Quantitation of the flow of soluble non-ammonia nitrogen entering the omasal canal of dairy cows fed grass silage based diets

Abstract

Four ruminally fistulated dairy cows were used to determine the flow of soluble non-ammonia N (SNAN) entering the omasal canal. Treatments in a 4×4 Latin square with 2×2 factorial arrangement were grass silage (GS) alone, or silage supplemented with barley (GB; 6.0 kg per day on dry matter (DM) basis), rapeseed meal (GR; 2.1 kg per day), or barley (6.0 kg per day) and rapeseed meal (2.1 kg per day) (GBR). Digesta was collected from the omasal canal at 1.5 h intervals, sequentially centrifuged to eliminate microbes and precipitated with trichloroacetic acid (TCA) followed by centrifugation. A sub-sample of digesta pooled across sampling intervals was subjected to analysis using ninhydrin assay (NHA), Kjeldahl procedure (KP) and amino acid (AA) analyser to determine SNAN. Different N fractions within omasal digesta (OD) were assessed according to NHA as (i) free AA in the supernatant, (ii) peptide in the supernatant hydrolysed with 6 M HCl minus the free AA and (iii) protein obtained following hydrolysis of TCA precipitate. In all cases, fractions were corrected for ammonia N. Digesta collected at each 1.5 h interval was subjected to the same determinations to study the extent of diurnal variation in SNAN fractions. The microbial contribution to SNAN was estimated using 15 N as a microbial marker. Free AA fraction determined using NHA and AA analyser were well correlated ( r 2=0.93) as were total SNAN determined by NHA and the KP ( r 2=0.88). Mean SNAN concentration was 15.8, 79.5 and 30.5 mg N l −1 for free AA, peptide and protein fractions, respectively. Peptides constituted the largest proportion of SNAN suggesting that hydrolysis of peptides to AA is the most limiting step in rumen proteolysis rather than hydrolysis of soluble proteins to peptides or deamination of AA to ammonia. The microbial contribution to SNAN was on average 0.61, indicating that a large proportion of the flow of SNAN leaving the rumen is of microbial origin. Mean soluble dietary NAN (SDNAN) flow was 10.8 g N per day, accounting for proportionately between 0.05 and 0.10 of total dietary NAN (TDNAN). Variations in SNAN concentration followed a clear diurnal pattern, with a maximum at 1.0 h post-feeding, that declined until 7.0 h post-feeding. In conclusion, these results suggest that, despite a proportion of microbial contribution, SDNAN represents a significant component of the flow of SNAN leaving the rumen.