Effects of Urea Supplementation of Diets Containing Lignosulfonate-Treated Soybean Meal on Bacterial Fermentation in Continuous Culture of Ruminal Contents

Eight dual-flow, continuous culture fermenters were used during four experimental periods to study effects of urea supplementation of diets containing ruminally protected soybean meal (SBM) protein and level of degraded N on bacterial metabolism of dietary nutrients. Treatments were arranged as a 2 × 4 factorial with two sources of protein and four levels of urea supplementation. Protein sources were untreated SBM (USBM) and calcium lignosulfonate-treated soybean meal (LSO3-SBM). Urea levels, expressed as a percentage of the total N input as urea-N, were 0, 10, 20 and 30%. Urea was added directly to fermenters via continuous infusion of a mineral buffer solution. Basal diets consisted of 10% alfalfa hay, 37% corn silage and 53% corn-SBM-based concentrate mix and had 16% CP (DM basis). Approximately 50% of the basal dietary N was derived from the SBM source. Protein source × urea level interactions were not significant for any measurements. Protein degradation and NH3-N concentrations were lower (P < .05) with LSO3-SBM diets (41.2% and 8.7 mg/100 ml, respectively) than with USBM diets (50.9% and 11.6 mg/100 ml, respectively), indicating that lignosulfonate treatment protected soybean protein from bacterial degradation. Ammonia-N concentrations, efficiency of bacterial protein synthesis, total bacterial N in the effluent, true OM digestion and cellulose digestion were higher (P < .05) with urea addition than with 0% urea. Linear responses (P < .05) in cellulose digestibility and efficiency of bacterial protein synthesis (g N/kg OM truly digested) and quadratic responses (P < .05) in true DM and OM digestibilities suggested that overall ruminal fermentation was enhanced as the level of degraded N supplied to the bacterial population was increased via increasing levels of urea supplementation.