Ruminal metabolism of grass silage soluble nitrogen fractions

Abstract

The present study was conducted to investigate ruminal N metabolism in dairy cows using N-labeled N sources and dynamic models. The data summarized in this study were obtained from 2 of 4 treatments whose effects were determined in a 4 × 4 Latin square design. Soluble N (SN) isolated from timothy grass silage labeled withN and ammonia N (AN) labeled with N were administered into the rumen contents of 4 ruminally cannulated dairy cows. Ruminal N pool sizes were determined by manual evacuation of rumen contents. The excessN-atom% was determined in N-fractions of rumen digesta grab samples that were collected frequently between 0 to 72 h and used to determine N metabolism in the rumen. Calculations of area under the curve ratios ofN were used to estimate proportions of N fractions originating from precursor N pools. A model including soluble nonammonia N (SNAN), AN, bacterial N, and protozoal N pools was developed to predict observed values of N atomic excess pool sizes. The model described the pool sizes accurately based on small residuals between observed and predicted values. An immediate increase inN enrichment of protozoal N suggests physical attachment of bacteria pool to protozoa pool. The mean proportions of bacterial N, protozoal N, and feed N in rumen solid phase were 0.59, 0.20, and 0.21, respectively. These observations suggest that protozoal N accounted for 0.25 of rumen microbial N. About 0.90 of the initial dose of AN was absorbed or taken up by microbes within 2 h. Faster N enrichment of bacterial N with SN than with AN treatment indicates a rapid adsorption of SNAN to microbial cells. Additionally, the recovery ofN as microbial and feed N flow from the rumen was approximately 0.36 greater for SN than for the AN treatment, indicating that SNAN was more efficiently used for microbial growth than AN. The present study indicated that about 0.15 of microbial N flowing to the duodenum was of protozoal origin and that 0.95 of the protozoal N originated from engulfed bacterial N. The kinetic variables indicated that 0.125 of SNAN escaped ruminal degradation, which calls into question the use of in situ estimations of protein degradation to predict the flow of rumen undegradable protein.

Invited review: Nitrogen in ruminant nutrition: A review of measurement techniques

“…Plant N can be further fractionated and studied using 15 N-labeled plants (e.g., Melgar and Hristov, 2004; who reported rates of microbial breakdown of alfalfa insoluble N, soluble protein N, NPN, neutral-detergent insoluble N, and neutral-detergent soluble N). Using 15 N-labeled grass silage and rumen modeling, Ahvenjärvi et al (2018) showed that a considerable proportion of silage soluble NAN (0.125 g/g) escapes ruminal degradation and contributes to postruminal feed NAN supply.…”

Extending Burk Dehority’s Perspectives on the Role of Ciliate Protozoa in the Rumen

“…However, these prior protozoal volumes derived using the geometric formula for cylinders were probably overestimated by 25 to 40% (Wenner et al, 2018). Based on these arguments, rather than 50%, perhaps a more accurate expectation should be 25% of the microbial biomass being derived from protozoa in the rumen compared with 15% in the ruminal outflow for lactating dairy cattle (Ahvenjärvi et al, 2018). Similar contributions to outflow in dairy cattle (about 17%) were derived using various approaches methods (Sok et al, 2017;Fessenden et al, 2019) that were not based on faulty microbial marker systems (Firkins et al, 1998).…”

In vitro investigation of the ruminal digestion kinetics of different nitrogen fractions of 15N-labelled timothy forage

“…The fractional degradation rates of SN fractions observed here were about two-fold lower than previously reported by Peltekova and Broderick [ 47 ]. In dairy cows, given a single dose of 15 N-labelled grass silage SN fraction, the SNAN disappearance rate was 1.24 /h, with the rate of SNAN degradation to ammonia N being about five-fold faster than direct bacterial uptake of SNAN [ 48 ]. In our study the degradation rate of silage SN fractions to ammonia N was only about 50% faster than the rate of microbial uptake of SN, and with SN from dried grass the rates of uptake and degradation were similar.…”