Effect of slowly and rapidly degraded protein sources on the concentrations of amino acids and peptides in the rumen of steers

Four steers were given diets of straw and tapioca supplemented with urea, casein, formaldehyde treated casein (FTC), decorticated groundnut meal (DCGM), soya bean meal (SBM), maize gluten meal (MGM) or fish meal (FM) to determine the effect of degradability on rumen amino acid and peptide concentrations. Maximum concentrations of amino acid and peptide-N in rumen ultrafiltrates occurred 1 h after feeding. Total amino acid-N concentrations (mg litre-1) at this time were low, ranging from 1.2 SE 0.8 (urea) to 15.6 SE 6.0 (casein) with little variation in individual amino acids. The peptide-N concentrations (mg litre-1) were higher, ranging from 2.4 SE 0.5 (urea) to 160.0 SE 21.0 (casein), but were poorly correlated with degradability and solubility. Apart from casein, the highest concentrations of peptide-N were for those protein supplements, MGM (136.0 SE 25.0), FM (90.0 SE 15.0), and FTC (87.0 SE 14.0), with the lowest degradability suggesting that other factors, such as hydrophobicity, are also involved in the control of rumen peptide concentrations. Pooled samples taken 1 h after the morning feed were separated into three molecular weight (MW) fractions, 5000-50 000 da, 250-5000 da and < 250 da by gel filtration chromatography. From amino acid analysis it was found that the highest proportion of peptide-N was in the 250-5000 da fraction, and although some high MW (5000-50 000) constituents were present, only the FTC and MGM diets gave rise to significant amounts.