Effects of carbohydrate and nitrogen supplementation on fermentation of cheatgrass (Bromus tectorum) in a dual-flow continuous culture system1

Abstract

Cheatgrass (CG; Bromus tectorum), an introduced winter annual grass, is an aggressive invader of the sagebrush community in the Western United States. Because of its greater flammability, mature CG constitutes a fire hazard leading to repeated wildfires. One fuel-reduction strategy is livestock grazing. The objective of this study was to evaluate the effects of urea, molasses, or a combination of urea and molasses supplementation of a CG-based diet on digestibility, microbial fermentation, bacterial protein synthesis, and nutrient flow using a dual-flow continuous culture system. Eight fermenters were used in a replicate 4 × 4 Latin square design with four 10-d experimental periods. Experimental treatments (DM basis) were 1) forage only (CON), 2) CG plus urea alone (URE; 1.36% urea), 3) CG plus molasses alone (MOL; 15.9% molasses), and 4) CG plus urea and molasses combined (URE+MOL; 1.28% urea plus 19.3% molasses). Each fermenter was fed 72 g/d of DM, and data were analyzed using the GLIMMIX procedure of SAS (SAS Inst. Inc., Cary, NC). The true digestibilities of NDF and ADF were not affected by diets (P > 0.05). Molasses-containing diets had greater true digestibility of OM (P = 0.02). However, true digestibility of CP was increased when molasses was fed alone (P < 0.01). Molasses-containing diets had lower pH (P < 0.01) and greater VFA concentrations (P < 0.01) compared to those of the other diets. The URE+MOL diet resulted in a greater VFA concentration (P < 0.01). Propionate concentration increased (P < 0.01), whereas acetate concentration decreased (P < 0.01) when molasses alone or in combination with urea was added to the diets. Supplying molasses alone resulted in greater (P = 0.03) total branched-chain VFA compared to the other diets. The concentration of NH3–N and total N flow increased (P < 0.01) in response to urea supplementation and was greater (P < 0.01) when urea alone was supplemented in the diet. On the other hand, molasses-supplemented diets yielded more non-ammonia N (P < 0.01) and bacterial N (P = 0.04). Supplementation had no effect (P = 0.83) on bacterial efficiency. Results from this study indicate that the addition of urea and molasses in a CG-based diet could improve nutrient supply to animals, notably VFA supply and microbial N supply; however, in the levels tested in this study, it did not improve CG utilization as assessed by NDF digestion