Effects of concentrations of peptides on microbial metabolism in continuous culture

Effects of peptide concentrations on microbial metabolism were investigated using a basal diet in which peptides replaced urea as a N source at levels of 0, 10, 20, and 30% of total N. The basal diet contained 17.8% CP and 46% nonstructural carbohydrates. Each diet was provided to continuous cultures of rumen contents operating at liquid and solids flow rates of 12 and 4.5%/h, respectively. Production of microbial CP and digestion of DM and protein were affected quadratically by peptide addition, with the highest values for each variable on the diet containing 10% peptides. Fiber digestion decreased linearly with peptide addition. The depressed fiber digestion and microbial CP production at peptide concentrations > 10% seemed related to a linear decrease in ammonia concentration in the fermenters as peptide levels increased. Peptide-amino acid N recovered in the supernatant fluid did not increase with increasing peptides, because peptide uptake by the microbes increased as peptide concentrations increased. Even though the efficiency of conversion of peptide N to microbial CP increased with increasing peptides, there was no change in grams of microbial N produced per kilogram of OMD. We suggest that in diets high in nonstructural carbohydrates, excessive peptides can depress protein digestion and ammonia concentrations, resulting in decreased OMD, fiber digestion, and total microbial CP production.