A high dietary protein level or poorly digestible protein source may lower voluntary feed intake and limit pre-cecal digesta flow and cecal protein fermentation in broilers

Cecal protein fermentation (PF) in broilers, as a result of increased pre-cecal protein flow, has been hypothesized to underly the negative effects on gut health when feeding high protein or poorly digestible protein diets. However, the effect of pre-cecal protein flow on cecal PF metabolites in broilers remains largely unknown. Also the effect of dietary fiber, which is hypothesized to increase saccharolytic fermentation and reduce PF, is still unclear in broilers. Male ROSS 308 broilers were fed either a low protein (LP, 19 %) or high protein (HP, 23 %) diet. Also, the digestibility level of the protein was varied, by using rapeseed meal (RSM; poorly digestible), rapeseed protein isolate (RPI; highly digestible), or a mixture of RSM and RPI. Finally, these 3 digestibility levels were fed with or without the addition of sunflower seed hulls (SFH), as a fiber source. HP vs. LP reduced feed intake of birds fed the RSM (106 vs. 121 g/d) or mixture (111 vs. 119 g/d) based diets. At the same time FCR was reduced when feeding HP vs. LP in RSM (1.35 vs. 1.42) and mixture (1.43 vs. 1.50) based diets. The apparent ileal N digestibility was higher in mixture and RPI diets than in RSM (78.7, 77.7, and 73.3 %, respectively). Pre-cecal digesta protein flows were not significantly increased by a higher protein level nor by reduced protein digestibility. The pre-cecal liquid AA flow, which was 3–8 % of the total AA flow, increased with HP vs. LP in the RSM-fed groups (415 vs. 167 mg/d). This increase corresponded with an increase in cecal histamine and tyramine contents (72 vs. 9 and 262 vs. 79 nmol/g freeze dried digesta). Addition of SFH in the diets did not reduce PF. A reduced feed intake of broilers fed HP and low digestible protein diets limited the total pre-cecal protein flow, which might explain the lack of a cecal PF metabolite response.