In over three decades the increasing trend on the use of feed additives gained momentum owing to a ban on antibiotic growth promoters. Total Mixed Rations form the main feed source for ruminants which are rich in fibrous portion and cannot be hydrolyzed by the endogenous enzymes and hence should depend on either the enzymes secreted by the rumen microbes or must be supplied externally as Exogenous Fibrolytic Enzymes (EFE). The ultimate function of these EFE is to supply maximum nutrients from the digestible, potentially digestible and the indigestible fractions of the cell walls. However the action of these enzymes differ from source to source, mode of application, dosage, type of substrate and other factors. Use of EFE is gaining importance as these improve the nutrient digestibility and performance without affecting the animal health. Wide knowledge on rumen eco system, biotechnology, plenty availability of EFE at cheaper rates, knowing the enzyme assay methods for assessing their activity also are the contributing factors for the wide usage of EFE. Addition of EFE can also be a crucial component in reducing the production of methane which is considered as a greenhouse gas. It was reported that the more productive the animal, the less is the greenhouse gas emission. But in tropical countries where most of the feed is from fibrous resources, addition of EFE is crucial since the production of methane and carbon dioxide is maximum with low yielding animals. However the function of EFE varies with various factors and various contradicting results were reported. These inconsistent results indicate that further studies need to be undertaken to demonstrate the effects of EFE on greenhouse gas emissions and microbial protein synthesis.

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