Ruminal metabolism of ammonia N and rapeseed meal soluble N fraction

摘要

The present study was conducted to investigate ruminalN metabolism in dairy cows using ~(15)N labeledN sources [ammonia N (AN), soluble non-ammoniaN (SNAN) from rapeseed meal, and insoluble nonammoniaN (NAN) from rapeseed meal]. To describethe observed pattern of ~(15)N transactions in the rumen,dynamic compartmental models were developed. Theexperiment consisted of 3 experimental treatments allocatedto 4 cows according to a changeover design.The results from 2 treatments (AN and rapeseed mealSNAN) are reported in this paper. Ammonia N andrapeseed SNAN, both labeled with ~(15)N, were administeredintraruminally. Rumen evacuations in combinationwith grab samples from the rumen contents wereused to determine ruminal N pool sizes. The ~(15)N-atom%excess was determined in N fractions of rumen digestasamples that were distributed between 0 and 82 h afterdosing. For the AN treatment, a 2-compartmentmodel was developed to describe the observed patternin ~(15)N-atom% excess pool sizes of AN and bacterialN and to estimate kinetic parameters of ruminal ~(15)Ntransactions. For the SNAN treatment, an additionalcompartment of SNAN was included in the model.Model simulations were used to estimate N fluxes in therumen. Both models described the observed pattern of~(15)N-atom% excess pool sizes accurately, based on smallresiduals between observed and predicted values. Immediateincreases in ~(15)N-atom% excess of bacterial Nwith AN treatment suggested that microbes absorbedAN from extracellular pools rapidly to maintain sufficientintracellular concentrations. Proportionally 0.69of the AN dose was recovered as NAN flow from therumen. A rapid disappearance of labeled SNAN fromrumen fluid and appearance in bacterial N pool indicatedthat, proportionally, 0.56 of SNAN was immediatelyeither adsorbed to bacterial cell surfaces or takenup to intracellular pools. Immediate uptake of labeledSNAN was greater than that of AN (proportionally0.56 vs. 0.16 of the dose). Degradation rate of SNANto AN was relatively slow (0.46/h), but only 0.08 of theSNAN dose was estimated to escape ruminal degradationbecause of rapid uptake by the bacteria. Overall,losses of the ~(15)N dose as AN absorption and outflowfrom the rumen were higher (P < 0.01) for the ANthan the SNAN treatment (0.31 and 0.11 of the dose,respectively). Consequently, recovery as NAN flow wasgreater for SNAN than for AN treatment (0.89 vs. 0.69of the dose). Estimated rate of bacterial N recyclingto AN was on average 0.006/h, which suggests that Nlosses due to intraruminal recycling are small in dairycows fed at high intake levels. We conclude that SNANisolated from rapeseed meal had better ruminal Nutilization efficiency than AN, as indicated by smallerruminal N losses as AN (0.11 vs. 0.31 of the dose) andgreater bacterial N flow (0.81 vs. 0.69 of the dose).Furthermore, the current findings indicate that rapidadsorption of soluble proteins to bacterial cells plays animportant role in ruminal N metabolism.