Whole stillage inclusion level influences in vitro fiber digestibility and ruminal fermentation of tall fescue hay

With the growing bourbon industry in the southeastern U.S. leading to increased production of liquid distillery byproducts, there is a pressing need to explore sustainable uses for whole stillage [containing residual grain (corn, rye, malted barley) and liquid after ethanol separation] in livestock nutrition. The objectives of this study were to evaluate the effects of increasing whole stillage inclusion on the in vitro fiber digestibility and ruminal fermentation of tall fescue hay. Ruminal contents were obtained from 2 ruminally cannulated Angus × Holstein steers (390 ± 4.49 kg BW) fed a basal diet consisting of 90% tall fescue hay and 10% cracked corn. Whole stillage was obtained from a local distillery, homogenized, and replaced water in the Goering and Van Soest buffer preparation at 0.00%, 9.06%, 18.1%, or 36.3% on a v/v basis to simulate ruminal fill of whole stillage under practical conditions. Tall fescue hay was used as the substrate and vessels were incubated for 48 h. Results were analyzed with the GLM procedure of SAS using polynomial contrast statements for statistical comparison. Increasing whole stillage inclusion linearly decreased (P = 0.002) apparent dry matter (DM) digestibility, with the lowest (quadratic: P = 0.03) coefficients for true DM and neutral detergent fiber (NDF) digestibility occurring at 36.3% whole stillage inclusion. The rate and extent of gas production, methane production, and total volatile fatty acid (VFA) concentration increased (P < 0.05) with increasing whole stillage inclusion. The final pH of the fermentation media linearly decreased (P < 0.001) with increasing whole stillage inclusion. The molar acetate, valerate, isovalerate, and isobutyrate proportions decreased (P < 0.05) with increasing whole stillage inclusion. The molar propionate proportion responded quadratically (P < 0.01), with the peak proportion occurring at 36.3% whole stillage inclusion. Increasing whole stillage inclusion linearly decreased (P < 0.001) the viable number of cellulolytic and 2-deoxyglucose-resistant cellulolytic bacteria in the fermentation media. Peptide- and amino acid-utilizing bacteria increased linearly (P < 0.001) and hyper-ammonia-producing bacterial concentration peaked (quadratic: P = 0.05) at 36.3% whole stillage inclusion. Increasing whole stillage inclusion in the in vitro ruminal media demonstrated negative effects on the fermentation of tall fescue hay, as indicated by decreased NDF digestibility, cellulolytic bacteria, pH, and branched-chain VFA proportions.