Effects of several protein supplements on flow of soluble non-ammonia nitrogen from the forestomach and milk production in dairy cows

Abstract

Five ruminally fistulated dairy cows were used to study the flow of soluble non-ammonia nitrogen (SNAN) from the rumen and to compare flow estimates based on ruminal fluid (RF) and omasal digesta (OD) samplings. The treatments in a 5×5 Latin square design consisted of a basal diet of grass silage and barley grain (50:50) and four protein supplemented diets. Nitrogen (N) content of the basal diet (24 g/kg dry matter (DM)) was increased to 28 g/kg DM by replacing the basal diet with skimmed milk powder (2.1 kg per day), wet distiller’s solubles (3.0 kg per day), untreated rapeseed meal (2.1 kg per day) or treated rapeseed meal (2.1 kg per day). Rumen liquid outflow was determined using a ruminal dose of LiCo–EDTA. For SNAN analysis, digesta was collected from both the rumen and omasal canal at 1 h intervals during a 12 h feeding cycle, sequentially centrifuged to eliminate microbes, and precipitated with trichloroacetic acid followed by centrifugation. Different N fractions, i.e. free amino acids (AA), peptides and protein in RF and OD were assessed using the ninhydrin assay. Protein supplements did not affect rumen liquid outflow, and only tended to increase milk yield. Protein supplements increased ( P≤0.10) SNAN fractions except for peptides. Mean flow of each SNAN fraction was higher in OD than that in RF ( P≤0.10), except for the soluble protein fraction. In both RF and OD, peptide N constituted the largest proportion of total SNAN suggesting that hydrolysis of peptides to AA rather than hydrolysis of soluble proteins to peptides or deamination of AA to ammonia is the most limiting step in rumen degradation. Estimated SNAN flow per kg of DM intake from the basal diet and protein supplemented diets indicated that 118, 113, 136 and 96 g/kg of N in skimmed milk powder, wet distiller’s solubles, untreated rapeseed meal and treated rapeseed meal, respectively, escaped ruminal degradation as SNAN.