Abstract

The present study was conducted to investigate ruminal N metabolism in dairy cows using 15N-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 x 4 Latin square design. Soluble N (SN) isolated from timothy grass silage labeled with N-15 and ammonia N (AN) labeled with N-15 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 excess N-15-atom% was determined in N-fractions of rumen digest a grab samples that were collected frequently between 0 to 72 h and used to determine N-15 metabolism in the rumen. Calculations of area under the curve ratios of N-15 were used to estimate proportions of N fractions originating from precursor N pools. A model including soluble non ammonia N (SNAN), AN, bacterial N, and protozoal N pools was developed to predict observed values of N-15 atomic excess pool sizes. The model described the pool sizes accurately based on small residuals between observed and predicted values. An immediate increase in N-15 enrichment of protozoal N suggests physical attachment of bacteria pool to protozoa pool, lire 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-15 enrichment of bacterial N with SN than with AN treatment indicates a rapid adsorption of SNAN to microbial cells. Additionally, the recovery of N-15 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.

Keywords

dairy cow; modeling; bacterial nitrogen; ammonia nitrogen

Published in

Journal of Dairy Science
2018, Volume: 101, number: 1, pages: 279-294

SLU Authors

• Vaga, Merko

  • Department of Agricultural Research for Northern Sweden, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Animal and Dairy Science
Agricultural Science


Publication identifier

DOI: https://doi.org/10.3168/jds.2016-12316

Permanent link to this page (URI)

https://res.slu.se/id/publ/91465