In vitro digestion of bacterial and plant fructans and effects on ammonia accumulation in cow and sheep rumen fluids
Journal of General and Applied Microbiology
One of the problems encountered with the pasture plants perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) as a food source for ruminants is their relatively low levels of non-structural carbohydrates (NSC) as compared to protein. As a consequence, a significant proportion of dietary protein is metabolized to meet energy requirements and large amounts of ammonia are produced, which need to be excreted (Moller et al., 1993) . This represents a loss of N and a wastage
... energy. Increasing the level of dietary carbohydrate is one of the means currently being explored to correct this protein/carbohydrate imbalance in pasture plants (Beever and Thorp, 1997) . We are investigating the potential of fructans to provide an alternate supply of NSC by genetic manipulation of pasture plants, as genes for levan biosynthesis are now available (van der Meer et al., 1994) . Fructans are a group of fructose polymers, containing either β(2-1) or β(2-6) fructosyl-fructose linkages, synthesized by some plants and certain bacteria. In general, plant fructans (inulins and phleins) are relatively small with an average degree of polymerization (DP) of around 40 (Pontis and del Campillo, 1985). In contrast, those from bacteria (levans) are very large (DPϾ100,000; Keith et al., 1991) . To be useful as a food source, any fructan must be metabolized at rates commensurate with the rate of passage through the rumen and must be linked to the utilization of ammonia from protein metabolism within the rumen. Although the degradability of some plant fructans by iso-lated rumen bacteria and protozoa has previously been reported (Anand, 1972; Paster and Canale-Parola, 1985; Thomas, 1960; Ziolecki et al., 1992) , there is no information on either the overall rate of inulin digestion or the degradability of the large bacterial levans within the rumen. Nor has the effect of levan or inulin on NH 3 metabolism in the rumen been reported. This study was undertaken to investigate the metabolism of fructans in rumen fluids from both cow and sheep. The results obtained indicate that inulin and novel bacterial levan have the potential for enhancing the food quality of forage plants for ruminants. Chicory root inulin a β(2-1) plant fructan, and levan, a β(2-6) bacterial fructan from Erwinia herbicola, were obtained from Sigma Chemicals, St. Louis, MO, USA. AnalAR grade fructose and sucrose were from BDH Limited (UK). Rumen fluid was collected from randomly selected four-year-old Romney sheep and from a 16-year-old Friesian cow grazed on identical white clover and ryegrass swards. Rumen samples were strained into an airtight container, sealed to exclude oxygen and placed into an anaerobic cabinet at 39°C in a 95% CO 2 : H 2 atmosphere. Fructan and other carbohydrates were dissolved in artificial saliva (Mc-Dougall, 1948) and aliquots of rumen fluid added to give final carbohydrate concentrations of 2 mg/ml, and a rumen fluid dilution of 1 in 5. The final concentration of sucrose was 4 mg/ml (fructose equivalent 2 mg/ml). Control samples contained diluted rumen fluid only. At sampling, 100 µl aliquots were removed, added to 12 µl of 1 M HCl to reduce the pH to about 2, and frozen.