Quantifying the interactions between dietary fibers and macronutrient digestibility in broiler chickens: The importance of considering fiber solubility

Abstract

Dietary fibers (DF) have been conventionally considered to limit nutrient utilization in broiler diets. However, the increasing attention towards the use of high-fiber by-products as feed ingredients to reduce costs and environmental impacts requires a better understanding of the role of fibers in the digestion of broiler chickens. Dietary fibers entail a highly heterogeneous group of polymers that differently affect nutrient digestibility. The aim of this study was to collect and analyze the data from published literature related to the inclusion of fibers into standard broiler diets and subsequent effects on digestion. A literature search was performed to find experimental studies that tested the effects of adding high-fiber ingredients on ileal and total tract digestibility of dry matter (DM), crude protein (CP), fat, and starch. A total of 45 studies and 198 experimental treatments were considered for the statistical analyses. For each diet, total DF (TDF), insoluble DF (IDF), and soluble DF (SDF) were calculated based on tabular data of each ingredient. The analysis unit was the effect size of digestibility expressed as the difference between control and treatment diets (% unit). For every digestibility variable, two multiple curvilinear regression models were fitted. Model 1 included the addition of TDF in the experimental diet in relation to the basal diet as a covariate, whereas model 2 included the addition of IDF and SDF as separate covariates. Model 2 better explained the variability in digestibility effect sizes for all nutrients, indicating that the consideration of DF solubility is required to better understand the effects of DF on nutrient digestibility. The inclusion of IDF showed improvements of up to 10 % units in the digestibility of DM, CP, fat, and starch at additional inclusion levels of up to 50 g/kg DM, beyond which the effects became negative. In contrast, the inclusion of SDF showed negative consequences for nutrient digestibility already beyond 20 g/kg DM. In conclusion, DF can affect digestibility of other nutrients in a positive or negative way, depending on DF characteristics and inclusion level. Solubility of DF was found to explain a large part of the variability in digestibility effects and therefore, we recommend to consider DF solubility in feed formulation. Furthermore, the additional inclusion of up to 50 g/kg DM of IDF to a standard cereal-soybean-based diet can be advised for improving the nutrient digestibility in broiler chickens, whereas the inclusion of SDF should be minimized and not exceed approximately 20 g/kg DM.