Relationship between efficiency of nitrogen utilization and isotopic nitrogen fractionation in dairy cows: contribution of digestion v. metabolism?

Abstract

Animal tissues are naturally 15 N enriched relative to their diet and the extent of this difference (Δ 15 N animal-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 N animal-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 N animal-diet ) in these different body pools. The Δ 15 N animal-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 N duodenal digesta-diet mean value = 0.42) compared with that observed in plasma proteins (Δ 15 N plasma protein-diet mean value = 2.41). These data support the idea that most of the isotopic N fractionation observed in ruminant proteins (Δ 15 N plasma protein-diet ) has a metabolic origin with very little direct impact of the overall digestion process on the existing relationship between Δ 15 N plasma 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 N plasma protein-diet ) could have a metabolic origin, but more studies are warranted to confirm this point with different diets and approaches. ImplicationsThe difference in the natural abundance of 15 N between an animal and its diet (Δ 15 N animal-diet ) is a good biomarker for predicting the efficiency of N utilization. Our results in dairy cows fed diets not exceeding recommended protein levels demonstrate that digestion processes contribute little to the r...

Fat accretion measurements strengthen the relationship between feed conversion efficiency and Nitrogen isotopic discrimination while rumen microbial genes contribute little

“…This correlation was reduced to 28.7% (P < 0.001) when excluding the effect of dietary additive. The general form of this relationship is commonly observed across different species and breeds of ruminants 4 , 5 , indicating the potential of Δ 15 N animal-diet as a biomarker for FCE. Previous work has shown that a significant proportion of N isotopic discrimination results from processes involved in N metabolism – particularly in the liver 1 , 2 , and rumen microbial metabolism 3 .…”

“…At the mechanistic level, differences in the rates of biochemical reactions causing N isotopic discrimination contribute to the overall observed discrimination (Δ 15 N). Previous studies have reported negative relationships between FCE and Δ 15 N 4 , 5 , suggesting that the relationship with FCE may stem from its strong association with N intake and growth, thus reflecting the incorporation of N into tissue versus its excretion in urine 6 – hence current interest in estimating the composition of gain.…”

Potential of milk mid-infrared spectra to predict nitrogen use efficiency of individual dairy cows in early lactation

“…Additionally, in the present data set, the correlation between predicted milk urea and NUE was only −0.3. Other works have predicted NUE from 15 N fractions in tissues (Cheng et al, 2011;Cantalapiedra-Hijar et al, 2016) but either resulted in models with low R 2 or concluded Grelet et al: MIR PREDICTION OF NITROGEN EFFICIENCY Figure 2. Plot of measured individual nitrogen use efficiency (NUE) of dairy cows compared with NUE values predicted by the support vector machines regression (SVM) model using mid-infrared (MIR) spectra, parity, and milk yield as predictors, in cross-validation with 10 subsets (n = 1,034).…”

“…Methods based on N isotopes have also been investigated. Although Cantalapiedra-Hijar et al (2016) obtained a model predicting NUE with a coefficient of determination (R 2 ) of 0.5, Cheng et al (2011) were unable to validate the link between 15 N isotopic fractionation and NUE for the diets used in their experiments. According to Herremans et al (2019), the 15 N isotopic fractionation method would be more appropriate for estimating urinary nitrogen excretion than overall N use efficiency.…”

Nitrogen metabolism in rumen bacteria can be characterised by their N isotopic signature

“…In ruminants, the natural 15 N enrichment of animal proteins over the diet has been proposed as a proxy for N use efficiency (Cantalapiedra-Hijar et al, 2018). The contribution of rumen microbiota to this link has been acknowledged (Cantalapiedra-Hijar et al, 2016) but the underlying mechanism remains unknown. Based on previous studies we hypothesized that the N isotopic signature of rumen bacteria (Δ 15 N bacteria-substrate ) depends on the balance between uptake and release of ammonia (Martinez-Fernandez et al, 2018), both reactions being catalysed by enzymes discriminating against 15 N. Because rumen bacteria differ in their N metabolism it is expected that Δ 15 N bacteria-substrate would reflect their prevailing ability to assimilate or release ammonia.…”