Inoculum source and transfer of rumen contents from bison to cattle improved in vitro gas production and feed digestibility, but not the responses to exogenous enzymes supplementation

We hypothesized that inoculating the rumen of beef cattle with rumen contents of bison and supplementing with exogenous fibrolytic enzymes (FE) would improve ruminal feed digestibility by enhancing microbiome activity in the rumen. The objectives of this study were to evaluate the effects of inoculum source (cattle versus bison), inocula after transfer from bison to cattle, FE supplementation, and their interaction on in vitro fermentation of barley straw, alfalfa hay and wheat dried distillers grain with solubles (DDGS). Rumen content transfers were performed using 16 beef heifers (body weight = 461 kg) twice at a 2-week interval by replacing 0.70 of total rumen contents of heifers with an equivalent amount of mixed rumen contents collected from plains bison. Three experiments were conducted using batch culture techniques. Barley straw and alfalfa hay were ground through 1-mm screen and DDGS was incubated as-is. In Exp. 1, inocula were prepared by combining rumen fluids (vol/vol) of cattle and bison at ratios of 100:0, 67:33, 33:67 and 0:100; the substrates were incubated for 96 h for determination of gas production (GP) kinetics. In Exp. 2, inocula with the same ratios as in Exp. 1 were incubated with substrates in vitro for 24 h to determine dry matter (DM) digestibility (DMD). In Exp. 3, incubations were conducted for 6, 24 and 48 h with the same substrates in four runs to compare the before versus after inocula effect. Each batch culture was a completely randomized design without or with FE addition at a rate of 2.0 μl/g substrate DM. Three different FE products (referred to FE1, FE2 and FE3) were used, with FE1 added in Exp. 1 and 2, while all 3 FE were added and compared in Exp. 3. Asymptotic GP of barley straw and alfalfa hay was not affected, whereas that of wheat DDGS responded quadratically (P < 0.04) with increasing bison contents in inocula. Rate of GP decreased (P < 0.03) for barley straw and alfalfa hay (quadratic), and for wheat DDGS (linear) with increasing bison contents in inocula (Exp. 1). In vitro DMD of barley straw at 24 h of incubation quadratically (P < 0.02) increased with increasing bison contents in inocula, whereas DMD of alfalfa hay and DDGS linearly (P < 0.01) decreased (Exp. 2). For Exp. 3, the DMD of barley straw, alfalfa hay and DDGS were greater (P < 0.05) at 6 h of incubation with the inocula collected after, as compared to before the rumen content transfer. In contrast, the DMD of alfalfa hay at 48 h and DDGS at 24 h and 48 h were less (P < 0.04) with inocula after versus before rumen content transfer. Addition of FE improved (P < 0.05) DMD of DDGS. The results demonstrated that the ruminal inocula from bison had an advanced capacity to degrade fibrous feed in vitro as compared to those from cattle. Enrichment of the cattle rumen microbiome with that of bison could potentially improve rumen digestion of fibrous feeds in beef cattle. Moreover, the results also suggest that the improvement in feed digestibility due to supplemental FE may be independent of inoculum source.