Fibrolysis in rumen mediated by a mixture of exogenous polysaccharidases

There is an increasing interest in the addition of fibrolytic enzymes to the rumen as a way of improving the utilization of the fibrous components of the diet. However, the mechanisms involved in such effects remain uncertain; direct enzymatic hydrolysis has been suggested, but indirect benefits through a modified rumen environment for cellulolytic bacteria have also been reported. In the work reported here, different carbohydrases and their interactions with ruminal cellulolytic activity were evaluated. Pure substrates for standard activity measurement were carboxymethyl-cellulose, birchwood xylans and citrus pectin; forage samples were alfalfa (Medicago sativa L.), berseem trefoil (Trifolium alexandrinum L.), oat fodder (Avena sativa L.) and annual ryegrass (Lolium multiflorum L.). Eleven commercial products, covering different polysaccharidase activities, were evaluated as single sources of enzymes and further pooled together and their individual activity assessed. The results indicated that no single enzyme could account for the overall fibre breakdown observed with the complete pool of enzymes. When six enzymes were selected according to their contribution towards fibre hydrolysis in the pool, essentially the same activity was observed as when all 11 were present. We concluded that the greatest effect was obtained when a mixture of cellulase, xylanase and pectinase was added, preferentially dosed according to the relative proportions of different polysaccharides in the substrate. Microscopic observations confirmed the hydrolytic capacity of the enzymes used. The incubation medium was modified with citric acid to obtain pH values of 6.8, 6.4, 6.0 and 5.6. The polysaccharidase activity of commercial enzymes was not affected by pH in a microbial free medium but in vitro rumen fluid breakdown was severely depressed as acidity increased. However, when commercial enzymes were added to the rumen fluid, hydrolysis was improved as the medium became more acidic. We concluded that exogenous and microbial enzymes compete for the same substrate.