Improvement of Ruminal Neutral Detergent Fiber Degradability by Obtaining and Using Exogenous Fibrolytic Enzymes from White-Rot Fungi

Simple Summary Neutral detergent fiber (NDF) quantifies the primary cell wall contents in ruminant feedstuff, and its degradability is related to intake and the quality of milk and meat production in ruminants. Overall, cellulases and xylanases, considered as endogenous fibrolytic enzymes (EFEs), can be obtained from white-rot fungi and utilized to increase ruminal NDF degradability. Although EFE can improve the NDF degradability and, therefore, animal productivity, results can be inconsistent. However, considering the ratio of cellulases to xylanases during the effect of EFEs on animal productive behavior has allowed us to find consistent correlations among previously published studies. The culture media where EFEs are obtained, in addition to the biotechnical tools-such as enzymatic activity analysis, metagenomics, metatranscriptomics, and enzyme immobilization-allow researchers to obtain or design products specific to ruminant feed applications. The present review examines the factors and variables that should be considered to obtain, design, and evaluate EFEs that might enhance ruminal NDF degradability. Different combinations of words were introduced in Google Scholar, then scientific articles were examined and included if the reported factors and variables addressed the objective of this review. One-hundred-and-sixteen articles were included. The fungal strains and culture media used to grow white-rot fungi induced the production of specific isoforms of cellulases and xylanases; therefore, EFE products for ruminant feed applications should be obtained in cultures that include the high-fibrous forages used in the diets of those animals. Additionally, the temperature, pH, osmolarity conditions, and EFE synergisms and interactions with ruminal microbiota and endogenous fibrolytic enzymes should be considered. More consistent results have been observed in studies that correlate the cellulase-to-xylanase ratio with ruminant productive behavior. EFE protection (immobilization) allows researchers to obtain enzymatic products that may act under ruminal pH and temperature conditions. It is possible to generate multi-enzyme cocktails that act at different times, re-associate enzymes, and simulate natural protective structures such as cellulosomes. Some EFEs could consistently improve ruminal NDF degradability if we consider fungal cultures and ruminal environmental conditions variables, and include biotechnological tools that might be useful to design novel enzymatic products.