Section G. General. Nitrogen metabolism and the rumen

A revised representation of urea and ammonia nitrogen recycling and use in the Molly cow model

Accurately predicting nitrogen (N) digestion, absorption, and metabolism will allow formulation of diets that more closely match true animal needs from a broad range of feeds, thereby allowing efficiency of N utilization and profit to be maximized. The objectives of this study were to advance representations of N recycling between blood and the gut and urinary N excretion in the Molly cow model. The current work includes enhancements (1) representing ammonia passage to the small intestine; (2) deriving parameters defining urea synthesis and ruminal urea entry rates; (3) adding representations of intestinal urea entry, microbial protein synthesis in the hindgut, and fecal urea-N excretion; and (4) altering existing urinary N excretion equations to scale with body weight and adding purine derivatives as a component of urinary N excretion. After the modifications, prediction errors for ruminal outflows of total N, microbial N, and nonammonia, nonmicrobial N were 29.8, 32.3, and 26.2% of the respective observed mean values. Prediction errors of each were approximately 7 percentage units lower than the corresponding values before model modifications and fitting due primarily to decreased slope bias. The revised model predicted ruminal ammonia and blood urea concentrations with substantially decreased overall error and reductions in slope and mean bias. Prediction errors for gut urea-N entry were decreased from 70.5 to 26.7%, which was also a substantial improvement. Adding purine derivatives to urinary N predictions improved the accuracy of predictions of urinary N output. However, urinary urea-N excretion remains poorly predicted with 69.0% prediction errors, due mostly to overestimated urea-N entry rates. Adding representations of undigested microbial nucleic acids, microbial protein synthesized in the hindgut, and urea-N excretion in feces decreased prediction errors for fecal N excretion from 21.1 to 17.1%. The revised model predicts that urea-N entry into blood accounts for approximately 64% of dietary N intake, of which 64% is recycled to the gut lumen. Between 48 and 67% of the urea recycled to the gut flows into the rumen largely depending on diet, which accounts for 29 to 54% of total ruminal ammonia production, and 65 to 76% of this ammonia-N is captured in microbial protein, which represents 17% of N intake. Based on model simulations, feeding a diet with moderately low crude protein and high rumen-undegradable protein could increase apparent ruminal N efficiency by 20%.

Recent advances in estimating protein and energy requirements of ruminants

Considerable efforts have been made in gathering scientific data and developing feeding systems for ruminant animals in the past 50 years. Future endeavours should target the assessment, interpretation and integration of the accumulated knowledge to develop nutrition models in a holistic and pragmatic manner. We highlight some of the areas that need improvement. A fixed metabolisable-to-digestible energy ratio is an oversimplification and does not represent the diversity of existing feedstock, but, at the same time, we must ensure the internal consistency and dependency of the energy system in models. For grazing animals, although data exist to compute energy expenditure associated with walking in different terrains, nutrition models must incorporate the main factors that initiate and control grazing. New equations have been developed to predict microbial crude protein (MCP) production, but efforts must be made to account for the diversity of the rumen microbiome. There is large and unexplained variation in the efficiency of MCP synthesis (9.81–16.3 g MCP/100 g of fermentable organic matter). Given the uncertainties in the determination of MCP, current estimates of metabolisable protein required for maintenance are biased. The use of empirical equations to predict MCP, which, in turn, is used to estimate metabolisable protein intake, is risky because it establishes a dependency between these estimates and creates a specificity that is not appropriate for mechanistic systems. Despite the existence of data and knowledge about the partitioning of retained energy into fat and protein, the prediction of retained protein remains unsatisfactory, and is even less accurate when reported data on the efficiency of use of amino acids are employed in the predictive equations. The integrative approach to develop empirical mechanistic nutrition models has introduced interconnected submodels, which can destabilise the predictability of the model if changed independently.

[The use of ribonucleic acid as a marker for measuring microbial protein yield in the rumen. 1. Chemical determination of ribonucleic acid]

The determination of ribonucleic acid (RNA) in the duodenal digesta and the rumen microbes was carried out colorimetrically with orcinol after extraction with hot NaCl solution, precipitation with phosphotungstic acid and alkaline hydrolysis. 96 +/- 1% of the RNA supplemented to the digesta content was recovered (y +/- s, n = 5). The methodic error (repetition accuracy) was 1.1%. The relation of RNA-N:N (mg/g) in the bacteria mass amounted to 93.8 +/- 3.99 (y +/- s(-y), n = 48) and to 16.1 +/- 0.8 (y +/- s(-y), n = 51) mg RNA/g DM in the duodenal content.

[Effect of type of starch and quantity in rations on apparent and true digestibility of nitrogen and nitrogen balance in sheep]

In a digestibility trial 5 semisynthetic rations were fed in 3 periods to 10 male sheep to examine the effects on N balance, components of faecal nitrogen and N digestibility. The rations contained constant amounts of nitrogen but different contents of cellulose and two different types of starch (untreated and steamflaked). Content and type of starch did not show any noticeable effect neither on excretion of undigested dietary nitrogen nor on true digestibility. There could not be noticed any effects on the apparent N digestibility by changing contents of cellulose or untreated starch. If the rations contained steamflaked corn starch, the animals excreted more faecal nitrogen and therefore showed a lower apparent N digestibility. Especially the water soluble N fraction of the faecal nitrogen was clearly higher. Compensation took place through a lower N excretion with the urine. The reason for increased faecal N excretion may be higher microbial protein synthesis in the rumen. This was accented by the allantoin excretion with the urine.

Protein supplement resistant against ruminal degradation as a factor improving utilization of urea in ruminant feeding

The metabolic and productive effects of the blood meal and formaldehyde (FA) treated casein supplements (5-10% of crude protein content) given with urea concentrates in sheep and fattening bulls were investigated. The blood meal has a similar composition of essential amino acids (EAA) to casein. The mean solubility of the FA treated casein and the blood meal after 6 hours of incubation in the sterilized rumen contents amounted 10.5% and 8.5% respectively. The average rumen ammonia concentration and plasma urea level was the highest in bulls fed urea ration without protected protein supplement. The supplementation of this ration with blood meal diminished the large daily fluctuation of plasma AA level and increased plasma EAA/NEAA ratio. The blood meal supplement improved the nitrogen retention in sheep (14%) and body gains in bulls (9%) but did not influence digestible coefficients and rumen protein synthesis in sheep.

The effect of diet on the mammalian gut flora and its metabolic activities

The review will encompass the following points: A brief introduction to the role of the gut flora in the toxicology of ingested food components, contaminants, and additives, including known pathways of activation and detoxication of foreign compounds and the implication of the flora in enterohepatic circulation of xenobiotics. The advantages and disadvantages of the various methods of studying the gut flora (classical bacteriological techniques, metabolic and enzymological methods) will be critically discussed with special reference to their relevance to dietary, toxicological, and biochemical studies. Sources of nutrients available to the gut flora will be described including host products (mucus, sloughed mucosal cells, hormones, proteins) and exogenous nutrients derived from diet. An account of the problems involved in studies of dietary modification with special reference to the use of stock laboratory animal diets, purified diets, and human dietary studies. The influence of dietary modification on the flora will be assessed on the basis of changes in numbers and types of bacteria and their metabolic activity, drawing on data from human and animal studies. The effects of manipulation of the quantity and quality of protein, fat, and indigestible residues (fiber) of the diet will be described together with their possible implications for toxicity of ingested compounds.

Effect of diet on amino and nucleic acids of rumen bacteria and protozoa

Amino acid composition and nucleic acid content of pure cultures of rumen bacteria (17 species) were analyzed. Amino acid composition between gram-positive and -negative organisms was not different. The total nitrogen content of gram-negative bacteria (10.8%) was significantly higher than gram-positive organisms (9.9%). Deoxyribonucleic acid-nitrogen: total nitrogen (mg/g) differed between gram-positive (8.8) and gram-negative (18.9) bacteria, but there was no significant difference in ratio of ribonucleic acid-nitrogen to total nitrogen. In a second experiment six rumen-fistulated cattle were fed either a high roughage (85% alfalfa hay plus 15% concentrate) or high concentrate diet (15% alfalfa hay and 85% concentrate). Cattle were adapted 14 days and rumen contents sampled on 3 consecutive days. Nitrogen content was higher in protozoa from cattle fed low concentrate (8.4%) than in protozoa from cattle fed high concentrate (7.9%) but was similar in bacteria for both diets. Deoxyribonucleic acid nitrogen: total nitrogen (mg/g in bacteria decreased from 27.2 in cattle fed the low concentrate diet to 20.9 in those fed the high concentrate diet. Differences between sampling days were significant for both bacteria and protozoa for ratio of deoxyribonucleic acid-nitrogen to total nitrogen but were significant only in protozoa for ratio of ribonucleic acid nitrogen to total nitrogen. Ribonucleic acid may serve as a marker for estimating microbial production in the rumen if sources of variation are recognized and corrected adequately.

Glycosyl ureides in ruminant nutrition. 2. In vitro studies on the metabolism of glycosyl ureides and their free component molecules in rumen contents

1. The fate of glucosyl urea (GU), lactosyl urea (LU) and corresponding mixtures of the free sugars and urea and their degradation products were examined during in vitro incubation of the compounds with rumen contents taken from donor sheep and steers at various stages of adaptation to these compounds. 2. The sugar-urea bond was virtually unattacked in rumen contents from unadapted sheep and steers but generally a slow release of the galactose moiety occurred. After feeding LU or GU to animals for a period of approximately 10 d, the rates of disappearance of both bound urea and sugar had increased, but were still markedly slower than those of the corresponding free sugars and urea. In vitro rates of degradation of both free lactose and urea also increased in response to the feeding of lactose and urea to rumen content donor animals. 3. Ammonia accumulation in rument contents when GU or LU were the substrates was notably lower than when equivalent amounts of glucose and urea or lactose and urea were the substrates. 4. Bacterial growth was estimated using an vitro method based on incorporation of 32P into bacterial nucleic acids. Markedly different patterns of bacterial growth were observed depending on whether LU or lactose and urea were the substrates.

Microbial ecology and activities in the rumen: part 1

This review describes the progress which has been made during the last 10 to 15 years in the field of rumen microbiology. It is basically an account of new discoveries in the bacteriology, protozoology, biochemistry, and ecology of the rumen microbial population. As such it covers a wide range of subjects including the isolation and properties of methanogenic bacteria, the role of rumen phycomycete fungi, anaerobic energy conservation, and general metabolic aspects of rumen microorganisms. It also attempts, however, to describe and develop new concepts in rumen microbiology. These consist principally of interactions of the microbemicrobe, microbe-food and microbe-host types, and represent the main areas of recent advance in our understanding of the rumen ecosystem. The development of experimental techniques such as chemostat culture and scanning electron microscopy are shown to have been instrumental in progress in these areas. The paper is concluded with an assessment of our present knowledge of the rumen fermentation, based on the degree of success of experiments with gnotobiotic ruminants inoculated with defined flora and in mathematical modeling of the fermentation. The efficacy of chemical manipulation of the fermentation in ruminant is also discussed in this light.

[Utilization of alimentary ribonucleic acid in calf N-metabolism]

In order to get hints concerning the utilisation of nucleic acids in the N-metabolism of ruminants, a N-balance experiment with not yet ruminating calves was carried out. An admixture of 0, 1.875 and 3.75% yeast ribonucleic acid (groups I-III) was given to a milk substitute feed with low purine and pyrimidine contents. The RNA admixture amounted to 10 resp. 20% of the diet protein-N. The daily feed per animal amounted to 1710 g. Apart from the N-balance the digestibility of RNA and some N-fractions in urine were registered. The excretion of nitrogen, RNA or desoxy ribonucleic acid in the feces of the calves was hardly changed by the RNA intake. Thus an extensive digestibility of RNA can be stated. The renale nitrogen excretion increased in group II by an amount which nearly corresponded to the RNA intake, and in group III it even increased overproportionally. As the N-balance showed, there was a low utilisation of the RNA-N of 12% (non-significant), whereas in group III the N-increase was negatively influenced. The N-excretion in urine connected with the RNA-intake could mainly be traced back to increased urea and allantoine values. The conclusions from these results were that nucleic acids of 10 to 20% of the feed protein are only of a low resp. no nutritive nitrogen value.

The contribution of protozoa to the protein entering the duodenum of sheep

1. Four sheep, each fitted with a rumen fistula and a re-entrant cannula at the proximal duodenum were fed a semi-purified diet containing urea as the only nitrogen source. The quantities of total protozoal amino acid-N (TPAN) present in the rumen and entering the duodenum were determined when the mean rumen dilution rate (D) was low (0.034/h) and when D was increased to 0.078/h by the intraruminal infusion of artificial saliva. 2. Increasing the dilution rate had no significant effect upon the proportions of TPAN present in the total microbial amino-acid-N (TMAN) of the rumen fluid and duodenal digesta. With both dilution rates the mean proportion of TPAN in the duodenal TMAN (0.24) was markedly less than the equivalent proportion (0.45) found in the rumen fluid. 3. The daily flow of TPAN, as measured at the duodenal cannula at both dilution rates were equivalent to only 41% of the flow of TPAN as predicted from measurements of rumen outflow, indicating that a substantial proportion of rumen protozoal protein was retained within the rumen.

The relationship between rumen bacterial growth, intake of dry matter, digestible organic matter and volatile fatty acid production in buffalo (Bos bubalis) calves

1. The production rates of bacteria in the rumen of buffalo (Bos bubalis) calves were estimated using an isotope-dilution technique. A series of fifteen experiments was done with animals given green maize and nine experiments with animals given cowpea (Vigna unguiculata). 2. The turnover time ranged from 205 to 567 min in the group given green maize and from 330 to 648 min in animals offered cowpea. The production rates of bacteria were (mean +/- SE; g/d) 145.77 +/- 7.240 and 237.09 +/- 11.847 in animals given green maize and cowpea respectively. 3. There was a significant correlation between bacterial production rates and dry matter intake, digestible organic matter and total volatile fatty acids formed in the rumen. 4. Regression equations obtained for the two foodstuffs were different suggesting that the bacterial growth rate may vary depending upon the quantity and quality of foodstuff digested and possibly the ratio nitrogen:energy of the foodstuff.

Determination of rumen microbial growth in vitro from 32P-labelled phosphate incorporation

1. The extracellular phosphate pool in incubations of rumen fluid or washed cell suspensions of mixed rumen bacteria (WCS) was labelled with 32P. From the constant extracellular phosphate pool specific activity and the amount of radioactivity incorporated during incubation, the amount of P incorporated in the microbial fraction was calculated. From the value for nitrogen: P determined in microbial matter, the amount of N incorporated was calculated as a measure of microbial growth. 2. Incorporation of soluble non-protein-N in incubations devoid of substrate protein was 50 and 80% of the values obtained using the isotope method for rumen fluid and WCS respectively. It is suggested that results obtained using the former method reflect 'net growth' of micro-organisms which is the result of simultaneous growth and degradation. The isotope method measures 'total growth', as isotope incorporation is not affected by degradation of non-growing cells. 3. Incorporation of 32P in P-containing microbial components (mainly nucleic acids) was compared with net synthesis of these components in incubations of WCS. The results showed different specific rates of synthesis and degradation for all components studied. It is concluded that the composition of microbial matter changed during growth. 4. When N incorporation, calculated from results obtained using the isotope method in incubations with rumen fluid, was compared with the amount of carbohydrate substrate fermented and the type of fermentation, values between 18-3 and 44-6 g N incorporated/kg of organic matter fermented were obtained. Low values were associated with large proportions of the substrate being fermented to lactate and the use of glucose instead of disaccharides as substrate. Part of the variation could also be attributed to differences in incubation period, reflected in different proportions of polysaccharide formed. 5. The use of isotopes for determination of rumen microbial growth in vitro is critically discussed.

[Studies on the determination of the part of bacterial nitrogen in relation to total nitrogen content in the duodenum of cows]

A method for estimation of content of bacterial nitrogen in duodenal intestine was described. The method of HUTTON et al. (1971) in modificated form for the own research was the best method. In two trials roughage (chopped dried forage) and ground corn were given to cows with duodenal bridge fistula in different order (90 min-intervall). Portion of bacterial nitrogen to total nitrogen was 72.5% (corn leads to roughage) and 64.7% (roughage leads to corn).

Determination of nitrogen requirement for microbial growth from the effect of urea supplementation of a low N diet on abomasal N flow and N recycling in wethers and lambs

1. Plasma urea entry rate, urinary area excretion and, by difference, urea recycling in the body, together with the flow of non-ammonia N through the abomasum and digestion of dry matter (DM) before the abomasum were determined in both wethers and lambs receiving cereal-starch diets supplemented with urea to give 60-120 g crude protein (N X 6-25)/kg DM. 2. Lambs excreted less urea in urine than wethers given the same diet. 3. Relationships between plasma urea entry rate or urine urea excretion rate and plasma urea concentration were different for lambs compared to wethers suggesting greater conservation of body N by renal control in lambs. 4. Recycling of urea was not related to plasma urea concentration in wethers but was related exponentially in lambs, suggesting recycling is controlled rather than the result of simple diffusion from the blood to the gastro-intestinal tract. 5. Abomasal non-ammonia-N flow was similar for wethers and lambs and increased linearly with urea supplementation. 6. DM digestion prior to the abomasum was not significantly altered, although there was a tendency for decreased digestion of the basal diet given to lambs. 7. Maximum microbial N flow to the abomasum was estimated as 30 g N/kg organic matter (OM) fermented in the rumen. 8. This work and the literature reviewed suggested maximum net microbial production can be obtained when the diet supplies an amount of fermentable N equal to the microbial N output. It is calculated the diet should supply approximately 26 g fermentable N/kg digestible OM or 1-8 g fermentable N/MJ metabolizable energy. This corresponds to a fermentable crude protein supply varying from 65 to 130 g/kg DM as digestible OM content increases from 400 to 800 g/kg DM.

Factors influencing rumen microbial growth rates and yields: effects of urea and amino acids over time

Washed cell suspensions of mixed rumen bacteria were used to evaluate effects of 100% urea-nitrogen and 75% urea-nitrogen plus 25% amino acid-nitrogen in growth media upon microbial growth rate and yield, specific rate of glucose consumption, and incorporation of glucose into mixed cells, carbon dioxide, and end products. Rumen microbial dry matter, nitrogen, ribonucleic acid, deoxyribonucleic acid, glucose disappearance, and production of volatile fatty acids were considerably higher in medium containing urea plus amino acids as compared with urea only. Specific growth rates of microbes were .104 and .203 and mean doubling times were 6.7 and 3.4 h in the urea and urea plus amino acid growth media. Microbial growth in mg per 100 mg glucose used, per mole glucose and per mole adenosine triphosphate (ATP), and specific rate of glucose consumption in mmol per mg cells-h were 19.3, 34.7, 15.4, and .016 with urea, and 24.4, 44.2, 20.6, and .014 with urea plus amino acids. Percentages of catabolized glucose incorporated into microbial cells, carbon dioxide, and end products did not differ between treatments and averaged 19.5, 7.8, and 64.4%.

Factors influencing rumen microbial growth rates and yields: effect of amino acid additions to a purified diet with nitrogen from urea

Effects of isonitrogenous urea and amino acid additions upon microbial growth in rumen contents from a cow fed a purified diet in which urea was the sole nitrogen source were studied. Incorporation of amino acids into microbial cells, volatile fatty acids, and carbon dioxide was estimated. Rates of microbial growth, volatile fatty acid production, and effects of amino acids upon microbial nitrogen yields were highest right after feeding and decreased with time after feeding. Microbial growth and amounts of amino acids incorporated into microbial cells, volatile fatty acids and carbon dioxide were related closely to quantity of starch remaining in the rumen. High amounts of starch increased microbial protein synthesis from carbon-14 labeled amino acids and reduced amounts of amino acid fermentation. Estimated microbial protein yields per day were 326.0, 444.4, 497.3, and 527.3 g when 0, 15, 30, and 45 mg amino acid nitrogen replaced urea nitrogen during incubation. Respective values for microbial cells per mole estimated adenosine triphosphate were 15.2, 19.2, 21.0, and 24.5. Microbial cell yields per kg carbohydrate digested were 139.0, 189.5, 212.0, and 224.8 g for 0, 15, 30, and 45 mg amino acid nitrogen. Addition of small amounts of amino acids to a diet containing urea as the sole nitrogen source improved considerably rumen microbial protein yields.

Relationship between ruminal ammonia and nonprotein nitrogen utilization by ruminants. I. Development of a model for predicting nonprotein nitrogen utilization by cattle

The influence of ration composition on mean ruminal ammonia concentration was studied by collecting samples of ruminal ingesta from cattle fed rations varying in crude protein and total digestible nutrient content. A minimum of four sampling times distributed throughout the day permitted calculation of mean ruminal ammonia concentrations. Mean ruminal ammonia concentration was positively related to dietary crude protein concentration and negatively related to total digestible nutrient concentration. It is postulated that mean ruminal ammonia concentration may be a useful criterion for predicting efficacy of nonprotein nitrogen supplementation. A quantitative approach for evaluating nonprotein nitrogen supplementation based upon determination of the point at which ruminal ammonia exceeds the requirement (5 mg ammonia nitrogen/100 ml rumen fluid) of the ammonia-utilizing bacteria is proposed. Dietary conditions expected to result in excessive concentrations of ruminal ammonia are defined and recommended upper limits for nonprotein nitrogen supplementation are presented. Theoretical relationships between composition of the unsupplemented ration, amount of nonprotein nitrogen added, and efficiency of nonprotein nitrogen utilization are discussed. The practice of adding nonprotein nitrogen so as to exceed 12 to 13% crude protein in typical dairy or feedlot rations needs to be reevaluated.

Nitrogen requirement and utilization in dairy cattle

Formulation of dairy cow rations should consider the following points regarding nitrogen utilization by lactating cows. (a) Maintenance of ruminal ammonia nitrogen in excess of 5 mg/100 ml rumen fluid has no effect on microbial protein production. (b)Supplemental nonprotein nitrogen is not utilized in typical dairy and feedlot beef rations containing more than 12 to 13% crude protein(dry matter basis). (c)Nonprotein nitrogen is approximately equal to true protein as a source of nitrogen in typical dairy and feedlot rations containing not more than 12 to 13% crude protein. (d)A scheme based upon metabolizable protein (absorbable protein) for calculating requirements and comparing protein sources is superior to crude or digestible protein designations. Ultimate expression of the requirement may be in terms of crude protein for the sake of simplicity. (e)One kilogram of crude protein, regardless of nitrogen source, equals about .75kg metabolizable protein in typical dairy and feedlot beef rations containing not more than 12 to 13% crude protein. One kilogram of plant protein (true protein) fed in excess of an amount equivalent to 12 to 13% dietary protein equals about .3 kg metabolizable protein. (f)Protein supplementation of lactating cows might be related more to stage of lactation than to milk production. (g)Lactating cows having above average lactational ability may benefit from dietary protein as high as 16 to 17% (dry matter basis) during the first third of lactation. (h)Cows in the latter two-thirds of lactation appear to require 12.5% dietary protein or less. (i)Plant protein (true protein) should be the supplemental sources of nitrogen during the first third of lactation, with NPN providing most, if not all, the supplemental nitrogen during the last two-thirds of lactation.

Rumen bypass and protection of proteins and amino acids

Potent rumen microbial proteases and deaminases rapidly degrade protein and amino acids which are soluble in the rumen liquid phase. Because protein sources vary in their solubility, the degree of degradation in the rumen is variable. Methods of decreasing protein and amino acid degradation in the rumen include heat treatment, chemical treatment, encapsulation, use of amino acid analogs, selective manipulation of balances of rumen metabolic pathways, and esophageal groove closure. It is important that procedures do not interfere with ruminal metabolism or post-ruminal digestion. Bypassing the rumen changes sites in the digestive tract of nutrient digestion and absorption and provides a mechanism for supplementing outflow of nutrients from the rumen. A feasible approach to production of animal protein from ruminants would be utilization of nonprotein nitrogen for rumen protein production, maximization of rumen bypass of dietary protein, and supplementation with rumen nondegradable amino acids.

Rumen modeling: rumen input-output balance models

Two models of rumen fermentative relationships expressed as systems of simultaneous linear equations and based on requirements for maintenance of balances of elementary imput and output and metabolic pathways are presented in matrix format consistent with solution by linear programs. Matrix entries defining the two models were verified carefully based upon a survey of the literature, and conceptual bases of the models were validated by comparisons of model outputs with experimental data not used in model construction. The models then were used to evaluate interactions among feed composition, volatile fatty acid yields and patterns, microbial growth yields and efficiencies, and microbial metabolic pathways.

Some factors influencing the chemical composition of mixed rumen bacteria

1. Sheep, cows and calves fitted with rumen cannulas were given diets mostly containing 10–16 g nitrogen/kg dry matter and consisting of roughage and cereals. Mixed bacteria were separated from samples of their rumen contents.

2. Bacteria taken 4–6 h after a feed from calves which were kept in an experimental calf-house with no contact with adult animals (environment A) contained more α-dextran, less total N and higher nucleic acid:total N ratios than similar bacteria from calves reared in contact with adult sheep (environment C) but otherwise treated in an identical way.

3. Mixed bacteria taken 4–6 h after a feed from sheep and cows were similar in composition, with respect to nitrogenous components, to those from the ‘environment C’ calves. This composition did not vary significantly when diets containing differing proportions of roughage were given.

4. The ‘environment A’ calves were free of ciliate protozoa. When they were placed in contact with, and were inoculated with rumen contents from, adult cattle (environment B), they rapidly developed a normal protozoal population and the chemical composition of their rumen bacteria became like that of the bacteria from the ‘environment C’ calves.

5. Mixed bacteria taken just before a feed, from either cows or ‘environment A’ calves, showed significantly lower RNA-N:total N ratios and slightly (but not usually significantly) higher DNA-N:total N ratios than bacteria taken 4–6 h after feeding. Total N contents of the bacteria did not change consistently with time after feeding.

6. The possible significance of these differences in relation to the nutrition of the host animal is discussed.

Carbohydrate metabolism in the ruminant. Bacterial carbohydrates formed in the rumen and their contribution to digesta entering the duodenum

1. Samples of mixed bacteria were separated from rumen digesta taken from calves, kept out of contact with adult animals, and from sheep and cows.

2. For calves receiving a diet made up of equal amounts of roughage and cereals with 13–16 g nitrogen/kg dry matter, samples of mixed bacteria taken 4–6 h after feeding contained, on average, 140 g glucose in α-linked polymers (α-dextran), 25 g galactose and a total of 25 g other non-glucose, non-galactose sugars (mainly rhamnose, ribose and mannose) in combined forms per kg dry matter.

3. The α-dextran content of similar bacteria samples from sheep or cows receiving diets of similar composition was 70 g/kg dry matter. Samples from animals receiving all-roughage diets contained only 25 g α-dextran/kg dry matter, but those from cows given more than 70% of their ration as concentrates (mainly cereal) contained 150 g α-dextran/kg dry matter.

4. Addition of supplementary protein or urea to cereal–roughage diets given to calves greatly depressed the amount of α-dextran in the rumen bacterial samples to an average value of 60 g/kg dry matter.

5. Samples taken before a morning feed (i.e. after 16 h fasting) contained less α-dextran than samples taken 4–6 h after feeding for both calves and cows.

6. Under different conditions, variations in the amounts of galactose in rumen bacteria sometimes paralleled variations in α-dextran. Amounts of other non-glucose sugars did not vary greatly.

7. It was estimated, from a comparison of the compositions of rumen bacteria and duodenal contents, that, in the latter, the rhamnose, ribose and mannose came mainly from the bacteria, the arabinose, xylose and cellulose-glucose mainly from the diet and the galactose and α-dextran-glucose from both sources.

Microbial phospholipid synthesis as a marker for microbial protein synthesis in the rumen

Phosphate uptake into intracellular inorganic phosphorus and cellular phospholipids and the relationship between cell growth and phospholipid synthesis were studied with suspensions of washed ruminal bacteria in vitro with (33)P-phosphorus. It was shown that ruminal bacteria accumulated inorganic phosphate at a low rate when incubated without substrate. Upon the addition of substrate, the rate of inorganic phosphorus uptake into the cells increased markedly, and phospholipid synthesis and cell growth commenced. There was a highly significant relationship (r = 0.98; P < 0.01) between phospholipid synthesis and cell growth. The specific activity of the intracellular inorganic phosphorus did not equilibrate with phosphorus medium. When ruminal contents from sheep fed a high or low protein diet were incubated in vitro, the rate of (33)P incorporation into microbial phospholipids was higher for the high protein diet. Since there was a high relationship between phospholipid synthesis and growth, rumen contents were collected before and various times after feeding and incubated with (33)P-phosphorus in vitro. The short-term, zero time approach was used to measure the rate of microbial phospholipid synthesis in whole rumen contents. In these studies the average specific activity of the intracellular inorganic phosphorus was used to represent the precursor pool specific activity. Microbial phospholipid synthesis was then related to protein (N x 6.25) synthesis with appropriate nitrogen-to-phospholipid phosphorus ratios. Daily true protein synthesis in a 4-liter rumen was 185 g. This represents a rate of 22 g of protein synthesized per 100 g of organic matter digested. These data were also corrected for ruminal turnover. On this basis the rate of true protein synthesis in a 4-liter rumen was 16.1 g of protein per 100 g of organic matter digested. This value represents a 30-g digestible protein-to-Mcal digestible energy ratio which is adequate for growing calves and lambs.