Relationship between efficiency of nitrogen utilization and isotopic nitrogen fractionation in dairy cows: contribution of digestion v. metabolism?. (24th September 2015)

• Record Type: Journal Article
• Authors: Cantalapiedra-Hijar, G.; Fouillet, H.; Huneau, J. F.; Fanchone, A.; Doreau, M.; Nozière, P.; Ortigues-Marty, I.
• Is Part Of: Animal. Volume 10:Number 2(2016:Feb.)
• Page Start: 221
• Page End: 229
• Publication Date: 2015-09-24
• Subjects: isotopic N fractionation, -- rumen, -- efficiency of N utilization, -- ruminant, -- 15N; Animal breeding -- Periodicals; Animal genetics -- Periodicals; Animal nutrition -- Periodicals; Animal physiology -- Periodicals; Environmental sciences -- Periodicals; 636.005
• DOI: 10.1017/S1751731115002025
• Languages: English
• ISSNs: 1751-7311

Abstract

Abstract : Animal tissues are naturally 15 N enriched relative to their diet and the extent of this difference (Δ 15 Nanimal-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 14 N over 15 N. However, in ruminants the contribution of rumen bacterial metabolism relative to animal tissues metabolism to naturally enrich animal proteins in terms of 15 N has been not assessed yet. The objective of this study was to assess the impact of rumen and digestion processes on the relationship between Δ 15 Nanimal-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 15 N 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 (Δ 15 Nanimal-diet ) in these different body pools. The Δ 15 Nanimal-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 15 N enrichment of duodenal digesta (Δ 15 Nduodenal digesta-diet mean value=0.42) compared with that observed in plasma proteins (Δ 15 Nplasma protein-diet mean value=2.41). These data support the idea that most of the isotopic N fractionation observed in ruminant proteins (Δ 15 Nplasma protein-diet ) has a metabolic origin with very little direct impact of the overall digestion process on the existing relationship between Δ 15 Nplasma protein-diet and MNE. The 15 N 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 (Δ 15 Nplasma protein-diet ) could have a metabolic origin, but more studies are warranted to confirm this point with different diets and approaches.