Effects of protein reduction and substitution of cassava for corn in broiler diets on growth performance, ileal protein digestibility and nitrogen excretion in feces

This experiment was conducted to investigate the effects of protein reduction and substitution of cassava for corn with maintained levels of methionine, lysine, threonine, and tryptophan in the diet for broilers from 1 to 42 days of age. A total of 2,688-day-old male Cobb-500 chicks were divided into 8 groups with 6 replicates each. The 2 × 4 factorial arrangement was applied to 2 energy sources, namely corn (CO) and corn substituted with 50% cassava (CC), and 4 protein levels (100, 95, 90, and 85% of the breed recommendation). Body weight, feed intake, ileal content and feces were recorded. Analyses were conducted for diet's nutrient composition, protein level in ileal content and nitrogen in feces. Apparent ileal protein digestibility coefficient, European Efficiency Factor (EEF) and feed cost per kg live weight were calculated. In the starter period, there was no interaction between energy sources and protein levels. No treatment effects were found on the final body weight, average daily gain (ADG), and feed intake among experimental groups but the feed conversion ratio (FCR) in 85% CP group was significantly higher than 90-100% CP groups (P = 0.009). The ileal protein digestibility was not affected by treatments. During the finisher period, interaction between energy sources and protein levels on feed intake was detected (P = 0.037). The protein level of 85% CP affected the final body weight, ADG and FCR (P < 0.000) as compared to the others. Birds in the CC group had significantly higher feed intake than the CO group (P = 0.001). Ileal protein digestibility was affected by both protein levels (P = 0.030), and energy sources (P < 0.000). The ileal protein digestibility of the group with the highest protein level was significantly different from 85 to 90% CP. Birds fed with CO diets had significantly higher ileal protein digestibility than those fed with CC diets (P < 0.000). Protein level affected the fecal nitrogen excretion in both periods (P < 0.000 and P = 0.002 in the starter and finisher periods, respectively). The decrease of protein from the breed recommendation by 10% or more could lower the nitrogen excretion in feces. In conclusion, the dietary crude protein could be reduced to 95% CP from the breed recommendation to retain the highest growth performance and the ileal protein digestibility with the highest EEF and lowest feed cost/kg live weight but no advantage on nitrogen excretion. For the alternative energy source, cassava could substitute corn at the level of 50% without impairing any measuring variables.