: one control diet (CT, 204 and 195 g/kg of CP in grower and finisher) and six experimental diets combining two levels of CP reduction (-15 and -30 g/kg) and three SBM inclusion strategies (no inclusion constraints, no SBM, and an intermediate inclusion). Body composition was measured by dual X-ray absorptiometry, and N volatilization was determined by mass balance. Over the entire experimental period (d 11-35), growth performance was not affected by CP content, whereas reducing SBM content decreased weight gain. Reducing SBM content decreased short-chain fatty acid production by the cecal microbiota, while dietary CP content had no effect. Body composition showed no effect from the dietary treatment. Reducing CP content decreased N excretion by 10.2% per 10 g/kg of CP reduction (P < 0.001). Nitrogen volatilization was reduced by decreasing SBM content (P < 0.001), which was also associated with a reduction in litter pH (P < 0.001). When CP is reduced by 30 g/kg, the total substitution of SBM impaired weight gain. These results demonstrate that it is possible to modulate dietary CP and SBM content without compromising animal performance while reducing N losses through volatilization.