Isotope techniques for studying the dynamics of nitrogen metabolism in ruminants

Selection for residual feed intake affects appetite and body composition rather than energetic efficiency

Residual feed intake (RFI) is the difference between an animal’s actual feed intake and that which would be expected based on production. This experiment was to test the hypothesis that part of the variation in RFI may be due to differences in energetic efficiency through changes in heat production, these being in part due to differences in protein metabolism. Following three generations of divergent selection for RFI, eight High and eight Low-RFI heifers were fed at both 105% and 180% of predicted maintenance feed requirements. Between-RFI line and feeding-level differences were assessed for energy intake, protein metabolism, heat production, body composition, energy and nitrogen balance and digestibility. The RFI lines did not differ in protein metabolism or heat production. The High-RFI heifers deposited 51% and 56% more subcutaneous fat at the P8 rump and 12/13th rib sites, respectively, with no difference in eye muscle area gain or average daily weight gain. The greater fat deposition of High-RFI heifers was due to a larger ad libitum feed consumption compared with the Low-RFI heifers. Energy and nitrogen balance did not differ between the RFI lines. The energy transactions indicated no difference in the efficiency of energy use on 105% maintenance, although when fed 180% of maintenance the differences in feed intake suggest variation in appetite as the mechanism contributing to RFI. All of the extra energy consumed by High-RFI heifers above maintenance and deposition of protein was associated with additional energy retained as fat. This study suggests that selection for RFI may not lead to improved efficiency of energy use.

Ruminal metabolism of grass silage soluble nitrogen fractions

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 × 4 Latin square design. Soluble N (SN) isolated from timothy grass silage labeled with ¹⁵N and ammonia N (AN) labeled with ¹⁵N 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 ¹⁵N-atom% was determined in N-fractions of rumen digesta grab samples that were collected frequently between 0 to 72 h and used to determine ¹⁵N metabolism in the rumen. Calculations of area under the curve ratios of ¹⁵N 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 ¹⁵N atomic excess pool sizes. The model described the pool sizes accurately based on small residuals between observed and predicted values. An immediate increase in ¹⁵N 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 ¹⁵N enrichment of bacterial N with SN than with AN treatment indicates a rapid adsorption of SNAN to microbial cells. Additionally, the recovery of ¹⁵N 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.

Stable isotope-labelled feed nutrients to assess nutrient-specific feed passage kinetics in ruminants

Knowledge of digesta passage kinetics in ruminants is essential to predict nutrient supply to the animal in relation to optimal animal performance, environmental pollution and animal health. Fractional passage rates (FPR) of feed are widely used in modern feed evaluation systems and mechanistic rumen models, but data on nutrient-specific FPR are scarce. Such models generally rely on conventional external marker techniques, which do not always describe digesta passage kinetics in a satisfactory manner. Here the use of stable isotope-labelled dietary nutrients as a promising novel tool to assess nutrient-specific passage kinetics is discussed. Some major limitations of this technique include a potential marker migration, a poor isotope distribution in the labelled feed and a differential disappearance rate of isotopes upon microbial fermentation in non-steady state conditions. Such limitations can often be circumvented by using intrinsically stable isotope-labelled plant material. Data are limited but indicate that external particulate markers overestimate rumen FPR of plant fibre compared with the internal stable isotope markers. Stable isotopes undergo the same digestive mechanism as the labelled feed components and are thus of particular interest to specifically measure passage kinetics of digestible dietary nutrients.

Energy expenditure, urea kinetics, and body weight gain within a segregating resource family population

Beef and dairy cattle represent divergent metabolic types that disseminate nutrients into either meat or milk and differ in nutrient accretion. To investigate nutrient flow and turnover in an animal model combining beef and dairy cattle, a crossbred experiment has been started. An F(2) resource population was generated from Charolais (beef breed) sires and German Holstein (dairy breed) cows as P(0) founders by consistent use of embryo transfer to establish the F(1) and F(2) generations, which accordingly comprised half- and full-sib offspring. In 64 bulls of 5F(2) families, dry matter intake and growth performance were measured monthly, and carcass composition was determined after slaughtering at 18 mo of age. Energy expenditure and urea kinetics were investigated via stable isotope tracer techniques using an intravenous single bolus dose of sodium [(13)C]bicarbonate [2.5 μmol/kg of body weight (BW), 99 atom% (13)C] at 8 and 18 mo of age and of [(15)N]urea (0.28 mg/kg of BW, 99 atom% (15)N) at 8 mo of age, respectively. Insulin responses were measured via glucose tolerances tests at the age of 8 mo. The results revealed significant differences between families for growth performance, energy expenditure, and urea kinetics. In summary, low energy expenditure was associated with high average body mass gain and high insulin response. A greater urea loss was associated with reduced muscle protein in carcass. In addition, corresponding half-sib and full-sib sisters from bulls with highest growth rate indicated highest milk production. In conclusion, we have demonstrated that differences in energy expenditure and urea kinetics result in differences in average daily gain and carcass traits and vice versa in F(2) crossbred bulls with common beef and dairy genetic backgrounds.

Whole-body protein metabolism and energy expenditure in sheep selected for divergent wool production when fed above or below maintenance

Rates of whole-body protein turnover and energy expenditure were measured in two groups of wethers differing in estimated breeding values (EBVs) for wool growth, but with similar EBVs for fibre diameter and liveweight (LW). The sheep were offered a pelleted diet at 1.2 times their metabolisable energy (ME) requirement for maintenance (1.2 M) followed by either 0.8 M or 1.8 M for 5 weeks. In the 5th week, whole-body protein metabolism was estimated by using intravenous injection of 15N-glycine (g N/day) and whole-body energy expenditure rate (EE) was predicted by the CO2 entry rate technique using intravenous injection of NaH13CO3. The higher N intake (8.7 v. 20.4 g N/day, P < 0.001) was associated with a higher whole-body protein flux (22.1 v. 34.2 g N/day, P < 0.001), and a higher whole-body protein synthesis rate (17.0 v. 25.5 g N/day, P < 0.001) and protein degradation rate (15.9 v. 20.7 g N/day, P < 0.001). Irrespective of feeding levels, sheep with high-fleece EBVs (F+) synthesised and degraded more body protein N (g N/day) than sheep with low-fleece EBVs (F–), and F+ sheep also retained more ingested protein N (P < 0.05) in wool and body tissue than F– sheep, but the significant differences due to genotypes disappeared when whole-body protein flux, synthesis and degradation were expressed as g N/kg LW0.75.day (metabolic weight). Estimates of EE were lower when the sheep were offered 0.8 M than when offered 1.8 M (5.85 v. 7.68 MJ/day, P < 0.001) and were higher in F+ than in F– sheep (6.95 v. 6.58 MJ/day; P < 0.05), but F+ sheep had a significantly lower (P < 0.05) EE (MJ/kg LW0.75.day) than F– sheep. F+ animals also retained more energy in wool and wool-free body tissue than F– animals (P < 0.05). The present study indicates that genetic selection for wool growth has resulted in increased efficiency of dietary protein and energy use for wool production and body-tissue growth in these sheep. Furthermore, there is no ‘trade off’ between deposition of nutrients in the body and wool in sheep with high EBVs for wool growth.

Characteristics of developmental changes in the kinetics of glucose and urea in Japanese Black calves: Comparison with Holstein calves1

The current study was conducted to clarify the characteristics of glucose and urea kinetics in male Japanese Black calves, using a glucose and urea dilution method with stable isotopes, at preweaning (3 wk of age) and postweaning (13 and 26 wk of age) stages, in comparison with the kinetics of glucose and urea in male Holstein calves. Six Japanese Black and 6 Holstein calves were arranged in a 2 (breed) x 3 (stage) factorial block design. These 12 suckling calves were fed only whole milk, offered concentrate and or chardgrass hay after 3 wk of age, and weaned at 6 wk of age. Under steady-state conditions, glucose challenges (1.0 mg/kg of BW; [U-13C]d-glucose) and urea challenges (both 2.2 mg/kg of BW; [13C]urea and [15N2]urea) were performed at the 3 stages examined. There were no significant differences in plasma glucose concentrations between Japanese Black and Holstein calves at any stage, but the glucose concentrations at 3 wk of age were greater (P <0.05) than those at 13 wk of age in both breeds. The glucose pool size in Japanese Black calves was smaller (P <0.05) than that in Holstein calves at all stages. Within each breed, there were no significant differences between the glucose pool sizes at 3 and 13 wk of age, but the glucose pool size was larger (P <0.05) at 26 wk of age. Moreover, Japanese Black calves had greater glucose irreversible loss rates (P <0.01) than Holstein calves at 3 wk of age, and the glucose irreversible loss rates were less (P <0.05) on and after 13 wk of age in both breeds. Japanese Black calves had greater plasma urea N concentrations (P <0.05) than Holstein calves at all stages, and in both breeds, the urea N concentrations increased (P <0.05) with age. The urea pool size was smaller (P <0.01) in Japanese Black calves than in Holstein calves at all stages, and in both breeds, the urea pool size increased (P <0.05) with age. In comparison with Holstein calves, Japanese Black calves had greater urea irreversible loss rates (P <0.05) on and after 13 wk of age and greater urea recycling rates (P <0.05) at 26 wk of age. In addition, in both breeds, urea irreversible loss rates and urea recycling rates increased (P <0.05) with age. We conclude that Japanese Black calves have partially different glucose and urea kinetics from Holstein calves and that the kinetics of these metabolites in both Japanese Black and Holstein calves are strongly influenced by weaning.

Effect of Barley and Its Amylopectin Content on Ruminal Fermentation and Nitrogen Utilization in Lactating Dairy Cows

The effect of type of grain (corn vs. barley) and amylopectin content of barley grain (normal vs. waxy) on ruminal fermentation, digestibility, and utilization of ruminal ammonia nitrogen for milk protein synthesis was studied in a replicated 3 x 3 Latin square design trial with 6 lactating dairy cows. The experimental treatments were (proportion of dietary dry matter): CORN, 40% corn grain, NBAR, 30% normal Baronesse barley:10% corn grain, and WBAR, 30% high-amylopectin (waxy) Baronesse barley:10% corn grain. All grains were steam-rolled and fed as part of a total mixed ration. The NBAR and WBAR diets resulted in increased ruminal ammonia concentrations compared with CORN (8.2, 7.4, and 5.6 mM, respectively), but other ruminal fermentation parameters were not affected. Ruminal digestibility of dietary nutrients and microbial protein synthesis in the rumen were also not affected by diet. Corn grain had greater in situ effective ruminal dry matter degradability (62.8%) than the barley grains (58.2 and 50.7%, respectively), and degradability of the normal barley starch was greater than that of the waxy barley (69.3 and 58.9%, respectively). A greater percentage of relative starch crystallinity was observed for the waxy compared with the normal barley grain. Total tract apparent digestibility of dry matter and organic matter were decreased by WBAR compared with CORN and NBAR. Total tract starch digestibility was greater and milk urea nitrogen content was lower for CORN compared with the 2 barley diets. In this study, the extent of processing of the grain component of the diet was most likely the factor that determined the diet responses. Minimal processing of barley grain (processing indexes of 79.2 to 87.9%) reduced its total tract digestibility of starch compared with steam-rolled corn (processing index of 58.8%). As a result of the increased ammonia concentration and reduced degradability of barley dry matter in the rumen, the utilization of ruminal ammonia nitrogen for microbial protein synthesis was decreased with the barley diets compared with the corn-based diet. In this study, waxy Baronesse barley was less degradable in the rumen and the total digestive tract than its normal counterpart. The most likely reasons for these effects were the differences in starch characteristics and chemical composition, and perhaps the different response to processing between the 2 barleys.

Developmental changes in the kinetics of glucose and urea in Holstein calves

Because weaning is the point when the nutrient composition of feed changes for the neonatal ruminant, the present experiment was conducted to assess the developmental changes in the kinetics of glucose and urea over this period, using stable isotopes of glucose and urea, at 4, 13, and 24 wk in calves. Plasma concentrations of nonesterified fatty acids, amino-N, urea-N, and insulin-like growth factor-I increased, but that of growth hormone decreased with age. The plasma glucose concentration increased at 13 wk of age and thereafter decreased at 24 wk of age. The glucose irreversible loss and recycling rates were significantly higher at 4 wk of age than at 13 and 24 wk of age. On the other hand, the irreversible loss and recycling rates of urea, as well as the urea pool size, were higher at 24 wk of age than at 4 and 13 wk. It is concluded that weaning at 6 wk is the pivotal time for the alteration of glucose kinetics. However, the aging process, but not weaning, is important for changes in the kinetics of urea in calves.

Feeding an energy supplement with white clover silage improves rumen fermentation, metabolisable protein utilisation, and milk production in dairy cows

Six rumen-fistulated Holstein-Friesian cows were used in a Latin square design to test the hypothesis that more frequent feeding of a high energy supplement to cows consuming high-protein white clover silage would improve microbial protein production, resulting in greater N retention and higher milk yields. The white clover silage (10.7 MJ metabolisable energy (ME)/kg DM) was fed to cows either alone (WCS) or with 4.5 kg DM of rolled barley grain (12.1 MJ ME/kg DM). The grain was offered either 24 times (WCS/24B) or twice daily (WCS/2B, at 0800 and 1700 hours). Cows offered the supplements, regardless of feeding frequency, had higher (P < 0.05) organic matter (17.3 v. 16.0 kg/day) and estimated ME (208 v. 189 MJ/day) intakes than cows offered white clover silage alone. Mean daily ruminal fluid pH (P < 0.05) and ammonia-N concentrations (P < 0.05) were lower in the supplemented treatments, with total VFA concentrations being highest (P < 0.05) in the WCS/2B treatment. Nitrogen intake and output in the faeces were similar for all 3 treatments. However, nitrogen excretion was lower (P < 0.05) in urine (174 v. 218 g/day) and higher (P < 0.05) in milk (115 v. 93 g/day) of cows offered the supplements. The crude protein consumed by cows on all 3 diets was estimated to be well in excess of cow requirements. The supplements reduced the calculated net losses of ammonia-N from the rumen from 25% of total crude protein intake for WCS to 14% in the 2 supplement treatments, and increased the metabolisable protein supply available for milk production. Increases in metabolisable protein were estimated to be due to a higher microbial crude protein contribution in the supplemented treatments compared with the WCS treatment. Grain supplements increased (P < 0.05) milk yield (22.4 v.19.6 kg/day) and although there were no significant differences in milk fat and protein concentrations between treatments, the latter tended to increase with grain supplementation. Milk yield was higher in the WCS/24B treatment than in the WCS/2B treatment, but neither the calculated nor the measured rumen variables were sufficiently different to explain this effect of frequency of feeding the grain. One possible explanation for the difference was the marked fluctuations in key rumen variables throughout the day in the WCS/2B compared with the WCS/24B treatment. Such fluctuations in the rumen environment are not accounted for in theoretical calculations since associative effects are not considered. The benefits of a higher milk production as a result of more frequent feeding of the supplement to cows should be considered in context of the additional effort or costs associated with more frequent feeding.

Effect of dietary crude protein level and degradability on ruminal fermentation and nitrogen utilization in lactating dairy cows1

The objectives of this experiment were to investigate the effects of two ruminally degradable protein (RDP) levels in diets containing similar ruminally undegradable protein (RUP) and metabolizable protein (MP) concentrations on ruminal fermentation, digestibility, and transfer of ruminal ammonia N into milk protein in dairy cows. Four ruminally and duodenally cannulated Holstein cows were allocated to two dietary treatments in a crossover design. The diets (adequate RDP [ARDP] and high RDP [HRDP]), had similar concentrations of RUP and MP, but differed in CP/RDP content. Ruminal ammonia was labeled with 15N and secretion of tracer in milk protein was determined for a period of 120 h. Ammonia concentration in the rumen tended to be greater (P = 0.06) with HRDP than with ARDP. Microbial N flow to the duodenum, ruminal digestibility of dietary nutrients, DMI, milk yield, fat content, and protein content and yield were not statistically different between diets. There was a tendency (P = 0.07) for increased urinary N excretion, and blood plasma and milk urea N concentrations were greater (P = 0.002 and P = 0.01, respectively) with HRDP compared with ARDP. Milk N efficiency was decreased (P = 0.01) by the HRDP diet. The cumulative secretion of ammonia 15N into milk protein, as a proportion of 15N dosed intraruminally, was greater (P = 0.003) with ARDP than with HRDP. The proportions of bacterial protein originating from ammonia N and milk protein originating from bacterial or ammonia N averaged 43, 61, and 26% and were not affected by diet. This experiment indicated that excess RDP in the diet of lactating dairy cows could not be efficiently utilized for microbial protein synthesis and was largely lost through urinary N excretion. At a similar MP supply, increased CP or RDP concentration of the diet would result in decreased efficiency of conversion of dietary N into milk protein and less efficient use of ruminal ammonia N for milk protein syntheses.

Effect of dietary carbohydrate composition and availability on utilization of ruminal ammonia nitrogen for milk protein synthesis in dairy cows

A trial with four ruminally and duodenally cannulated, late-lactation dairy cows was conducted to investigate the effect of dietary carbohydrate (CHO) composition and availability on ruminal ammonia N utilization and transfer into milk protein. Two diets were fed at 8-h intervals in a crossover design. The diets differed in CHO composition: the ruminally fermentable non-structural carbohydrates (RFSS) diet (barley and molasses) contained a larger proportion of ruminally available CHO in the nonstructural carbohydrate fractions and the ruminally fermentable fiber (RFNDF) diet (corn, beet pulp, and brewer's grains) contained a larger proportion of CHO in ruminally available fiber. Nitrogen-15 was used to label ruminal ammonia N and consequently microbial and milk N. Fermentation acids, enzyme activities, and microbial protein production in the rumen were not affected by diet. Ruminal ammonia concentration was lowered by RFNDF. Ruminal and total tract digestibility of nutrients did not differ between diets except that apparent ruminal degradability of crude protein was lower for RFNDF compared with RFSS. Partitioning of N losses between urine and feces was also not affected by diet. Milk yield and fat and protein content were not affected by treatment. Average concentration of milk urea N was lower for RFNDF than for RFSS. Proportion of milk protein N originating from ruminal microbial N (based on the areas under the 15N-enrichment curves) was higher for RFNDF than for RFSS. Cumulative recovery of 15N in milk protein was 13% higher for RFNDF than for RFSS indicating enhanced transfer of 15N-ammonia into milk protein with the former diet. The results suggested that, compared to diets containing higher levels of ruminally fermentable starch, diets providing higher concentration of ruminally fermentable fiber may enhance transfer of ruminal ammonia and microbial N into milk protein.

Comparison of the ruminal metabolism of nitrogen from 15N-labeled alfalfa preserved as hay or as silage

Alfalfa (Medicago sativa L. cv. AC Blue J) was labeled with 15N during growth in a greenhouse, harvested at early bloom, and preserved as silage (19% dry matter) or as sun-cured hay. The labeled silage and hay were given as single-pulse doses to two lactating Holstein cows fed diets comprising 30% concentrate and 70% alfalfa forage (preserved either as silage or as hay). Labeled forage and ruminal content samples collected for 72 h after dosing were partitioned into N fractions and analyzed for 15N-enrichment. Pool sizes of N compartments and kinetics in the rumen were derived by isotope dilution and by gravimetric measurements. The rate of outflow of total N, determined gravimetrically, was 21% higher with the silage diet than with the hay diet. On both diets, the largest individual flux was associated with the nonprotein, nonammonia, nonmicrobial nitrogen (NPAM-N) pool. As related to the flux of 15N through the acid detergent insoluble N pool, less tracer passed through the solid-phase nonfiber N and the soluble protein-N pools, and more passed through the NPAM-N pool, with silage than with hay. The solid-phase nonfiber N pool, which includes readily available feed N and adherent bacterial- and protozoal-N, constituted the largest N entity in the rumen, followed by the NPAM-N pool. When the forage component of the diet was alfalfa silage, N flux through the NPAM-N pool was remarkably high, and with both methods of preserving alfalfa forage, the exchange of tracer was most intensive through this pool.

Synthesis of microbial protein in ruminally cannulated cows fed alfalfa silage, alfalfa hay, or corn silage

Six ruminally cannulated cows were used in an experiment with a 3 x 3 Latin square design. Three all forage diets-alfalfa silage, alfalfa hay, or corn silage plus 2.2% urea (DM basis)-were fed for ad libitum intake four times daily. The microbial protein marker 15NH3 and the liquid marker Cr-EDTA were infused continuously into the rumen for 72 and 48 h, respectively; the solid marker, Yb-labeled forage, was dosed into the rumen twice daily for 60 h. Pool sizes of ruminal NAN were determined by emptying the rumen. Proportions of bacterial N formed from NH3 were 57, 46, and 82% for the alfalfa silage, alfalfa hay, and corn silage diets, respectively. For all diets, flows of microbial NAN with the liquid and solid phases were about equal. Although feed NAN in the liquid pool was only 12% of ruminal feed NAN, 30% of feed NAN that escaped the rumen flowed with the liquids. Flow of microbial NAN was highest for corn silage (243 g/d) and lowest for alfalfa hay (212 g/d); microbial NAN represented 50% (alfalfa silage and hay) and 76% (corn silage) of total NAN flow. Proportions of NAN intake that were degraded in the rumen were 61, 56, and 57% for alfalfa silage, alfalfa hay, and corn silage (without urea N), respectively; these values were lower than those reported by the NRC. Total flows of NAN from the rumen were 472, 424, and 321 g/d for the alfalfa silage, alfalfa hay, and corn silage diets, respectively. Use of liquid (Cr-EDTA) and solid (Yb) markers to compute the rate of passage of microbial protein proved to be less variable than regression of 15N enrichment of bacterial NAN over time.

Evaluation of chemical and physical properties of feeds that affect protein metabolism in the rumen

The goal of the NC-185 Cooperative Regional Research Project is to provide the information needed to improve the nutrition and feeding of dairy cattle, a major factor determining composition of milk and cost of milk yield. Emphasis is placed on understanding how energy and protein nutrition of lactating cows can be manipulated to increase the quantity and improve the profile of AA passing to the small intestine and to improve yield of milk and milk protein. To achieve this goal, one of the major objectives of this project has been to evaluate quantitatively the chemical and physical properties of protein and energy sources that determine AA availability to lactating cows. Reliable measurements of microbial protein synthesis and protein degradation in the rumen are critical in the evaluation process. Therefore, one of the ongoing areas of investigation of this research project has been to determine the most appropriate methods for estimating microbial protein synthesis and dietary protein degradation in the rumen. Other areas have been investigated, using continuous culture fermenters and ruminally and duodenally cannulated cows, including factors that alter microbial metabolism of N in the rumen and subsequently protein supply to the small intestine, such as sources of carbohydrate, protein, and fat and interrelationships of protein and carbohydrate. Findings of the NC-185 Cooperative Regional Research Project Committee and other investigators are summarized in this review.

Urease (EC 3.5.1.5) inhibition in the sheep rumen and its effect on urea and nitrogen metabolism

The urease (EC 3.5.1.5) inhibitor, phenylphosphoryldiamidate (PPDA), was given by continuous infusion into the rumen of two sheep nourished by intragastric infusion and into either the rumen or abomasum of four sheep given a pelleted diet containing 119 g crude protein (nitrogen x 6.25)/kg dry matter. PPDA was given at 1 g/d in infusion sheep and 1.5 g/d in the normally-fed sheep. Measurements of urea kinetics were made using single injections of [14C]urea. Urease inhibition was complete within 24 h of starting PPDA infusions to the rumen; in this time-period, urea concentration in rumen contents reached equilibrium with that in plasma and this situation persisted until infusions were terminated. Relative to the control periods, plasma urea and rumen ammonia concentrations were unchanged but urea irreversible loss rate decreased by 26% in infusion sheep and 33% in fed sheep when PPDA was given. Urinary urea excretion was not affected, hence urea degradation, measured by difference, decreased by 77 and 58% respectively in response to urease inhibition. Administration of PPDA to the abomasum resulted in a reduction in rumen urease activity to about 40% of control values but had no effect on urea metabolism. Differences between treatments in daily nitrogen retention were not significant, indicating that under the dietary conditions imposed in these experiments, even substantial changes in urea recycling had only minor effects on the overall N economy of the animal.

Ruminal nitrogen metabolism in steers as affected by feed intake and dietary urea concentration

Four multiple-cannulated steers (340 kg) were used in a 4 X 4 Latin square design with a 2 X 2 factorial arrangement of treatments. Steers were fed a diet of 50% ground hay and 50% concentrate at two intakes (1.4 and 2.1% of BW), with urea and 15N-enriched ammonium sulfate infused continuously into the rumen at .4 or 1.2% of diet DM. Ratios of purines and diaminopimelic acid-N to N in fluid-associated and particulate-associated bacteria and in protozoa were similar among treatments but were lower for protozoa than for bacteria. Diaminopimelic acid-N:N was higher for fluid-associated vs. particulate-associated bacteria. Enrichment of 15N was similar between bacteria among treatments and was 30% lower for protozoa. Turnover rates of 15N in bacteria, NH3N, and non-NH3N pools were faster for steers infused with 1.2 than those infused with .4% urea, indicating less efficient usage of ammonia with higher urea. A method is described to estimate the proportion of duodenal nitrogen comprising bacterial and protozoal nitrogen.

Nitrogen and carbon flows between the caecum, blood and rumen in sheep given chopped lucerne (Medicago sativa) hay

1. Experiments involving 15N and 14C tracers were made in sheep consuming 800 g air-dry chopped lucerne (Medicago sativa) hay/d and providing 20.4 g N/d to study N and C flows within the caecal digesta and between the caecum, blood and rumen. 2. Continuous infusions of 15N tracers were made into the caecal ammonia, blood urea and rumen NH3 pools. The concentration and enrichment of caecal digesta NH3-N, caecal microbial N, caecal digesta non-urea, non-ammonia-N (NU-NAN), faecal NU-NAN, blood urea-N, rumen digesta NH3-N and rumen bacterial N were estimated at intervals during the infusions. A three-pool open-compartment model was solved to estimate N flows between the caecal digesta NH3-N, blood urea-N and rumen digesta NH3-N pools. 3. The rate of irreversible loss from the caecal digesta NH3-N pool was 2.17 (SE 0.623) g N/d. On average 0.9 (SE 0.56) g N/d of caecal digesta NH3-N was derived from blood urea and 0.1 (SE 0.08) g caecal digesta NH3-N/d was apparently derived from the fermentation of undigested rumen microbes in the caecum. The amount of NH3-N produced by proteolysis and deamination of dietary and endogenous N was 1.1 (SE 0.13) g/d. 4. There was net incorporation of 0.56 (SE 0.306) g caecal digesta NH3-N/d into caecal microbes. The microbial N synthesized de novo in the caecum was not determined, but 2.9 (SE 0.52) g microbial N/d of both rumen and caecal origin flowed out of the caecum and constituted 0.48 of the NU-NAN flow. The majority (mean 0.83 (SE 0.044] of this microbial N was excreted in faeces. 5. On average 1.8 (SE 0.80) g caecal digesta NH3-N/d were absorbed. Of this NH3-N, 0.92 (SE 0.054) was converted to blood urea, contributing 0.10 (SE 0.031) of blood urea-N. Only 0.012 (SE 0.0041) of rumen digesta NH3-N and 0.005 (SE 0.0009) of rumen bacterial N were derived from caecal digesta NH3-N. 6. Infusions of 14C tracers were made into the caecal digesta bicarbonate, blood bicarbonate, rumen digesta bicarbonate and blood urea pools, and samples were obtained at intervals to determine the specific radioactivity of each pool. A four-pool open-compartment model was solved to estimate C flows between these pools. 7. The rate of irreversible loss of blood urea estimated with [14C]urea (17.1 (SE 1.18) g N/d) was greater (P less than 0.01) than that estimated with [15N]urea (14.0 (SE 0.87) g N/d).(ABSTRACT TRUNCATED AT 400 WORDS)

The digestion of fresh perennial ryegrass (Lolium perenne L. cv. Melle) and white clover (Trifolium repens L. cv. Blanca) by growing cattle fed indoors

1. Pure swards of perennial ryegrass (Lolium perenne L. cv. Melle) or white clover (Trifolium repens L. cv. Blanca) were harvested daily at three and two stages of growth respectively, and offered to housed cattle. The grass diets comprised primary growth (May) and two later regrowths of contrasting morphology (i.e. leaf: stem values of 1.54 and 2.84 respectively), and were characterized by high contents of water-soluble carbohydrate and neutral-detergent fibre and comparable in vitro dry matter (DM) digestibilities (mean 0.80). Total nitrogen content was higher on primary growth grass (34 g/kg DM) than on regrowths (23 g/kg DM) but lower than values obtained for the two clover diets (38 and 43 g/kg DM, respectively). The clover diets had lower water-soluble carbohydrate contents than the grasses, comparable cellulose, but lower neutral-detergent fibre contents and in vitro DM digestibilities of 0.70 and 0.77 respectively. 2. The experiment lasted from May until August, during which time a total of twenty-one young Friesian steers (initial average live weight 130 kg) were used to determine both nutrient supply to the small intestine (twelve animals) and apparent digestibility (nine animals). Each diet was offered at three levels of DM intake (i.e. 18, 22 and 26 g/kg live weight). A further six steers, all fed at the rate of 22 g DM/kg live weight, were used to determine the metabolizable energy contents of the five diets by means of open-circuit calorimetry. 3. The three grass diets and the later-cut clover had, as intended, quite similar in vivo organic matter digestibilities, but that of the earlier-cut clover was lower, and this was associated with a large number of flower heads in this crop at the time of feeding. 4. On the clover diets, proportionately less of the ingested organic matter appeared to be digested in the rumen (0.40) compared with the grass diets (0.58) (P less than 0.001). On the high-N primary grass and the clover diets, substantial rumen losses of N were detected (P less than 0.01) compared with regrowth grasses. 5. The metabolizable energy content of the primary growth of grass was 12.2 MJ/kg DM, whilst the values for the other two grass diets were lower (11.6 MJ/kg DM), despite no marked decline in overall energy digestibility. Values for the two clover diets (mean 10.5 MJ/kg DM) were considerably lower than all values noted for the grasses.(ABSTRACT TRUNCATED AT 400 WORDS)

Estimation of the degradability of dietary protein in the sheep rumen by in vivo and in vitro procedures

Estimates of degradability of nitrogen in the sheep rumen for a basal hay diet and for soya-bean meal (SBM), groundnut meal (GNM) and fish meal (FM), when given together with the hay, were determined from measurements of duodenal N flow, ammonia kinetics and rumen N disappearance from polyester bags and rumen outflow rate. The ability of various in vitro procedures to predict in vivo N degradability was also examined. Four sheep were given a basal hay diet (800 g dry matter (DM) and 19 g N/d) either alone or supplemented with isonitrogenous amounts (15 g N/d) of SBM, GNM or FM. Duodenal non-ammonia-N flow (g/d) was increased more by FM (8.0) than by GNM (5.9) and SBM (5.8), whilst microbial N flow (g/d) was increased more by SBM (3.9) than by GNM (2.3) and FM (1.6). N degradability values calculated from these results were 0.88, 0.76 and 0.57 for the SBM, GNM and FM respectively. The corresponding value for hay was calculated to be 0.76. The irreversible loss of ammonia in the forestomachs (g N/d) was increased more by SBM (11.9) than by GNM (7.2) and FM (5.8), whilst ammonia outflow from the rumen (g N/d) was increased to a similar extent by all supplements (1.1, 0.9 and 0.8 respectively), as was the amount of microbial N (g/d) synthesized from sources other than rumen ammonia (1.8, 2.0 and 1.9 respectively). N degradability values calculated from these results were 0.84, 0.54 and 0.45 for the SBM, GNM and FM respectively. The fractional rate of N disappearance (/h) when the feedstuffs were incubated in polyester bags in the rumen of sheep receiving the basal hay diet (800 g DM/d) was the highest for SBM (0.145) and lowest for FM (0.037), with the hay (0.082) and GNM (0.071) intermediate, whilst the fractional outflow rates from the rumen (/h) of the three supplements were similar (0.034, 0.038 and 0.030 for SBM, GNM and FM respectively). N degradability values calculated from these results were 0.82, 0.67 and 0.60 for the SBM, GNM and FM respectively; the value for the hay was 0.73. Of a number of in vitro procedures tested, only N solubility in sodium hydroxide and ammonia or total non-protein-N (NPN) production during incubation with rumen fluid in the absence of hydrazine sulphate ranked the supplements, although not the hay, in the same order as in the vivo degradability procedures.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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. The amounts of organic matter (OM) consumed and flowing at the duodenum and ileum and excreted in the faeces were similar (P greater than 0.05) with both diets. Each diet supplied 23 g digestible OM/d per kg live weight 0.75, which was sufficient to maintain body-weight. There were no differences (P greater than 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 less than 0.01), and molar proportion of butyrate lower (P less than 0.001) when the silage was given. 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 greater than 0.05) between diets. Rumen microbial NAN flow at the duodenum, based on 15N as the marker, also did not differ (P greater than 0.05) between diets but the efficiency of microbial N synthesis in the rumen (g/kg OM apparently digested) was higher (P less than 0.05) with the dried grass. When the sheep were consuming silage they had a higher concentration of ammonia in rumen fluid (P less than 0.01), a higher rate of irreversible loss of ammonia from the rumen (P less than 0.05) and a higher rate of absorption of ammonia across the rumen wall (P less than 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). 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.(ABSTRACT TRUNCATED AT 400 WORDS)

Urea turnover and transfer to the digestive tract in the rabbit

14C and 15N isotopes of urea were infused intravenously into rabbits for 6-8 h in order to measure urea synthesis and the extent of degradation in the digestive tract. The results indicate that 0.62 of the urea flux was excreted in the urine and that re-incorporation of urea-N following hydrolysis in the gut represented 0.3 of the urea synthesis rate. Sampling of metabolites from the caecum by dialysis provided an opportunity to assess the contribution of urea-N to the caecal ammonia pool. This contribution is calculated to be 0.25 of caecal ammonia turnover. Infusion of a urease (EC 3.5.1.5) inhibitor during a continuous infusion of [14C]urea into the caecum permitted the measurement of urea turnover within the caecum. Results obtained for urea entry into the caecum are contrasted with the measured urea degradation rate in the gut.

Nitrogen metabolism in the rumen

Nitrogen metabolism is reviewed with emphasis on methods for quantitating various nitrogen-transactions in the rumen of animals on a variety of diets. Ammonia kinetics, microbial cell synthesis, the inputs of endogenous nitrogen, degradation of dietary protein, and availability to the animal of dietary bypass protein are discussed. The efficiency of microbial protein from the rumen is discussed in relation to the ratio of protein to energy in the nutrients available to meet the requirements of the animal. The ratio is determined largely by the maintenance requirements of microbes and the breakdown of microbial materials, which result in the recycling of microbial nitrogen in the rumen. Emphasis is placed on the role of rumen protozoa in decreasing the ratio of protein to energy in absorbed nutrients in ruminants on diets that are marginally deficient in protein. Recent studies of the dynamics of protozoa in the rumen and their contribution to microbial protein outflow are summarized.

Studies of the large intestine of sheep. 3. Nitrogen kinetics in sheep given chopped lucerne (medicago sativa) hay

A study was made of nitrogen kinetics in the large intestine of sheep given 800 g chopped lucerne (Medicago sativa) hay/d. Four sheep were continuously infused with (15NH4)2SO4 into the caecum and three other sheep were infused intravenously with [15N]urea. A digesta marker, 51Cr complexed with EDTA (51Cr-EDTA), was infused into the rumen of each sheep to allow estimation of the rates of digesta constituents. Infusions were continued until tracer concentrations reached plateaux in digesta and blood pools, after which the sheep were anaesthetized and slaughtered. Pre-infusion samples and samples on plateau were obtained before slaughter for subsequent analysis to give plasma urea and rumen ammonia-N concentration and enrichment. At slaughter, digesta were obtained from the ileum and segments of the large intestine. These were analysed for 51Cr-EDTA content and concentration and enrichment of ammonia-N, microbial N and non-urea non-ammonia-N (NU-NAN). N flows in segments of the large intestine were calculated and represented in a quantitative eight-pool model. Transfer of plasma urea across the wall of the caecum and proximal colon was negligible but there was an input of 0.8 g endogenous NU-NAN/d. Flow of urea plus ammonia-N in digesta from the ileum into the caecum contributed 1.0 g N/d to the caecal ammonia pool. Proteolysis and deamination produced a further 3.0 g ammonia-N/d in the caecum and proximal colon. The net absorption of N between the ileum and the rectum was 2.8 g N/d but 3.0 g ammonia-N/d was absorbed from the caecum and proximal colon and, in addition, at least 0.9 g ammonia-N/d from the distal colon and rectum. Ammonia-N was incorporated into caecal microbes (0.6 g N/d) and approximately 57% of the NU-NAN in caecal digesta was microbial N. The majority of the microbial N flowing from the caecum was excreted in faeces. The rate of irreversible loss of urea-N from plasma, measured by intravenous infusion of [15N]urea, was 13.6 g/d. On average 83 (SE 6.8)% of the 15NH3 apparently absorbed from the caecum was incorporated into plasma urea; caecal ammonia contributed 9-19% of the N in plasma urea and 0.2-3.1% of the N in rumen ammonia.

The appearance of re-cycled urea in the digestive tract of goats during the final third of a once daily feeding of a low-protein ration

1. An experiment was carried out with goats fed on a low-protein ration to clarify the importance of the rumen and significance of saliva in the appearance of re-cycled urea in the digestive tract during the final third of a once daily feeding regimen. The isotope-dilution method with [15N]urea and 15NH4Cl was used. 2. When the serum urea level was 58 mg N/l, the amount of urea transferred from the blood urea pool to the rumen ammonia pool was 48.6 mg N/h, which was estimated to be approximately 43% of the total amount of urea having appeared in the NH3 pool of the digestive tract. When the serum urea level was 106 mg N/l, the corresponding amount of NH3 was 77.7 mg N/h, which was estimated to be approximately 46% of this total amount. 3. The amount of saliva secreted was measured directly by the oesophageal fistula method. Salivary secretion serves as a mode of transfer of blood urea to the rumen NH3 pool. Then the ratio, salivary secretion:diffusion through the rumen wall during the final third of the cycle was calculated to be 1:4-1:6. 4. In goats fed on a low-protein diet, the rumen is an important site of appearance of blood urea in the digestive tract. It was verified that the principal mode of transfer of blood urea to the rumen was the direct diffusion through the wall of the rumen.

Aspects of urea metabolism in ruminants with reference to the goat

In goats and other ruminants, urea functions as a source of nitrogen for protein biosynthesis in the digestive tract. Ammonia can be absorbed in the digestive system when formed in excessive quantitites and enhance formation of urea, or it can be derived from urea of blood plasma when its formation from feed sources is small. Entry rates of urea into plasma may vary from 4 to 80 mumol/min per kg.75 body weight depending on dietary conditions. Urea formation is related to nitrogen intake of which approximately 70% passes into the urea pool of plasma. Irreversible losses of urea of plasma into the digestive tract vary between 10 and 90% depending on the protein to energy ratios of the diet. Entry of urea from plasma into the rumen appears to be a passive process which is sensitive to short-term changes of urea concentrations in plasma. Permeability of ruminal epithelium to urea may be altered by fermentation products of rumen (ammonia, carbon dioxide, volatile fatty acids). The influx of nitrogen into the rumen is related to needs for nitrogen of microbial populations and is associated with changes of renal excretion and tubular reabsorption of urea. Combined gastrointestinal and renal responses exert a synergistic effect on improved utilization of urea of plasma when uptake of dietary nitrogen is limited in goats and other ruminants.

Nitrogen digestion in sheep given poor-quality indigenous hill herbages

1. In two experiments, the sites of digestion of non-ammonia nitrogen (NAN) and the amounts of urea N recycled to the rumen were measured in mature wether sheep given diets of indigenous hill herbage (Agrostis-Festuca and heather). 2. Duodenal and ileal flow values were obtained using 103Ru-phenanthroline and 51Cr-EDTA markers in animals prepared with simple (T-shaped) cannulas. Amounts of urea N recycled to the rumen were estimated from measurements of the transfer of plasma urea carbon into rumen bicarbonate and the production rate of rumen bicarbonate using 14C-labelled urea and bicarbonate respectively. 3. The flows of NAN at the duodenum and ileum were linearly related to the intake of herbage (P less than 0.001). There was a net gain of non-ammonia N anterior to the duodenum on both diets (at an intake of 460 g organic matter (OM)/d, 3.7 g NAN/d on Agrostis-Festuca and 3.3 g NAN/d on heather). 4. Net digestibility of NAN entering the small intestine was within a normal range on the Agrostis-Festuca (0.58 at 460 g OM intake) diet but low on the heather diet (0.43 at 460 g OM intake). 5. It was calculated that at 460 g OM intake only 0.9 and 1.1 g/d respectively of the duodenal NAN on the Agrostis-Festuca and heather diets could have been derived from urea-N recycled to the rumen. Thus 2.8 g and 2.2 g/d had to be accounted for as non-urea endogenous NAN.

Protein nutrition of growing lambs. 2. Effect on nitrogen digestion of supplementing a low-protein-cellulosic diet with either urea, casein or formaldehyde-treated casein

1. Lambs with cannulas in the duodenum and ileum were allowed free access to one of four diets: a basal diet of oat hulls and solka floc, or the basel diet supplemented with either urea, urea plus casein or urea plus formaldehyde-treated (HCHO)-casein. Mean nitrogen intake was 1.9 g N/d for the basal diet and 15.0. 32.4 and 36.9 g N/d respectively for the other diets. 2. The rate of irreversible loss of ammonia from the rumen pool estimated using 15NH4+ was highest on the casein diet (33 g NH3-N/d) by comparison with 18 g NH3-N/d for the urea and HCHO-casein diets and 7 g NH3-N/d for the basal diet. 3. The proportions of bacterial and protozoal N in the rumen derived from rumen ammonia did not differ significantly between the supplemented diets and were 0.66 and 0.52 respectively. 4. Estimation of 15N flowing to the duodenum during continuous infusions of 15NH4+ into the rumen indicated considerable ammonia absorption from the rumen on all the diets. Greatest absorption of ammonia (21 gN/d) apparently occurred in animals on the diet supplemented with urea and casein. 5. The estimated microbial non-ammonia-N (NAN) flowing out of the rumen per unit organic matter fermented in the rumen (FOM) was similar on all diets, i.e. 21.3 (+/- 1.09) g N/kg Fom. the requirement for dietary fermentable N for microbial N production on these diets was 1.2 (+/- 0.07) g N/MJ ME. 6. The flow of NAN into the duodenum and through the ileum, and total N in the faeces was significantly influenced by the form of N supplementation. The flow of NAN into the duodenum for the HCHO-casein diet (27 g N/d) was more than twice that for the other diets (11 g N/d). The flow of NAN through the ileum and excretion of total N in the faeces was also greater with the HCHO-casein diet than with all other diets. The apparent digestibility of NAN in the small intestine ranged between 0.62--0.66 for all diets. 7. Urea and casein supplements were apparently completely degraded in the rumen. In contrast, the HCHO-casein was almost completely resistant to degradation in the rumen and only 65% of the HCHO-casein was digested in the small intestine. 8. Protein absorbed : energy absorbed (expressed as NAN digested in the small intestine/MJ ME) was calculated to be 5.5 (+/- 0.70) for the basal, urea and urea-plus-casein diets, and 11.6 (+/- 1.71) for the urea-plus-HCHO-casein diet.

Fermentation and nitrogen dynamics in Merino sheep given a low-quality-roughage diet

1. Fermentation in the rumen and nitrogen dynamics in the body were studied in mature Merino sheep given a maintenance ration of a low-quality-roughage diet containing mainly chopped wheat straw.

2. Intake of metabolizable energy was 3.49 MJ/d and of total N 6.2 g/d.

3. From measurements of volatile fatty acid (VFA) production rates and stoichiometric principles, it was calculated that 75% of the digestible organic matter intake was fermented in the rumen, making an estimated 44 g/68d microbial dry matter available to the animal.

4. The total flux of ammonia through the rumen NH3 pool, estimated by 15NH3 dilution methods, was 8.2 g N/d of which 3.5 g N/d was irreversibly lost; thus 4.7 g N/d was recycled, partly within the rumen (approximately 3.8 g N/d) and partly via endogenous secretions (approximately 0.9 g N/d). The extensive recycling of NH3-N within the rumen indicated that turnover of microbial N was considerable, and the total production of micro-organisms was at least twice the net outflow.

5. The proportion of the N in rumen bacteria derived from rumen ammonia was 62% and thus 38% was derived from other nitrogenous compounds such as peptides and amino acids.

6. The rates of transfer of blood urea into the rumen, estimated from the appearance of 14CO2 or 15NH3 in the rumen after intravenous single injections of [14C]-and [15N]urea, did not differ significantly and the mean transfer was 2.3 urea-N/d.

7. Estimates of the rate of irreversible loss of urea-C (i.e. urea synthesis in the body) were obtained by analysis of samples of either blood or urine obtained after a single, intravenous injection of [14C]urea. The two methods gave results that did not differ significantly. The estimated rate of urea synthesis in the body was 5.3 g N/d. Urea excretion rate was relatively low, i.e. 1.2 g N/d, and thus transfer of urea to the digestive tract was approximately 4.1 g N/d. Approximately 53% of the latter was transferred to the rumen, and 47% to the rest of the digestive tract. These results are discussed in relation to similar studies with sheep given other diets.

8. Various aspects of isotope-tracer methods and the errors that could occur in this type of study are discussed.

Effects of cold exposure on digestion, microbial synthesis and nitrogen transformations in sheep

1. Six closely shorn sheep were given brome grass (Bromus inermis) pellets at the rate of 59 or 98 g dry matter (dm)/h and maintained at ambient temperatures of 2–5° and 22–25° for 35 d. Measurements of digestion, rate of passage of digesta, and nitrogen transformations were made during the last 13 d of temperature exposure.

2. Cold exposure at the lower level of intake reduced the apparent digestibility of dm and organic matter (om) approximately 0.055 units. Apparent digestibility of dm and om was further decreased approximately 0.03 units with the higher level of food intake in the cold. Apparent N digestibility was significantly depressed from 0.62 to 0.59–0.60 for sheep exposed to cold at both levels of intake.

3. Exposure of sheep to cold resulted in a decrease in the turnover time of the particulate marker, 103Ru, from 19 h to 10.12 h in the rumen, a decrease in rumen volume, and a significant increase in dm and om which escaped digestion in the stomach. Volatile fatty acid and methane production in the rumen were highly correlated with the amount of om digested in the stomach. Methane production in the rumen comprised 0.81 of total production in warm sheep, and 0.68–0.74 of total production in cold-exposed sheep.

4. More om and non-ammonia-N were apparently digested in the intestines of sheep exposed to cold than in warm sheep at the same food intake, but the apparent digestibilities in the intestines of dm, om and non-ammonia-N leaving the abomasum did not change significantly between treatments. The retention time of 103Ru in the intestines was 17.18 h in sheep given 59 g dm food/h at both exposure temperatures, but was reduced to 12 h for cold-exposed sheep given 98 g dm/h. Methane production in the postruminal tract was increased at the higher food intake, but there was no difference between warm and cold-exposed sheep at the same food intake.

5. The rate of irreversible loss of plasma urea and rumen ammonia was measured by infusion of [15N]urea and [15N]ammonium chloride. Exposure to cold reduced the irreversible loss of plasma urea from 0.85 to 0.75–0.77 g N/g N intake, and the irreversible loss of rumen ammonia from 0.66 to 0.57–0.61 g N/g N intake. The transfer of plasma urea-N to the rumen ammonia pool was significantly greater (9.5 g N/d) in the cold-exposed sheep than the value (7.3 g N/d) in warm sheep.

6. The efficiency of microbial synthesis in the rumen was increased in cold-exposed sheep, and was related to the amount of N recycled through the rumen ammonia pool from intraruminal sources. The effect of dilution rate and fermentation patterns on efficiency of microbial synthesis is discussed.

Further studies of the dynamics of nitrogen metabolism in sheep

1. A study of ammonia and urea metabolism in sheep was made using isotope dilution techniques with (15NH4)2SO4,[15N]urea and [14C]urea in order to determine quantitatively the movements of urea-N and NH3-N throughout the body of normal, feeding sheep. 2. Single injections of 15N-labelled compounds were made into the rumen fluid NH3, caecal fluid NH3 and the blood urea pools, in order to estimate the rates of flux through, and the transfer of N between, these and other nitrogenous pools in the body. 51CrEDTA was injected into the rumen and caecum with (15NH4)2SO4 to allow estimation of fluid volumes and to provide an indication of mixing, and of times of transit of isotopes between different sampling sites in the digestive tract. 3. The sheep ate approximately 22 g lucerne chaff/h and the mean dietary N intake was 16-3 g/d. 4. The rate of flux of NH3 through the rumen NH3 pool was 15-0 g/d (i.e. 90% of the dietary N ingested; however, this amount also included N from plasma urea (1-1 g/d) and other endogenous sources including NH3 derived from caecal NH3 (0-4 g/d). 5. Only 40% of the N in isolated rumen bacteria was derived from NH3, indicating that a considerable proportion of their N requirements were obtained from compounds other than NH3 (e.g. peptides and amino acids). 6. There was evidence of recycling of N between nitrogenous pools in the rumen, probably through rumen NH3 leads to microbial N leads to NH3. 7. It was estimated that 5-3 g blood urea-N/d entered the digestive tract; 20% of this urea was degraded in the rumen, 25% in the caecum and the remainder was apparently degraded elsewhere; there was evidence of urea degradation in the large intestine posterior to the caecum and it is suggested that urea degradation and absorption of the resultant NH3 may occur in the ileum. 8. Of the 4-8 g N/d entering the caecal NH3 pool, 4-2 g N/d left and did not return and the difference (0-6 g N/d) was recycled, possibly through caecal NH3 leads to microbial N leads to NH3. 9. A large proportion of the NH3 entering the caecal NH3 pool (70% or 3-2 g N/d) was apparently derived from degradation of nitrogenous products, other than urea, including rumen microbial N (1-0 g N/d) passing undigested from the small intestine. 10. Less than half the NH3-N of caecal origin entering the rumen passed through the blood urea pool; the remainder was apparently transported as other nitrogenous compounds in the blood or body fluids. 11. The results of the three experiments were combined in a general three-pool, open-compartment model which formally recognizes an unlimited number of other unspecified, interconnected pools together comprising the whole-animal system. Rates of flux through, and transfer of N between these and other nitrogenous pools in the body were calculated by solving this model and the information derived has been applied to whole-animal models with a view to subsequently using these models in computer simulation studies.