# Ruminal utilization of nitrogen derived from 15N-labeled faba beans in vitro

## TL;DR

This study examines how nitrogen from faba bean protein is processed in the rumen, finding that much of it breaks down into ammonia and bacterial nitrogen.

## Contribution

The study introduces new insights into the ruminal metabolism of different nitrogen fractions in faba bean protein using 15N labeling.

## Key findings

- Soluble protein nitrogen (SPN) in faba beans is highly degraded into ammonia and bacterial nitrogen.
- Insoluble nitrogen fractions like NDIN show minimal ruminal metabolism.
- Faba bean protein has a high proportion of soluble true protein nitrogen.

## Abstract

Summary: This study aimed to investigate ruminal metabolism of 15N-enriched faba bean protein and its nitrogen fractions (acid detergent insoluble N [ADIN], insoluble N [INSN], neutral detergent insoluble N [NDIN], nonprotein N [NPN], and soluble protein N [SPN]). The results confirmed the high content of soluble true protein nitrogen in faba bean protein. The 10-hour batch culture incubation of 15N-enriched faba bean protein showed that it was largely degraded to ammonia (12%) and partly incorporated into bacterial nitrogen (15%). Future studies should further evaluate the degradation rate and utilization of soluble faba bean protein in the rumen and its contribution to postruminal amino acid supply and nitrogen utilization.

Summary: This study aimed to investigate ruminal metabolism of 15N-enriched faba bean protein and its nitrogen fractions (acid detergent insoluble N [ADIN], insoluble N [INSN], neutral detergent insoluble N [NDIN], nonprotein N [NPN], and soluble protein N [SPN]). The results confirmed the high content of soluble true protein nitrogen in faba bean protein. The 10-hour batch culture incubation of 15N-enriched faba bean protein showed that it was largely degraded to ammonia (12%) and partly incorporated into bacterial nitrogen (15%). Future studies should further evaluate the degradation rate and utilization of soluble faba bean protein in the rumen and its contribution to postruminal amino acid supply and nitrogen utilization.

•High rumen degradability of faba bean often results in low nitrogen efficiency.•Majority of faba bean protein is soluble, with the potential to contribute to metabolizable protein supply.•Soluble protein is degraded to NH3 and partly incorporated into bacterial nitrogen.•The flow of soluble faba bean protein into the lower gut remains to be investigated.

High rumen degradability of faba bean often results in low nitrogen efficiency.

Majority of faba bean protein is soluble, with the potential to contribute to metabolizable protein supply.

Soluble protein is degraded to NH3 and partly incorporated into bacterial nitrogen.

The flow of soluble faba bean protein into the lower gut remains to be investigated.

The aim of this short-term in vitro study was to characterize and evaluate ruminal utilization of N fractions of faba bean protein. Faba bean seeds were grown using a 15N-enriched N-fertilizer and fractionated into insoluble N (INSN), NDIN, NPN, and soluble protein N (SPN). Unfractionated ground 15N-labeled faba bean seeds (FB) were used as a reference treatment. Batches of the N fractions and FB were incubated for 10 h in an automatic Gas Endeavor in vitro batch-culture system with 3 replicated incubations. Purified mono-, di-, and polysaccharides were used as an energy source. The ratio of buffer to rumen fluid, obtained from 2 rumen-cannulated cows, was 2:1. The incubation medium was sampled at 0, 0.5, 1, 2, 5, and 10 h after the start of incubation. Subsequently, the incubation medium was separated into low- (i.e., feed and protozoa) and high-speed centrifugate (i.e., bacterial) N pools and ammonia-N pool, and 15N enrichment of each pool was measured. Due to low gas and VFA production, data for NPN fraction were not included in the dataset. The faba bean seeds contained (% of CP) 54% SPN, 18% NPN, 17% NDIN, 10% true protein soluble in neutral detergent, and 2% ADIN. At the end of incubation, the recovery of 15N in the ammonia-N pool was greatest for FB (12%) and SPN (7%) compared with the INSN (2%) and NDIN (0.06%) fractions. The recovery of 15N in the bacterial-N pool was numerically greatest for the FB treatment (15%), followed by INSN and SPN (6%), and was lowest for the NDIN treatment (0.75%). In line with our hypothesis, ruminal metabolism of N differed between the N fractions of faba bean being greatest for the SPN fraction, and minimal for the NDIN. Future studies should confirm the findings from this short-term in vitro batch-culture study, and further investigate the degradation and ruminal utilization of faba bean soluble N and its contribution to MP supply in dairy cows fed diets with varying protein and energy contents.