Effect of starchy or fibrous carbohydrate supplementation of orchardgrass on ruminal fermentation and methane output in continuous culture

A 4-unit dual-flow continuous culture fermentor system was used to assess the effects of supplementing orchardgrass (Dactylis glomerata L.) with 2 levels [5 and 10% of total dry matter (DM) fed] of starchy (barley grain, BAR) or fibrous (beet pulp, BP) carbohydrate sources on nutrient digestibility, volatile fatty acid (VFA) production, bacterial protein synthesis, and CH4 output. Treatments were randomly assigned to fermentors in a 4 × 4 Latin square design with a 2 × 2 factorial arrangement using 7 d for microbial adaptation and 3 d for sample collection. Treatments included (1) 57 g of DM orchardgrass + 3 g of DM BAR, (2) 54 g of DM orchardgrass + 6 g of DM BAR, (3) 57 g of DM orchardgrass + 3 g of DM BP, or (4) 54 g of DM orchardgrass + 6 g of DM BP. Feedings occurred at 0900, 1030, 1400, and 1900 h throughout four 10-d periods. Gas samples for CH4 analysis were collected 6 times daily at 0725, 0900, 1000, 1355, 1530, and 1630 h. Fermentor samples for pH, NH3-N, and VFA analysis were taken on d 8, 9, and 10. Samples were also analyzed for DM, organic matter, crude protein, purines, neutral detergent fiber, and acid detergent fiber to determine nutrient digestibilities and estimation of bacterial protein synthesis. Apparent and true DM and organic matter digestibilities were not affected by supplement source. Apparent neutral and acid detergent fiber digestibilities were greater for BAR than BP. Conversely, apparent crude protein digestibility was greater for BP than BAR. Mean and maximum pH tended to be greatest for BAR than BP. Minimum pH was greater at the lower level (5% of diet DM) of supplementation. Barley produced greater concentrations of total VFA and acetate, whereas BP had greater daily outputs of CH4. Significant supplement type × level interactions were found for bacterial N flow and efficiency. Overall, supplementing orchardgrass with BP improved crude protein digestibility, reduced fiber digestibility and total VFA concentration, but increased CH4 output compared with BAR.