In vivo production and molar percentages of volatile fatty acids in the rumen: a quantitative review by an empirical approach

Despite their major contribution to the energy supply of ruminants, the production of volatile fatty acids (VFA) in the rumen is still poorly predicted by rumen models. We have developed an empirical approach, based on the interpretation of large bibliographic databases gathering published in vivo measurements of ruminal VFA production rate (PR), rates of duodenal and faecal digestion and molar percentages of VFA in the rumen. These databases, covering a wide range of intake levels and dietary composition, were studied by meta-analysis using within-experiment models. We established models to quantify response laws of total VFA-PR and individual VFA molar percentages in the rumen to variations in intake level and dietary composition. The rumen fermentable organic matter (RfOM) intake, estimated from detailed knowledge of the chemical composition of diets according to INRA Feed Tables, appears as an accurate explanatory variable of measured total VFA-PR, with an average increment of 8.03 ± 0.64 mol total VFA/kg RfOM intake. Similar results were obtained when total VFA-PR was estimated from measured apparent RfOM (total VFA-PR/RfOM averaging 8.3 ± 1.2 mol/kg). The VFA molar percentages were related to dry matter intake and measured digestible organic matter (OM), digestible NDF and rumen starch digestibility, with root mean square error of 1.23, 1.45, 0.88 and 0.41 mol/100 mol total VFA for acetate, propionate, butyrate and minor VFA, respectively, with no effect of pH on the residuals. Stoichiometry coefficients were calculated from the slopes of the relationships between individual VFA production (estimated from measured apparent RfOM and individual VFA molar percentages) and measured fermented fractions. Coefficients averaged, respectively, 66, 17, 14 and 3 mol/100 mol for NDF; 41, 44, 12 and 4 mol/100 mol for starch; and 46, 35, 13 and 6 mol/100 mol for crude protein. Their use to predict VFA molar percentages appear relevant for most dietary conditions, that is, when the digested NDF/digested OM ratio exceeded 0.12. This study provides a quantitative review on VFA yield in the rumen. It contributes to the development of feed evaluation systems based on nutrient fluxes.     

Glucagon-like peptide-2 (GLP-2) increases net amino acid utilization by the portal-drained viscera of ruminating calves

Glucagon-like peptide-2 (GLP-2) increases small intestinal mass and blood flow in ruminant calves, but its impact on nutrient metabolism across the portal-drained viscera (PDV) and liver is unknown. Eight Holstein calves with catheters in the carotid artery, mesenteric vein, portal vein and hepatic vein were paired by age and randomly assigned to control (0.5% bovine serum albumin in saline; n = 4) or GLP-2 (100 μg/kg BW per day bovine GLP-2 in bovine serum albumin; n = 4). Treatments were administered subcutaneously every 12 h for 10 days. Blood flow was measured on days 0 and 10 and included 3 periods: baseline (saline infusion), treatment (infusion of bovine serum albumin or 3.76 μg/kg BW per h GLP-2) and recovery (saline infusion). Arterial concentrations and net PDV, hepatic and total splanchnic fluxes of glucose, lactate, glutamate, glutamine, β-hydroxybutyrate and urea-N were measured on days 0 and 10. Arterial concentrations and net fluxes of all amino acids and glucose metabolism using continuous intravenous infusion of [U¹³-C]glucose were measured on day 10 only. A 1-h infusion of GLP-2 increased blood flow in the portal and hepatic veins when administered to calves not previously exposed to exogenous GLP-2, but after a 10-day administration of GLP-2 the blood flow response to the 1-h GLP-2 infusion was substantially attenuated. The 1-h GLP-2 infusion also did not appreciably alter nutrient fluxes on either day 0 or 10. In contrast, long-term GLP-2 administration reduced arterial concentrations and net PDV flux of many essential and non-essential amino acids. Despite the significant alterations in amino acid metabolism, glucose irreversible loss and utilization by PDV and non-PDV tissues were not affected by GLP-2. Fluxes of amino acids across the PDV were generally reduced by GLP-2, potentially by increased small intestinal epithelial growth and thus energy and amino acid requirements of this tissue. Increased PDV extraction of glutamine and alterations in PDV metabolism of arginine, ornithine and citrulline support the concept that GLP-2 influences intestine-specific amino acid metabolism. Alterations in amino acid metabolism but unchanged glucose metabolism suggests that the growth effects induced by GLP-2 in ruminants increase reliance on amino acids preferentially over glucose. Thus, GLP-2 increases PDV utilization of amino acids, but not glucose, concurrent with stimulated growth of the small intestinal epithelium in post-absorptive ruminant calves.     

Comparison of energy evaluation systems and a mechanistic model for milk production by dairy cattle offered fresh grass-based diets

Grass-based diets are of increasing social-economic importance in dairy cattle farming, but their low supply of glucogenic nutrients may limit the production of milk. Current evaluation systems that assess the energy supply and requirements are based on metabolisable energy (ME) or net energy (NE). These systems do not consider the characteristics of the energy delivering nutrients. In contrast, mechanistic models take into account the site of digestion, the type of nutrient absorbed and the type of nutrient required for production of milk constituents, and may therefore give a better prediction of supply and requirement of nutrients. The objective of the present study is to compare the ability of three energy evaluation systems, viz. the Dutch NE system, the agricultural and food research council (AFRC) ME system, and the feed into milk (FIM) ME system, and of a mechanistic model based on Dijkstra et al. [Simulation of digestion in cattle fed sugar cane: prediction of nutrient supply for milk production with locally available supplements. J. Agric. Sci., Cambridge 127, 247–60] and Mills et al. [A mechanistic model of whole-tract digestion and methanogenesis in the lactating dairy cow: model development, evaluation and application. J. Anim. Sci. 79, 1584–97] to predict the feed value of grass-based diets for milk production. The dataset for evaluation consists of 41 treatments of grass-based diets (at least 0.75 g ryegrass/g diet on DM basis). For each model, the predicted energy or nutrient supply, based on observed intake, was compared with predicted requirement based on observed performance. Assessment of the error of energy or nutrient supply relative to requirement is made by calculation of mean square prediction error (MSPE) and by concordance correlation coefficient (CCC). All energy evaluation systems predicted energy requirement to be lower (6–11%) than energy supply. The root MSPE (expressed as a proportion of the supply) was lowest for the mechanistic model (0.061), followed by the Dutch NE system (0.082), FIM ME system (0.097) and AFRC ME system (0.118). For the energy evaluation systems, the error due to overall bias of prediction dominated the MSPE, whereas for the mechanistic model, proportionally 0.76 of MSPE was due to random variation. CCC analysis confirmed the higher accuracy and precision of the mechanistic model compared with energy evaluation systems. The error of prediction was positively related to grass protein content for the Dutch NE system, and was also positively related to grass DMI level for all models. In conclusion, current energy evaluation systems overestimate energy supply relative to energy requirement on grass-based diets for dairy cattle. The mechanistic model predicted glucogenic nutrients to limit performance of dairy cattle on grass-based diets, and proved to be more accurate and precise than the energy systems. The mechanistic model could be improved by allowing glucose maintenance and utilization requirements parameters to be variable.     

Predicting the profile of nutrients available for absorption: from nutrient requirement to animal response and environmental impact

Current feed evaluation systems for dairy cattle aim to match nutrient requirements with nutrient intake at pre-defined production levels. These systems were not developed to address, and are not suitable to predict, the responses to dietary changes in terms of production level and product composition, excretion of nutrients to the environment, and nutrition related disorders. The change from a requirement to a response system to meet the needs of various stakeholders requires prediction of the profile of absorbed nutrients and its subsequent utilisation for various purposes. This contribution examines the challenges to predicting the profile of nutrients available for absorption in dairy cattle and provides guidelines for further improved prediction with regard to animal production responses and environmental pollution.The profile of nutrients available for absorption comprises volatile fatty acids, long-chain fatty acids, amino acids and glucose. Thus the importance of processes in the reticulo-rumen is obvious. Much research into rumen fermentation is aimed at determination of substrate degradation rates. Quantitative knowledge on rates of passage of nutrients out of the rumen is rather limited compared with that on degradation rates, and thus should be an important theme in future research. Current systems largely ignore microbial metabolic variation, and extant mechanistic models of rumen fermentation give only limited attention to explicit representation of microbial metabolic activity. Recent molecular techniques indicate that knowledge on the presence and activity of various microbial species is far from complete. Such techniques may give a wealth of information, but to include such findings in systems predicting the nutrient profile requires close collaboration between molecular scientists and mathematical modellers on interpreting and evaluating quantitative data. Protozoal metabolism is of particular interest here given the paucity of quantitative data.Empirical models lack the biological basis necessary to evaluate mitigation strategies to reduce excretion of waste, including nitrogen, phosphorus and methane. Such models may have little predictive value when comparing various feeding strategies. Examples include the Intergovernmental Panel on Climate Change (IPCC) Tier II models to quantify methane emissions and current protein evaluation systems to evaluate low protein diets to reduce nitrogen losses to the environment. Nutrient based mechanistic models can address such issues. Since environmental issues generally attract more funding from governmental offices, further development of nutrient based models may well take place within an environmental framework.     

Net nutrient absorption and liver metabolism in lactating dairy cows fed supplemental dietary biotin

The effect of feeding supplemental biotin on net absorption and metabolism of nutrients by the portal-drained viscera (PDV; the gut, pancreas, spleen and associated fat) and liver of lactating dairy cows was measured. Three cows in early to mid-lactation catheterised for measurements of net nutrient absorption and metabolism by the PDV and liver were fed a total-mixed ration with or without supplemental biotin at 20 mg/day using a switch-back design (ABA v. BAB) with three 2-week periods. There were no effects of feeding biotin on dry matter intake (22.2 kg/day), milk yield (29.5 kg/day) or milk composition. There was also no effect of feeding biotin on net release of glucose by the liver, net liver removal of glucose precursors (propionate, alanine, lactate) or net liver release of β-hydroxybutyrate. Feeding biotin increased net PDV release of ammonia. Reasons for the response are not certain, but a numerical increase in net PDV release of acetate suggests that rumen or hindgut fermentation was altered. Results of the present study do not support the hypothesis that supplemental biotin increases liver glucose production in lactating dairy cows.     

The effect of jugular or abomasal infusion of amino acids on milk yield in lactating cows fed a protein deficient diet

The influence of first-pass splanchnic metabolism was investigated by comparing the response of 5 lactating cows to an infusion of an amino acid mixture into the abomasum or a jugular vein over 5 d according to a complete block design. The basal diet and the amino acid infusion provided 71% and 14% of crude protein requirements, respectively. The jugular infusion increased (P = 0.01) milk yield by 0.80 kg in comparison to the abomasal infusion, but milk protein yield was not altered. The jugular infusion tended to increase (P = 0.06) the arterial concentration of total essential amino acids by 11% relatively to the abomasal infusion. Mammary plasma flow and net fluxes of amino acids and glucose were not affected by the infusion sites. Variations in essential amino acid concentrations suggest that splanchnic metabolism alters peripheral delivery of amino acids but the recirculation of amino acids within the animal decreased the impact of the first-pass splanchnic metabolism on lactational performances.     

Milk production is unaffected by replacing barley or sodium hydroxide wheat with maize cob silage in rations for dairy cows

Starch is an important energy-providing nutrient for dairy cows that is most commonly provided from cereal grains. However, ruminal fermentation of large amounts of easily degradable starch leads to excessive production and accumulation of volatile fatty acids (VFA). VFA not only play a vital role in the energy metabolism of dairy cows but are also the main cause of ruminal acidosis and depressed feed intake. The aim of the present study was to compare maize cob silage (MCS) as an energy supplement in rations for dairy cows with highly rumen-digestible rolled barley and with sodium hydroxide wheat (SHW), which has a higher proportion of by-pass starch than barley. Two studies were carried out: (1) a production study on 45 Danish Holstein cows and (2) an intensive study to determine digestibilities, rumen fermentation patterns and methane emission using three rumen-cannulated Danish Holstein cows. Both studies were organised as a 3×3 Latin square with three experimental periods and three different mixed rations. The rations consisted of grass-clover silage and maize silage (~60% of dry matter (DM)), rapeseed cake, soybean meal, sugar beet pulp and one of three different cereals as a major energy supplement: MCS, SHW or rolled barley (~25% of DM). When MCS replaced barley or SHW as an energy supplement in the mixed rations, it resulted in a lower dry matter intake; however, the apparent total tract digestibilities of DM, organic matter, NDF, starch and protein were not different between treatments. The energy-corrected milk yield was unaffected by treatment. The fat content of the milk on the MCS ration was not different from the SHW ration, whereas it was higher on the barley ration. The protein content of the milk decreased when MCS was used in the ration compared with barley and SHW. From ruminal VFA patterns and pH measures, it appeared that MCS possessed roughage qualities with respect to rumen environment, while at the same time being sufficiently energy rich to replace barley and SHW as a major energy supplement for milk production. The environmental impact, expressed as methane emissions, was not different when comparing MCS, SHW and barley.     

Modelling the implications of feeding strategy on rumen fermentation and functioning of the rumen wall

The present study gives a critique of the mechanisms involved with the formation of volatile fatty acid (VFA) formed in the lumen of the reticulo-rumen, the absorption of VFA across the reticulo-rumen wall, and the intra-epithelial metabolism of VFA by reticulo-rumen epithelium. In contrast to the empirical treatment of these aspects in previous rumen modelling studies, a mechanistic model was developed which represents each of these aspects separately. Because tissues of the reticulo-rumen may strongly adapt to changing nutritional conditions, this adaptive response was included in the model. The model enabled an evaluation of the implications of VFA yield on the development of the rumen wall, on the transport of VFA, on the extent of intra-epithelial metabolism of VFA, and on the consequences for the supply of VFA to the ruminant. The current modelling effort allowed the integration of existing knowledge on each of these aspects and the model reproduced some essential characteristics of experimental observations on VFA absorption and metabolism. Although further development is still needed, the model appears helpful to distinguish elements that require specific consideration when evaluating rates of net portal appearance of VFA, or when testing hypothesis on the interaction between formation, absorption and intra-epithelial metabolism of VFA under various experimental conditions.     

Energy metabolism in the digestive tract and liver of cattle: influence of physiological state and nutrition

Major functions of portal-drained viscera (PDV) and liver of cattle include absorption of digestion products and modification of the body's supply of intermediary metabolites. The disproportionately high metabolic rate of PDV and liver (7-13% of body tissues) is exemplified by their oxygen uptake (40-50% of whole body). Extensive metabolism of glucose, volatile fatty acids and amino acids by PDV modulates nutrient supply from the diet such that most responses to diet or physiological state are a function of level of diet intake. Similarly, blood flow through PDV is highly correlated with energy intake across a range of body weight, physiological state or diet composition. Most common dietary responses in metabolite uptake by PDV are changes in uptake of ammonia and volatile fatty acids, which emphasize the strong energy: nitrogen interrelationship in the rumen and subsequently the rest of the body. The liver (tissue in series with PDV) removes glucose precursors and ammonia from its blood supply as part of its functions in gluconeogenesis, ammonia detoxification and urea synthesis. The liver also alters amounts and proportions of amino acids supplied by PDV. Accountable percentages of metabolizable energy from net PDV supply include: organic acids, 41-59%; amino acids, 5-13%; and heat energy (from oxygen uptake), 11-22%.     

Use of in vitro gas production models in ruminal kinetics.

Physiological systems models for ruminant animals are used to predict the extent of ruminal carbohydrate digestion, based on rates of intake, digestion, and passage to the lower tract. Digestion of feed carbohydrates is described in these models by a first-order rate constant. Recently, an in vitro gas production technique has been developed to determine the digestion kinetics in batch fermentation, and nonlinear mathematical models have been fitted to the cumulative gas production data from these experiments. In this paper, we present an analysis that converts these gas production models to an effective first-order rate constant that can be used directly in rumen systems models. The analysis considers the digestion of an incremental mass of substrate entering the rumen. The occurrence of passage is represented probabilistically, and integration through time gives the total mass of substrate and total rate of digestion in the rumen. To demonstrate the analysis, several gas production models are fitted to a sample data set for corn silage, and the effective first-order rate constants are calculated. The rate constants for digestion depend on ruminal passage rate, an interaction that arises from the nonlinearity of the gas production models.     

Empirical prediction of net splanchnic release of ketogenic nutrients,acetate, butyrate and β-hydroxybutyrate in ruminants: a meta-analysis

For energy feeding systems for ruminants to evolve towards a nutrient-based system, dietary energy supply has to be determined in terms of amount and nature of nutrients. The objective of this study was to establish response equations of the net hepatic flux and net splanchnic release of acetate, butyrate and β-hydroxybutyrate to changes in diet and animal profiles. A meta-analysis was applied on published data compiled from the FLuxes of nutrients across Organs and tissues in Ruminant Animals database, which pools the results from international publications on net splanchnic nutrient fluxes measured in multi-catheterized ruminants. Prediction variables were identified from current knowledge on digestion, hepatic and other tissue metabolism. Subsequently, physiological and other, more integrative, predictors were obtained. Models were established for intakes up to 41 g dry matter per kg BW per day and diets containing up to 70 g concentrate per 100 g dry matter. Models predicted the net hepatic fluxes or net splanchnic release of each nutrient from its net portal appearance and the animal profile. Corrections were applied to account for incomplete hepatic recovery of the blood flow marker, para-aminohippuric acid. Changes in net splanchnic release (mmol/kg BW per hour) could then be predicted by combining the previously published net portal appearance models and the present net hepatic fluxes models. The net splanchnic release of acetate and butyrate were thus predicted from the intake of ruminally fermented organic matter (RfOM) and the nature of RfOM (acetate: residual mean square error (RMSE)=0.18; butyrate: RMSE=0.01). The net splanchnic release of β-hydroxybutyrate was predicted from RfOM intake and the energy balance of the animals (RMSE=0.035), or from the net portal appearance of butyrate and the energy balance of the animals (RMSE=0.050). Models obtained were independent of ruminant species, and presented low interfering factors on the residuals, least square means or individual slopes. The model equations highlighted the importance of considering the physiological state of animals when predicting splanchnic metabolism. This work showed that it is possible to use simple predictors to accurately predict the amount and nature of ketogenic nutrients released towards peripheral tissues in both sheep and cattle at different physiological status. These results provide deeper insight into biological processes and will contribute to the development of improved tools for dietary formulation.     

A simulation model “CTR Dairy” to predict the supply of nutrients in dairy cows managed under discontinuous feeding patterns

A simulation rumen model has been developed to function under non-steady state conditions in order to allow prediction of nutrient availability in dairy cows managed under discontinuous feeding systems. The model simulates availability of glycogenic, aminogenic and lipogenic nutrients to lactating dairy cows fed discontinuously. The model structure considers input of up to three different feeds fed independently at any time during the day. Feeds are described by their nitrogen (N), carbohydrate and fatty acid fractions. The N containing feed fractions include ruminally undegraded crude protein (CP), ruminally insoluble but potentially degradable CP, ruminally soluble CP and ammonia N. The feed carbohydrate fractions include ruminally undegradable neutral detergent fibre (NDF), ruminally degradable NDF, ruminally insoluble starch, ruminally soluble starch and sugars. The fatty acids in the feeds are divided between long chain fatty acids and volatile fatty acids (VFA). Additionally four pools were defined representing absorption of amino acids, glucose, long chain fatty acids and volatile fatty acids. The rumen microbial population is represented as a single pool. Besides a flexible structure, new features to the extant model include adoption of the concept of chewing efficiency (or chewing effectiveness) during eating, variable fractional ruminal absorption rates of VFA and variable fractional ruminal degradation rates of NDF as a function of rumen liquid pH, as well as a variable rumen volume which directly affects rumen concentrations of metabolites. The model continuously (i.e., by minute) predicts release of soluble components from the feeds in the rumen, concentration and absorption of fermentation end products in the rumen, rumen pools of nutrients and microbial biomass dynamics, as well as passage of microbial biomass and non-fermented nutrients from the rumen, in response to various feeding strategies. Model evaluation covered a wide range of feeding strategies that included pasture and housed feeding systems. Overall, the mean square prediction error (MSPE) as a percentage of the observed mean was relatively low (<10%) with a high amount of the total variation explained by random variation (>65%). Deviation from unity varied between 23% (rumen dry matter content) and 25% (NDF), indicating some consistent over and/or under prediction. A more detailed evaluation was done based on studies available that reported diurnal behaviour of key model outputs such as rumen pools, rumen pH, and rumen VFA. The predictions broadly simulated the observed values quantitatively, relative to general diurnal patterns, and relative to differences between treatments in the predicted diurnal patterns. Results show that the model provides a tool to assess potential outcomes of changing feeding strategies which may be particularly valuable in assessing selection of feeds, amounts and times of the day to offer the feeds. The continuous nature of the simulated output also allows determination of the time(s) of the day that ruminal (and/or post-ruminal) delivery of nutrients may limit ruminal output of nutrients (and/or availability of nutrients) to support milk nutrient synthesis.     

Effect of 2-bromoethanesulfonic acid and Peptostreptococcus productus ATCC 35244 addition on stimulation of reductive acetogenesis in the ruminal ecosystem by selective inhibition of methanogenesis.

Evidence is provided that reductive acetogenesis can be stimulated in ruminal samples during short-term (24-h) incubations when methanogenesis is inhibited selectively. While addition of the reductive acetogen Peptostreptococcus productus ATCC 35244 alone had no significant influence on CH4 and volatile fatty acid (VFA) production in ruminal samples, the addition of this strain together with 2-bromoethanesulfonic acid (BES) (final concentration, 0.01 or 0.03 mM) resulted in stimulation of acetic acid production and H2 consumption. Since acetate production exceeded amounts that could be attributed to reductive acetogenesis, as measured by H2 consumption, it was found that P. productus also fermented C6 units (glucose and fructose) heterotrophically to mainly acetate (> 99% of the total VFA). Using 14CH3COOH, we concluded that addition of BES and BES plus P. productus did not alter the consumption of acetate in ruminal samples. The addition of P. productus to BES-treated ruminal samples caused supplemental inhibition of CH4 production and stimulation of VFA production, representing a possible energy gain of about 13 to 15%.     

Predicting In Vitro Rumen VFA Production Using CNCPS Carbohydrate Fractions with Multiple Linear Models and Artificial Neural Networks

The objectives of this trial were to develop multiple linear regression (MLR) models and three-layer Levenberg-Marquardt back propagation (BP3) neural network models using the Cornell Net Carbohydrate and Protein System (CNCPS) carbohydrate fractions as dietary variables for predicting in vitro rumen volatile fatty acid (VFA) production and further compare MLR and BP3 models. Two datasets were established for the trial, of which the first dataset containing 45 feed mixtures with concentrate/roughage ratios of 10∶90, 20∶80, 30∶70, 40∶60, and 50∶50 were used for establishing the models and the second dataset containing 10 feed mixtures with the same concentrate/roughage ratios with the first dataset were used for testing the models. The VFA production of feed samples was determined using an in vitro incubation technique. The CNCPS carbohydrate fractions (g), i.e. CA (sugars), CB₁ (starch and pectin), CB₂ (available cell wall) of feed samples were calculated based on chemical analysis. The performance of MLR models and BP3 models were compared using a paired t-test, the determination coefficient (R²) and the root mean square prediction error (RMSPE) between observed and predicted values. Statistical analysis indicated that VFA production (mmol) was significantly correlated with CNCPS carbohydrate fractions (g) CA, CB₁, and CB₂ in a multiple linear pattern. Compared with MLR models, BP3 models were more accurate in predicting acetate, propionate, and total VFA production while similar in predicting butyrate production. The trial indicated that both MLR and BP3 models were suitable for predicting in vitro rumen VFA production of feed mixtures using CNCPS carbohydrate fractions CA, CB₁, and CB₂ as input dietary variables while BP3 models showed greater accuracy for prediction.     

Mengen- und Spurenelementausnutzung durch Küken bei unterschiedlicher Vitamin-B6-Zufuhr

The need for improvement of feed information in future is discussed from the perspective for implementation in practice. Weaknesses of the current feed evaluation systems, energy systems in particular, are described: prediction of voluntary feed intake, additivity of digestibilities of individual feedstuffs and dynamics of digestion, efficiency of utilization of energy and variation in maintenance requirements. A brief inventory of models to quantify energy and nutrient metabolism by farm animals is followed by an overview of aspects required to fulfill the demands for a nutrient‐oriented feed evaluation. These include feed characteristics to determine and animal aspects, as well as involvement of environmental and management factors. The need for better harmonization of methods and quality of feed information and for international coordination in this matter is underlined.     

Effects of monensin on volatile fatty acid metabolism in periparturient dairy cows using compartmental analysis

Eight multiparous periparturient Holstein cows fitted with ruminal cannulas were used in a split plot design to evaluate effects of monensin on ruminal volatile fatty acid (VFA) metabolism. Diets were supplemented with 300 mg/day of monensin, or no monensin, both prepartum and postpartum. Isotopic tracers, Na-1-¹³C-acetate (Ac), Na-1-¹³C-propionate (Pr), or Na-1-¹³C-butyrate (Bu) were used as markers to describe VFA kinetics in the rumen. The Windows version of SAAM software (WinSAAM) was used to develop a steady state VFA model. A 9-compartment model was adequate to comprehensively describe ruminal VFA metabolism. The main VFA compartments consisted of Ac, Pr and Bu. The model estimated lower Bu and Ac interconversions with monensin, postpartum (Bu to Ac; 0.14 versus 0.12; P=0.04, and Ac to Bu; 0.32 versus 0.25; P=0.11) compared to when measured prepartum. Results demonstrate that dilution studies employing stable isotopes of VFA can be used to provide information on VFA metabolism of the periparturient dairy cow. A time frame of 320 min of labeled VFA infusion employing a single injection allows accurate quantification of VFA metabolism in the rumen. Compartmental kinetic analysis of major VFA in the rumen indicate that monensin reduced about 0.125 the portion of the Ac that contributes to Bu by reducing movements of Bu originated carbons to the Ac pool. Monensin may affect certain biochemical pathways of interconversion of Bu and Ac in the rumen. Propionate kinetic data suggests that Pr behaves as a single pool in the rumen. Monensin did not affect Pr production in the rumen suggesting that monensin improves the metabolic status of the transition cow in a way other than increasing Pr production in the rumen.     

Interorgan ammonia, glutamate, and glutamine trafficking in pigs with acute liver failure

Ammonia reduction is the target for therapy of hepatic encephalopathy, but lack of quantitative data about how the individual organs handle ammonia limits our ability to develop novel therapeutic strategies. The study aims were to evaluate interorgan ammonia metabolism quantitatively in a devascularized pig model of acute liver failure (ALF). Ammonia and amino acid fluxes were measured across the portal drained viscera (PDV), kidneys, hind leg, and lungs in ALF pigs. ALF pigs developed hyperammonemia and increased glutamine levels, whereas glutamate levels were decreased. PDV contributed to the hyperammonemic state mainly through increased shunting and not as a result of increased glutamine breakdown. The kidneys were quantitatively as important as PDV in systemic ammonia release, whereas muscle took up ammonia. Data suggest that the lungs are able to remove ammonia from the circulation during the initial stage of ALF. Our study provides new data supporting the concept of glutamate deficiency in a pig model of ALF. Furthermore, the kidneys are quantitatively as important as PDV in ammonia production, and the muscles play an important role in ammonia removal.     

Dynamic model of the lactating dairy cow metabolism

The whole-animal model described in this paper is intended to be a research model with an intermediary structure between sophisticated and simple cow models. The mechanistic model structure integrates the main metabolic pathways of the lactating dairy cow. Milk yield and related feed intake for varying production potentials were considered to be the driving forces and were empirically defined. The model was designed to explain the main metabolic flows and variations in body reserves associated with the push of nutrients from dry matter intake and their pull by the mammary gland to synthesise milk components throughout lactation. The digestive part of the model uses either known feed unit systems (e.g. PDI system for protein) or published empirical equations for the prediction of digestive flows of organic matter, starch and fatty acids and ruminal volatile fatty acid production. The metabolic sub-system is made up of four tissue compartments (body protein, body lipid, protein in the uterus and triglycerides in the liver) and five circulating metabolites (glucose, amino acids, acetate + butyrate, propionate, and triglycerides and non-esterified fatty acids). A major original aspect of the model is its homeorhetic control system. It was assumed that flows relating to adipose and protein tissue anabolism and catabolism were driven by a couple of linked theoretical hormones controlling mobilisation and anabolism. The evolution in body composition and body weight were outcomes of this control. Another originality of the model is its suitability for milk yields varying from 10 to 50 kg at peak production. This was achieved by homeorhetic control of milk potential acting on several key metabolic flows. Homeostatic regulation was also introduced in order to confine the behaviour of the model within realistic physiological values. For some basic aspects (e.g. Uterine involution, standard kinetics of body protein and fat), it was necessary to build specific databases from the literature and interpret these by meta-analysis. Fairly realistic simulated kinetics were obtained for body composition, liver triglycerides, blood plasma metabolite concentrations, milk protein and fat contents, and also for major groups of milk fatty acids.     

Quantitative in vivo nuclear magnetic resonance studies of hybridoma metabolism

Carbon-13 nuclear magnetic resonance (NMR) spectroscopy was used to study the metabolism of a murine hybridoma cell line at two feed glutamine concentrations, 4.0 and 1.7 mM. Carbon-13 labeling patterns were used in conjunction with nutrient uptake rates to calculate the metabolic fluxes through the glycolytic pathway, the pentose shunt, the malate shunt, lipid biosynthesis, and the tricarboxylic acid (TCA) cycle. Decreasing the feed glutamine concentration significantly decreased glutamine uptake but had little effect on glucose metabolism. A significant incrase in antibody productivity occurred upon decreasing the feed glutamine level. The increased antibody productivity in concert with decreased glutamine uptake and no apparent change in glucolytic metabolism suggests that antibody production was not energy limited. Metabolic flux calculations indicate that (1) approximately 92% of the glucose consumed proceeds directly through glycolysis with 8% channeled through the pentose shunt; (2) lipid biosynthesis appears to be greater than malate shunt activity; and (3) considerable exchange occurs between TCA cycle intermediates and amino acid metabolic pools, leading to substantial loss of (13)C label from the TCA cycle. These results illustrate that (13)NMR spectroscopy is a powerfulf tool in the calculation of metabolic fluxes, particularly for exchange pathways where no net flux occurs. (c) 1994 John Wiley & Sons, Inc.