Relationship between efficiency of nitrogen utilization and isotopic nitrogen fractionation in dairy cows: contribution of digestion v. metabolism?
Animal tissues are naturally 15N enriched relative to their diet and the extent of this difference (D15Nanimal diet) has been correlated to the efficiency of N assimilation in different species. The rationale is that transamination and deamination enzymes, involved in amino acid metabolism are likely to preferentially convert amino groups containing 14N over 15N. However, in ruminants the contribution of rumen bacterial metabolism relative to animal tissues metabolism to naturally enrich animal proteins in terms of 15N has been not assessed yet. The objective of this study was to assess the impact of rumen and digestion processes on the relationship between D15Nanimal diet and efficiency of N utilization for milk protein yield (milk N efficiency (MNE) milk N yield/N intake) as well as the relationship between the 15N natural abundance of rumen bacteria and the efficiency of N use at the rumen level. Solid and liquid associated rumen bacteria, duodenal digesta, feces and plasma proteins were obtained (n=16) from four lactating Holstein cows fed four different diets formulated at two metabolizable protein supplies (80% v. 110% of protein requirements) crossed by two different dietary energy source (diets rich in starch v. fiber) We measured the isotopic N fractionation between animal and diet (D15Nanimal diet) in these different body pools. The D15Nanimal diet was negatively correlated with MNE when measured in solid associated rumen bacteria, duodenal digesta, feces and plasma proteins, with the strongest correlation found for the latter. However, our results showed a very weak 15N enrichment of duodenal digesta (D15Nduodenal digesta diet mean value=0.42) compared with that observed in plasma proteins (D15Nplasma protein diet mean value=2.41) These data support the idea that most of the isotopic N fractionation observed in ruminant proteins (D15Nplasma protein diet) has a metabolic origin with very little direct impact of the overall digestion process on the existing relationship between D15Nplasma protein diet and MNE. The 15N natural abundance of rumen bacteria was not related to either rumen N efficiency (microbial N/available N) or digestive N efficiency (metabolizable protein supply/CP intake) but showing a modest positive correlation with rumen ammonia concentration. When using diets not exceeding recommended protein levels, the contribution of rumen bacteria and digestion to the isotopic N fractionation between animal proteins and diet is low. In our conditions, most of the isotopic N fractionation (D15Nplasma protein diet) could have a metabolic origin, but more studies are warranted to confirm this point with different diets and approaches.
Animal an international journal of animal bioscience