There are unknown interactions between supplements of exogenous fibrolytic enzymes (EFE) and the cell walls of feedstuff in ruminal conditions. The quantitative effects of using EFE in ruminant diets were evaluated using meta-analysis. Records (586) were extracted from 74 journal articles from a list of published papers (2000–2012). Statistical analyses were performed considering fixed [type of forage-based diet, forage-to-concentrate ratio (F:C ratio) and primarily enzyme activities in the EFE], and random effects [Experiment(Article)]. In dairy cows fed high-forage (F:C ≥50%), the supplementation of primarily mixtures of cellulases and xylanases (Cel:Xyl: 1:4–1:1) increased milk production and milk composition of legume-based diets, and primarily xylanases (Xyl) EFE improved those variables of grass-based diets. In F:C <50% grass-based diets, Cel:Xyl improved the average daily gain (ADG) and feed conversion [FC:DM intake (DMI)/ADG] of beef cattle. DMI of dairy cows was not affected by EFE supplementation, but EFE improved the DMI of beef cattle. EFE effects were inconsistent in sheep productive performance variables. Cellulases (Cel) and Xyl enhanced in vivo dry matter (DM) digestibility (DMD) in low-forage (F:C <50%) grass-based diets. In F:C ≥50% legume-based diets, EFE enhanced the in situ DM disappearance (ISDMD), and mainly Cel:Xyl improved the in situ neutral detergent fibre (NDF) disappearance (ISNDFD), but there were no effects in those variables in F:C ≥50% grass-based diets. Regardless of the type of ruminal liquid (RL) or forage, in F:C ≥50% diets, in vitro DM degradability (IVDMD) was improved mainly by Cel, but fibre degradability only was improved by Cel:Xyl when sheep RL was used for in vitro evaluations. Overall, EFE could improve the productive performance of dairy cows and beef cattle, but the response depends upon the proper mixture of Cel and Xyl according to the diet composition. Abbreviations: ADF: acid detergent fiber; ADG: average daily gain; A:P: acetate:propionate ratio; BW: initial body weight; Cel:Xyl: cellulases:xylanases; DM: dry matter; DMD: in vivo dry matter digestibility; DMI: dry matter intake; EA: enzyme activities; EFE: exogenous fibrolytic enzymes; F: type of forage; FC: feed conversion; F:C: forage-to-concentrate ratio; ISDMD: in situ dry matter disappearance; ISNDFD: in situ neutral detergent fiber disappearance; IVADFD: in vitro acid detergent degradability; IVDMD: in vitro dry matter degradability; IVNDFD: in vitro neutral detergent fiber degradability; NDF: neutral detergent fiber; VFA: in vitro volatile fat acids

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.