Ruminal metabolism of ammonia N and rapeseed meal soluble N fraction

The present study was conducted to investigate ruminal N metabolism in dairy cows using ¹⁵N labeled N sources [ammonia N (AN), soluble non-ammonia N (SNAN) from rapeseed meal, and insoluble non-ammonia N (NAN) from rapeseed meal]. To describe the observed pattern of ¹⁵N transactions in the rumen, dynamic compartmental models were developed. The experiment consisted of 3 experimental treatments allocated to 4 cows according to a changeover design. The results from 2 treatments (AN and rapeseed meal SNAN) are reported in this paper. Ammonia N and rapeseed SNAN, both labeled with ¹⁵N, were administered intraruminally. Rumen evacuations in combination with grab samples from the rumen contents were used to determine ruminal N pool sizes. The ¹⁵N-atom% excess was determined in N fractions of rumen digesta samples that were distributed between 0 and 82 h after dosing. For the AN treatment, a 2-compartment model was developed to describe the observed pattern in ¹⁵N-atom% excess pool sizes of AN and bacterial N and to estimate kinetic parameters of ruminal ¹⁵N transactions. For the SNAN treatment, an additional compartment of SNAN was included in the model. Model simulations were used to estimate N fluxes in the rumen. Both models described the observed pattern of ¹⁵N-atom% excess pool sizes accurately, based on small residuals between observed and predicted values. Immediate increases in ¹⁵N-atom% excess of bacterial N with AN treatment suggested that microbes absorbed AN from extracellular pools rapidly to maintain sufficient intracellular concentrations. Proportionally 0.69 of the AN dose was recovered as NAN flow from the rumen. A rapid disappearance of labeled SNAN from rumen fluid and appearance in bacterial N pool indicated that, proportionally, 0.56 of SNAN was immediately either adsorbed to bacterial cell surfaces or taken up to intracellular pools. Immediate uptake of labeled SNAN was greater than that of AN (proportionally 0.56 vs. 0.16 of the dose). Degradation rate of SNAN to AN was relatively slow (0.46/h), but only 0.08 of the SNAN dose was estimated to escape ruminal degradation because of rapid uptake by the bacteria. Overall, losses of the ¹⁵N dose as AN absorption and outflow from the rumen were higher (P < 0.01) for the AN than the SNAN treatment (0.31 and 0.11 of the dose, respectively). Consequently, recovery as NAN flow was greater for SNAN than for AN treatment (0.89 vs. 0.69 of the dose). Estimated rate of bacterial N recycling to AN was on average 0.006/h, which suggests that N losses due to intraruminal recycling are small in dairy cows fed at high intake levels. We conclude that SNAN isolated from rapeseed meal had better ruminal N utilization efficiency than AN, as indicated by smaller ruminal N losses as AN (0.11 vs. 0.31 of the dose) and greater bacterial N flow (0.81 vs. 0.69 of the dose). Furthermore, the current findings indicate that rapid adsorption of soluble proteins to bacterial cells plays an important role in ruminal N metabolism.