Can cell-free enzymes replace rumen micro-organisms to model energy and protein suppty?

Abstract

AbstractNew ruminant feeding systems being developed in Europe and North America have greatly increased the need for rapid measurement of rate and extent of ruminal digestion of complex carbohydrates and proteins in individual foods. Application of these systems is lagging because of a lack of reliable kinetic digestion data. This has increased interest in using cell-free enzymes to assess ruminal breakdown of carbohydrates and proteins. The simplest kinetic model describing ruminal digestion divides carbohydrates and proteins into three fractions: A (that already digested and/ or that soluble), B (that digestible) and C (that completely indigestible). Fractions A, B and C sum to 1·0; proportion of digestible fraction B that is degraded is given by: [kd/ (kd+ kp], where kpand kdare, respectively, rates of ruminal passage and digestion. Some models divide fraction B into two or three subfractions while others include a digestion lag. Fully successful, cell-free enzyme systems would yield accurate estimates of digestion fractions and rates and, where appropriate, lag time. As a minimum, results with enzymes should be well correlated to in situ extents of digestion. In vivo total tract digestibility of organic matter (OM) and energy was predicted accurately when food samples were pre-treated with acid-pepsin or neutral-detergent solution, followed by treatment with cellulase from Trichoderma viride (T. reesei). Pepsin-cellulase and neutral detergent-cellulase methods have predicted in vivo digestibility more precisely than Tilley and Terry methods in some cases. Two-stage cellulase assays have given good results with many forages and, when including an amylase treatment, with mixed foods; OM digestibility of straws was not well predicted. Many different cell-free proteases have been tested to estimate ruminal protein degradability. Generally, effectiveness of proteases was assessed by correlating proportions of food nitrogen solubilized after specific incubation times with extents of in situ protein degradation. The broad specificity protease fromStreptomyces griseushas been used most extensively for this purpose in incubations at pH 8·0. However, somewhat better correlations have been reported for ficin, bromelain, papain and neutral proteases of fungal and bacterial origin in incubations at pH more similar to the rumen. Prior treatment with amylases improved correlations for concentrate foodstuffs. As yet, cell-free proteases have not accurately predicted rates and extents of protein degradation observed in ruminal in vitro and in situ systems.