Effect of protozoa on rumen protein degradation in sheep

Abstract

1. We have studied the contribution of ciliate protozoa to the degradation of dietary protein, utilizing 5 defaunated and 6 faunated sheep. Rumen samples from these animals were used as inocula for in vitro determination of the degradability of different protein sources: lupine grain, peanut and soybean cake, and fish meal. We also applied the in sacco method to measure the digestibility of soybean proteins. Two pore sizes (50 and 100 μm) were used in the in sacco study. The nitrogen degradation curve was constructed according to the mathematical model: P(t) = a + b (1-e(-ct)), and the parameters a, b, and c were determined for both faunated and defaunated animals. The proteolytic activity of the rumen contents was also determined using azocasein. 2. The in vitro degradability of proteins was significantly less in defaunated than in faunated animals: 17, 19, 28 and 64% with lupine grain, peanut cake, soybean cake and fish meal proteins, respectively. The protozoal effect was greater when protein solubility was low. 3. The in sacco study showed that neither the protozoa nor bag pore size had any effect on the protein fraction (a) immediately soluble in the rumen juice. However, both factors increased the insoluble, potentially degradable fraction 'b' (P ≤ .01). Protozoa had a positive effect on the rate of degradation (c) of the fraction b. This effect was most clear with the 100 μm bags. The significant interaction between protozoa and pore size on factor 'c' showed that large ciliate protozoa (100 μm) were directly involved in determining this parameter. 5. Taking into account the very soluble azocasein utilized in this study to determine the protease activity of rumen juice, we confirmed the dominant effect of bacteria on the degradation of very soluble proteins. Conversely, protozoa, particularly large Ophryoscolecidae (100 μm), increased the ruminal degradation or relatively insoluble proteins.