THE EFFECT OF SACCHAROMYCES-CEREVISIAE AND ASPERGILLUS-ORYZAE ON THE DIGESTION OF THE CELL-WALL FRACTION OF A MIXED DIET IN DEFAUNATED AND REFAUNATED SHEEP RUMEN

The objective of this study was to determine the effect of two probiot ics, Saccharomyces cerevisiae (SC) and Aspergillus oryzae (AO), withou t their culture medium, on the digestion of plant cell wall components in sheep that had been successively defaunated and refaunated. Six sh eep fitted with large rumen cannulae were used to study 1) defaunated sheep with no probiotic, 2) defaunated sheep with SC or AO, 3) refauna ted sheep with no probiotic, 4) refaunated sheep with SC or AO. The ap parent digestibility of the plant cell walls was not altered (P > 0.05 ) by the probiotics in defaunated sheep but was increased (P < 0.05) w ith SC (+ 16 %) in refaunated sheep. Simultaneously, SC stimulated the growth of the protozoal population in the rumen. As noted in most pre vious experiments, the positive effect of the presence of protozoa on plant cell wall digestion (P < 0.001) was confirmed here. The effect ( P > 0.05) of SC or AO on in situ ADF digestion was either not signific ant or negative in defaunated rumens, whereas it became positive in re faunated rumen after a residence time of 12 h. The improvement of in s itu ADF digestion due to the presence of protozoa was significant (P < 0.05 for NDF; P < 0.001 for ADF). However, we could not determine whe ther this was a direct effect of protozoa or an indirect effect operat ing via bacteria. The limits of the nylon bag technique for evaluating the microbial activity are discussed in relation to the ability of pr otozoa to enter and grow in the bags, and the pH regulation inside the bags. The pH values below 5.5 noted inside the bags can significantly alter the bacterial and protozoal populations and limit the validity of the technique. SC increased (P < 0.05) the specific activity of CMC ase and xylanase of the solid-adherent bacteria (SAB) isolated from th e rumen digesta of refaunated sheep. Stimulation of both the bacterial activity and protozoa numbers could explain the positive effect of SC on cell wall digestion in the rumen since the modelling of marker exc retion in faeces showed that the ruminal mean retention time of hay me asured from the model of Danhoa et al. was not modified (P > 0.05) by either refaunation or the presence of probiotics. AO increased (P < 0. 05) the total retention time of the solid particles in the whole diges tive tract and increased the ruminal liquid volume in the refaunated a nimals but it had no effect on the protozoa population or on the polys accharidase activity of the SAB, which could explain the absence of ef fect of AO on the total digestibility of plane cell walls. Both probio tics decreased the liquid turnover in defaunated rumens (P < 0.05) but neither had any effect (P > 0.05) on this parameter after refaunation . The improvement of plant cell wall degradation in the whale digestiv e tract is probably due to a stimulation of digestion at the rumen lev el as indicated by the higher activity of the SAB in rumen digesta and the growth of protozoa. (C) Inra/Elsevier, Paris.