Influence of peptides and amino acids on fermentation rate and de novo synthesis of amino acids by mixed micro-organisms from the sheep rumen

The influence of different N sources on fermentation rate and de nova amino acid synthesis by rumen micro-organisms was investigated in vitro using ru men fluid taken from four sheep receiving a mixed diet comprising (g/kg DM) : grass hay 500, barley 299.5, molasses 100, fish meal 91, minerals and vit amins 9.5. Pancreatic casein hydrolysate (P; comprising mainly peptides wit h some free amino acids; 10 g/l), free amino acids (AA; casein acid hydroly sate + added cysteine and tryptophan; 10 g/l), or a mixture of L-proline, g lycine, L-valine and L-threonine (M; 0.83 g/l each) were added to diluted(1 :3, v/v), strained rumen fluid along with (NH4Cl)-N-15(A; 1.33 g/l) and 6.7 g/l of a mixture of starch, cellobiose and xylose (1 : 1 : 1, by weight). P and AA, but not M, stimulated net gas production after 4 and 8 h incubati on (P < 0.05) in comparison with A alone. P increased microbial-protein syn thesis (P < 0.05) compared with the other treatments. All of the microbial- N formed after 10 h was synthesized de nova from (NH3)-N-15 in treatment A, and the addition of pre-formed amino acids decreased the proportion to 0.3 7, 0.55, and 0.86 for P, AA, and M respectively. De novo synthesis of amino acids (0.29, 0.42 and 0.69 respectively) was lower than cell-N. Enrichment of alanine, glutamate and aspartate was slightly higher than that of other amino acids, while enrichment in proline was much lower, such that 0.83-0. 95 of all proline incorporated into particulate matter was derived from pre -formed proline. Glycine, methionine, lysine, valine and threonine tended t o be less enriched than other amino acids. The form in which the amino acid s were supplied, as P or AA, had little influence on the pattern of denovo synthesis. When the concentration of peptides was decreased, the proportion of microbial-N formed from NH3 increased, so that at an initial concentrat ion of 1 g peptides/l, similar to the highest reported ruminal peptide conc entrations, 0.68 of cell-N was formed from NH3. Decreasing the NH3 concentr ation at 1.0 g peptides/l caused proportionate decreases in the fraction of cell-N derived from NH3, from 0.81 at 0.53 g NH3-N/l to 0.40 at 0.19 g NH3 -N/l. It was concluded that different individual amino acids are synthesize d de novo to different extents by mixed rumen micro-organisms when pre-form ed amino acids are present, and that the source of N used for synthesis of cell-N and amino acids depends on the respective concentrations of the diff erent N sources available; however, supplementing only with amino acids who se synthesis is lowest when pre-formed amino acids are present does not sti mulate fermentation or microbial growth.