Effect of Saccharomyces cerevisiae and Aspergillus oryzae on the digestion of nitrogen in the rumen of defaunated and refaunated sheep

The objective of this study was to determine the effect of two probiotics, Saccharomyces cerevisiae (SC) and Aspergillus oryzae (AO) without their culture medium, on the digestion of nitrogen (N) in sheep that have been successively defaunated and refaunated. Six sheep fitted with a large rumen cannulae were fed 1350 g day−1 of a mixed diet composed of hay (600 g), barley (600 g) and soybean meal (150 g). We studied: (i) defaunated sheep receiving no probiotics (six sheep); (ii) defaunated sheep receiving either SC (three sheep) or AO (three sheep); (iii) refaunated sheep receiving no probiotics (six sheep); (iv) refaunated sheep receiving SC (three sheep) or AO (three sheep). The probiotics had no effect (p>0.05) on ruminal feed protein degradation estimated by in vitro ammonia (NH3) production, whereas protozoa increased it (p<0.001). The total N pool in the rumen was increased by protozoa, and by both probiotics in defaunated and refaunated sheep. The main contribution of this increase came from the NAN pool, although the significant threshold (p<0.05) was reached only for SC in the refaunated sheep. Probiotics had no effect on the ruminal NH3–N pool in defaunated sheep, but a large positive effect when the sheep were refaunated. A similar evolution of NH3–N concentration in the rumen was noted during the 6-h period following feed intake. The contribution of NH3–N pool to the total N pool was low (10%). Probiotics and protozoa did not affect the net microbial protein flow in the duodenum estimated from the urinary excretion of purine derivatives. This result was confirmed by the absence of any statistical difference in NH3 uptake by bacteria during the in vitro incubations. We assumed that yeasts favour the release of peptides or amino acids into the NAN pool in the rumen. This is supported by other literature data and may open a new direction of research to determine the composition of the NAN pool and its origin, and explain the real impact of probiotics in relation with the animal N metabolism and the dietary N supply. Neither SC nor AO had any effect (p>0.05) on blood urea, total proteins, albumin or free amino acids or glycaemia, but they significantly decreased the concentration of blood non-esterified fatty acids (NEFA) in refaunated sheep. The tested probiotics had no effect on N digestibility nor on urinary N excretion.