Carbohydrases, specifically xylanases and beta-glucanases, are used routinely in wheat- and barley-based diets, respectively, throughout Europe, Canada, Australia and many parts of the Middle East. Whilst their use is well accepted, there is still considerable debate regarding their mode of action. Use of such enzymes results in increments in performance that far exceed the nutritive value of the sugars they release from cell walls. Evidently, the substrates they degrade are either anti-nutrients or reduce the efficiency of the digestive process. As a consequence, both the viscosity and the cell wall encapsulation hypothesis are discussed as likely candidates for the mode of action of carbohydrases. The bulk of the evidence supports the former, although the cell wall mechanism cannot be ruled out entirely. Whilst enzyme use in wheat and barley diets is well established and accepted, it is only recently that maize- and sorghum-based diets have been shown to respond to the relevant enzymes. Maize starch, in particular, is incompletely digested by the time it exits the terminal ileum. In addition to the cell wall degrading enzymes, amylases and proteases have been found to enhance the rate of starch digestion in the ileum of the broiler and hence improve overall performance. Carbohydrase enzymes effectively increase the rate of diet digestibility whether the target grain is barley, wheat or maize. The net result is more nutrients for the bird and fewer for the resident intestinal flora. As a result, the use of carbohydrases has been shown to reduce the populations of ileal flora. This response in itself may well be responsible for the bulk of the response to feed enzymes. Carbohydrases also produce many oligomeric and smaller products, which act as substrates for many bacteria in the caecum. This results in an increase in total fermentation and an alteration in volatile fatty acid profiles towards propionate, which has implications both for energy conservation and pathogen control. The response to addition of a carbohydrase is dependent upon many different factors that are discussed, and as more is understood of their mode of action, it is likely that new enzymes will be designed rather than empirically derived.
