Continuous efforts are being made to utilize the fibre feeds to the maximum extent in ruminants which form a staple feed in tropical countries. Though only about 65% of the fibre is effectively digested in the rumen, need to supplement certain exogenous fibrolytic enzymes (EFE) has become mandatory. The main principle with which EFE is added is to specifically target the increased bacterial multiplication of certain species which enhances rumen degradation of cellulose which constitutes the major part of the forages. Effectiveness of enzymes in animal nutrition depends on (i) type, (ii) source, (iii) level of supplemented enzymes, (iv)the type of diet fed, (v) animal health and (vi) animal productivity. Exogenous enzymes are classified into cellulolytic, amylolytic, proteolytic and others based on the type of substrate on which they act. The mode of application of EFE, incubation with the substrate is also crucial. Solubulized enzymes incubated with the substrate is more effective since the enzymes starts acting on the substrate forming a stable complex resistant to endogenous enzymes even before entering into the rumen. Supplementation of White-rot fungi in recent times improved the digestibility of the so called tough lignocellulosic bonded substrates fed to ruminants. Supplementation of EFE to silage can hydrolyze plant cell walls into fermentable substrates through enhanced homolactic fermentation and thus improves digestion. It was reported that EFE works in three ways - 1) improved feed conversion efficiency 2) release of entrapped nutrients from the cell wall portions of diets and 3) release of oligosaccharides which support beneficial microflora while suppressing pathogenic bacteria. It was known that the microbial mass in the rumen meets 80 to 90% energy and 60 to 70% protein needs in sheep for which efficient utilization of the fibrous crop residues is mandatory for which the use of EFE is required. However, animal responses to the enzyme supplementation are inconsistent and are possibly be attributed to four main factors: enzyme characteristics (differences in enzyme preparations, enzyme activity, units of activity added, pH and temperature effect on activity), forage (type, maturity), animal (species, age) and management (diet, mode of enzyme application, application rate, interaction time of enzymes applied to feed). It can be concluded that enzyme action is species specific, substrate specific, dosage specific, application mode specific, incubation period specific, source specific and hence long term trials are further suggested for not having consistent results and there is a great need to develop certain of the enzymes to more thermo stable and can resist the rumen pH and temperature. Since the any forage has three different digestible fractions viz- digestible, potentially digestible, and indigestible and the action of EFE which may be of any origin depends upon the extent of the digestibility of the potentially digestible fraction and this determines the magnitude of the response of the EFE towards forages.

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