Nitrogen Metabolism in the Rumen

Highly fermentable diets are rapidly converted to organic acids [i.e., short-chain fatty acids (SCFA) and lactic acid] within the rumen. The resulting release of protons can constitute a challenge to the ruminal ecosystem and animal health. Health disturbances, resulting from acidogenic diets, are classified as subacute and acute acidosis based on the degree of ruminal pH depression. Although increased acid production is a nutritionally desired effect of increased concentrate feeding, the accumulation of protons in the rumen is not. Consequently, mechanisms of proton removal and their quantitative importance are of major interest. Saliva buffers (i.e., bicarbonate, phosphate) have long been identified as important mechanisms for ruminal proton removal. An even larger proportion of protons appears to be removed from the rumen by SCFA absorption across the ruminal epithelium, making efficiency of SCFA absorption a key determinant for the individual susceptibility to subacute ruminal acidosis. Proceeding initially from a model of exclusively diffusional absorption of fermentation acids, several protein-dependent mechanisms have been discovered over the last 2 decades. Although the molecular identity of these proteins is mostly uncertain, apical acetate absorption is mediated, to a major degree, via acetate-bicarbonate exchange in addition to another nitrate-sensitive, bicarbonate-independent transport mechanism and lipophilic diffusion. Propionate and butyrate also show partially bicarbonate-dependent transport modes. Basolateral efflux of SCFA and their metabolites has to be mediated primarily by proteins and probably involves the monocarboxylate transporter (MCT1) and anion channels. Although the ruminal epithelium removes a large fraction of protons from the rumen, it also recycles protons to the rumen via apical sodium-proton exchanger, NHE. The latter is stimulated by ruminal SCFA absorption and salivary Na(+) secretion and protects epithelial integrity. Finally, SCFA absorption also accelerates urea transport into the rumen, which via ammonium recycling, may remove protons from rumen to the blood. Ammonium absorption into the blood is also stimulated by luminal SCFA. It is suggested that the interacting transport processes for SCFA, urea, and ammonia represent evolutionary adaptations of ruminants to actively coordinate energy fermentation, protein assimilation, and pH regulation in the rumen.

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RUMINANT NUTRITION SYMPOSIUM: Role of fermentation acid absorption in the regulation of ruminal pH

Nitrogen Recycling in the Ruminant: A Review

The ability of ytterbium acetabe (Yb acetate) to fulfil the requirements of a particulate-phase digesta-flow marker in a dual-phase marker system, and of the indigestible acid-detergent-fibre fraction of the feed (IADF) to act as a digesta flow marker, were examined using six mature wether sheep given a diet of dried grass (1 kg dry matter (DM)/d). CrEDTA was continuously infused (240 mg chromium/d) into the rumen of all sheep and Yb acetate was also continuously infused (100 mg Yb/d) into the rumen of three of the sheep. At this level of infusion the equilibrium concentration of Yb in rumen, duodenal and ileal digesta and in faeces could be reliably measured by atomic absorption spectrometry. Estimates of faecal DM excretion based on either Yb or IADF did not differ (P greater than 0.05) from that determined by total collection, whereas estimates based on Cr were significantly (P less than 0.05) lower. Urinary excretion accounted for 3.1% of the infused Cr but no Yb was detected in urine. Estimates of ileal DM flow, assuming total marker recovery, were similar (P greater than 0.05) with all three markers, whereas the estimate of duodenal DM flow based on IADF was lower (P less than 0.05) than the estimates based on either Cr or Yb. Compared with the infusion of Cr alone, the infusion of Cr and Yb had no effect (P greater than 0.05) on nutrient flows at the duodenum, ileum and in faeces nor on microbial degradative activity volatile fatty acid production and N metabolism in the rumen. Polyester bag and in vitro studies showed that pre-labelling the dried grass with up to 285 mg Yb/g DM did not affect its susceptibility to microbial degradation. The Yb in rumen, duodenal and ileal digesta was predominantly (greater than 90%) associated with the particulate matter but was not uniformly distributed and its concentration increased as particle size decreased. The use of CrEDTA and Yb acetate as a dual-phase marker system proved more reliable in estimating 'true' duodenal flow than the use of the individual markers when the digesta sample was unrepresentative.

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Ytterbium acetate as a particulate-phase digesta-flow marker

W. N. Aldridge (Chairman) H. Gutfreund (Deputy Chairman) V. P. Whittaker (Deputy Chairman) J. R. Quayle (Deputy Chairman) J. L. Bailey R. V. Brooks R. H. Burdon R. A. Cox M. J. Crumpton D. F. Elliott J. R. S. Fincham P. B. Garland J. N. Hawthorne J. R. Knowles P. N. Magee D. J. Manners J. H. Moore C. J. 0. R. Morris 1. H. M. Muir A. C. T. North* C. A. Pasternak A. R. Peacocke* H. R. Perkins E. R. Redfearn D. N. Rhodes D. S. Robinson D. B. Roodyn R. E. Strange J. R. Tata P. K. Tubbs D. G. Walker

The Biochemical Journal

1. Nitrogen kinetics were studied in six sheep (45–55 kg live weight) consuming either a high-N grass silage or a low-N dried grass made from swards of perennial ryegrass (Lolium perenne). The diets were fed hourly at a level of 600 g dry matter/d and supplied 19.5 and 11.0 g N/d respectively.2. The amounts of organic matter (OM) consumed and flowing at the duodenum and ileum and excreted in the faeces were similar (P < 0.05) with both diets. Each diet supplied 23 g digestible OM/d per kg live eight0.75, which was sufficient to maintain body-weight.3. There were no differences (P < 0.05) between diets in rumen fluid volume, fractional outflow rate of fluid from the rumen, total concentration of volatile fatty acids or molar proportion of acetate in the rumen. The pH and molar proportion of propionate in rumen fluid were higher (P < 0.01), and molar proportion of butyrate lower (P < 0.001) when the silage was given.4. There was a net loss of N (4.0 g/d) between mouth and duodenum when the silage was consumed but a net gain (5.5 g/d) when the dried grass was consumed. As a result, total non-ammonia-N (NAN) flow at the duodenum did not differ (P / 0.05) between diets. Rumen microbial NAN flow at the duodenum, based on 15N as the marker, also did not differ (P < 0.05) between diets but the efficiency of microbial N synthesis in the rumen (g/kg OM apparently digested) was higher (P < 0.05) with the dried grass.5. When the sheep were consuming silage they had a higher concentration of ammonia in rumen fluid (P < 0.01), a higher rate of irreversible loss of ammonia from the rumen (P < 0.05) and a higher rate of absorption of ammonia across the rumen wall (P < 0.01). The rate of absorption was found to be more closely related to the unionized ammonia concentration in rumen fluid (r2 0.85) than to the total ammonia concentration (r2 0.36).6. Endogenous N entry into the forestomachs was calculated to be 5.5 g/d when the silage was given and 9.4 g/d when the dried grass was given, of which 1.7 and 3.5 g/d respectively were in the form of urea. Thus, approximately 4–6 g N/d were derived from non-urea materials.7. Within the small intestine the apparent absorption coefficient of rumen microbial NAN (0.72) did not differ (P < 0.05) between diets but the apparent absorption coefficient of total NAN was lower (P < 0.05) when the I silage was given, owing to a lower (P < 0.01) absorption coefficient of the non-microbial NAN fraction (undegraded feed and endogenous).8. Within the large intestine, diet had no effect (P < 0 05) on the apparent absorption coefficients of total N (0.22) and rumen microbial NAN (0.63).9. Plasma urea concentration, the rate of urea synthesis in the body and urinary urea excretion were higher (P < 0.001) when the silage was consumed. However, the transfer of urea to the whole digestive tract and to the post-ruminal part of the tract did not differ (P < 0.05) between diets; urea transfer to the rumen was higher (P < 0.01) when the dried grass was given.10. The results were used to construct a whole-animal model of N flows between the digestive tract and the tissues.

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Nitrogen digestion and metabolism in sheep consuming diets containing contrasting forms and levels of N

1. Six sheep, each fitted with a rumen cannula and re-entrant cannulas in the proximal duodenum and distal ileum, were given two diets (600 g dry matter (DM)/d) consisting of either grass silage (32.1 g nitrogen/kg DM) or dried grass (18.3 g N/kg DM). A net loss of N occurred between mouth and duodenum with the silage diet, indicating extensive ruminal degradation of dietary N, compared with a net gain on the dried-grass diet. Consequently, despite higher N intakes when silage was given, N flow at the duodenum was similar for both diets. 2. The proportion of microbial N in duodenal digesta N was estimated using diaminopimelic acid (DAPA), [35S]methionine (35S), 15N-enriched non-ammonia-N (15NAN) and amino acid profiles (AAP) as microbial markers. Isotopic labelling of rumen micro-organisms was achieved by intraruminal infusions of Na2 35SO4 and (15NH4)2SO4. 3. A comparison of all methods was made based on the marker concentrations in microbial fractions isolated by differential centrifuagation of strained rumen contents. With both diets, DAPA gave the highest estimates and AAP the lowest. Estimates based on 35S and 15NAN were intermediate and did not differ significantly (P greater than 0.05). 4. For the 15NAN, 35S and AAP methods, the effect of site of sampling of the microbial fraction, i.e. from rumen contents or duodenal digesta, was examined and in all instances mean estimates based on duodenally-derived microbes were higher. However, the differences were significant for only 15NAN with both diets (P less than 0.001), for 35S with the dried grass (P less than 0.05), and for AAP with the silage (P less than 0.05). Estimates based on duodenally-derived microbes were higher (P less than 0.05) using 15 NAN than those obtained using 35S with both diets. 5. Depending on the method used for estimating microbial N, estimates of the efficiency of microbial N synthesis in the rumen (g microbial N flow at duodenum/kg organic matter apparently digested in the rumen) ranged between 16 and 38 for the silage diet and 10 and 46 for the dried grass diet. Similarly, estimates of feed N degradability in the rumen ranged between 0.62 and 0.97 for the silage and 0.00 and 0.93 for the dried grass.

/pdf/a-comparison-of-methods-for-the-estimation-of-microbial-2ogr7w98a5.pdf

A comparison of methods for the estimation of microbial nitrogen in duodenal digesta of sheep.

https://hal.archives-ouvertes.fr/hal-00901154/document

D. E. Beever

Papers

Nitrogen digestion and metabolism in sheep consuming diets containing contrasting forms and levels of N

A comparison of methods for the estimation of microbial nitrogen in duodenal digesta of sheep.

Estimation of microbial protein in duodenal digesta.