1. The fatty acid compositions of muscle and brain phospholipids of foetal calves, neonatal lambs, deer calves and piglets, and mature cattle, sheep, deer and pigs were determined. The cattle, sheep and deer had previously grazed ryegrass–clover pastures, and the pigs had been given rations based on barley. Two steers and four sheep had been given protected polyunsaturated lipid-protein supplements.

2. In muscle phospholipids the values for triene:tetraene were 1.5 for neonatal lambs and 0.3 for foetal calves. Levels of linoleic acid were low compared with those in older animals but levels of the fatty acids 22:5ω3 and 22:6ω3 were comparatively high. For arachidonic acid there was little difference between young and mature animals.

3. In muscle phospholipids of neonatal piglets and deer calves values for triene:tetraene were low. The piglet also had a low value for 22:5ω3:22:6ω3 compared with those in deer, calves or lambs. This ratio showed a proportionately greater increase with maturity in the pig than in cattle and sheep. Whilst the neonatal deer had higher linoleic acid levels than the other young ruminants, the fatty acid composition of muscle phospholipids of mature deer was rather similar to that in other ruminants.

4. Phospholipids of brain showed little difference in fatty acid composition between foetuses or neonates and the mature animals. There was higher 22:4ω6 content in the adult ruminant with even higher levels in sheep given protected polyunsaturated fat. Linoleic acid was barely evident in any animal. The 22:6ω3 content was as high in the foetal or neonatal ruminant brain as in the adult, and higher than in the piglet. The fatty acid composition of brain phospholipids of young deer was similar to that in other ruminants.

5. In other tissue phospholipids in foetal or neonatal ruminants and piglets there were high levels of 22:6ω3 in liver and low levels in lung. The neonatal animals, in particular, had high palmitic acid levels in lung. Hearts of young ruminants contained high levels of 20:5ω3 and C16-aldehyde derived from plasmalogens. Piglet heart contained higher linoleic acid and arachidonic acid, possibly due to increased entry of linoleic acid across the placenta from the sow.

1. Information on the fatty acid composition of tissues of foetal calves, neonatal lambs, deer and piglets reported by Payne (1978) has been quantified by the use of an internal standard during analysis, to give concentrations of total polyunsaturated fatty acids (PUFA) derived from linoleic acid (ω6) and linolenic acid (ω3) expressed on a per kg tissue basis. The total concentration of both acids (ω6+ω3) was similar in all tissues examined except brain. Because muscle, the main constituent of the soft tissues of young animals, contains about 40–50 % of the total body content of these acids, it is considered that muscle concentrations are a reflection of total body status of these acids.

2. Concentrations in muscle of both ω6 derivatives and total PUFA were significantly lower in the neonatal lamb and foetal calf than in the mature animal whereas in pigs and deer the concentrations in the young animal were similar to those in the mature animal. Concentrations of ω6 derivatives and total PUFA in lambs were significantly lower than those in calves; the presence of ω3 derivatives reduced the level of significance for total PUFA. Again, total PUFA content did not differ significantly between the piglet and the young ruminants.

3. There was a substantial placental transfer, with apparently a preferential transfer of ω3 derivatives.

4. In brain the levels of ω3 acids were as high in the foetal and neonatal animals as in mature animals. The levels of ω6 acids were lower in young animals.

5. Calculations of ω6 intake from milk showed that the total deficit of ω6 could be made up within a few days.

6. It was concluded that the extent of deficiency of ω6 in young ruminants raised in a grazing situation, as in New Zealand, is marginal and any feeding to overcome this is unlikely to be of any benefit.

1. 65Zn uptake by blood cells in vitro has been compared with plasma Zn concentration and plasma alkaline phosphatase (EC 3.1.3.1) activity as indicators of an animal's Zn status.

2. Dietary Zn deficiency, low food intake, reduced dietary protein content and endotoxin administration all reduced plasma Zn concentration in the rat. In each case there was a parallel reduction in plasma alkaline phosphatase activity and an increase in 65Zn uptake in vitro by cells of whole blood.

3. A similar relationship between the three measurements existed in sheep with lowered plasma Zn concentrations whether these were caused by dietary deficiency or by post-surgical stress.

4. 65Zn uptake by cells of whole blood did not differentiate dietary Zn deficiency from the other factors which reduce plasma Zn under ‘field’ conditions.

5. 65Zn uptake by the cells in whole blood in vitro was three to five times less rapid in blood of ruminant origin than in that from non-ruminants. This difference related to the erythrocytes rather than to the leukocytes or the plasma.

1. Studies were done on the effects of giving calves two types of concentrate diet, both in a liquid form from an open pail or in a dry pelleted form, on the closure of oesophageal groove, body-weight gains and the digestibility of rations.

2. Twenty-four Holstein calves were used in an experiment by dividing into two groups of twelve calves each. The ‘dry-fed’ group (DFG) was given a pelleted concentrate, F1, from 25 to 42 d of age, and then given another type of pelleted concentrate, F2, from 43 to 87 d of age, and again given F1 from 88 to 112 d of age. The ‘liquid-fed’ group (LFG) was given from a pail a liquid suspension of the equivalent amount of the same concentrates as those fed to DFG calves, for the same periods. F1 contained skim milk and glucose with lower levels of maize and soya-bean meal than F2 which did not contain skim milk or glucose; F1 and F2 were both supplemented with 10 g methionine/kg and also supplemented with lysine at 5 and 10 g/kg respectively. All calves were allowed free access to roughage, and digestibilities were measured at 12 and 16 weeks of age.

3. Wet weights of reticulo-rumen and abomasal tissues (% whole stomach weight) were respectively 69.8 and 17.6 in DFG, and 59.0 and 26.4 in LFG at 42 d of age; at 87 d of age, the corresponding values were 68.0 and 16.3 in DFG, and 52.8 and 36.6 in LFG, and further, LFG had significantly more developed caecums and large intestines as well as significantly less developed reticulo-rumens and omasums than DFG.

4. Free amino acid patterns in plasma did not vary in DFG, but varied in LFG in response to the change in diet from F1 to F2 and to the difference between diets in lysine supplementation.

5. Feeding F1 in a liquid form and F2 in a dry form significantly improved calf gains. The intake of roughage in DFG exceeded that in LFG especially when F2 was fed.

6. The digestibility of diethyl ether extracts did not vary with feeding procedures, but that of crude protein (nitrogen × 6.25) was significantly higher in DFG independent of the type of diet. Digestibilities of dry matter and N-free extracts were significantly higher in LFG for F1, while higher in DFG for F2. The digestibility of acid-detergent fibre did not differ significantly between groups at 12 weeks of age, but was significantly higher in LFG at 16 weeks of age, suggesting a possible compensatory contribution of the large intestine to the digestion of fibre in LFG.

7. The efficiency of liquid feeding of a concentrate diet to ruminants compared with that of dry feeding is discussed.

1. Effects of incorporating 30 g leucine/kg into diets on quinolinate phosphoribosyl transferase (QPRT; EC 2.4.2.19) activity and leucine aminotransferase (EC 2.6.1.6) activity were studied in groups of rats receiving 5, 30 and 60 μg of pyridoxine/10 g diet.

2. The results indicated that 30 g leucine/kg diet significantly reduced the QPRT activity when the diets provided 5 μg pyridoxine/10 g and that the effect was only marginal when the diet included 30 μg pyridoxine/ 10 g. The inhibitory effect was completely absent when the diet provided higher amounts of pyridoxine (60 μg/10g).

3. These results suggest that additional amounts of pyridoxine are necessary to counteract the effects of excess of leucine in the diet.

4. Leucine aminotransferase activity was increased in rats given diets containing higher amounts of pyridoxine; supplementary leucine also increased the enzyme activity.

1. Food intake and rates of protein, lipid and energy deposition during growth were measured for lean and congenitally obese (fatty) Zucker rats offered from 34 d of age to slaughter at 66 d of age, one of six semi-synthetic diets containing casein (C) in the following amounts (g C/kg): 40C, 100C, 150C, 300C, 500C and 700C.

2. In Expt 1, groups of four male rats were offered each diet to appetite. The digestibility of dietary protein and metabolizability of dietary energy were determined. Total carcasses were analysed for protein, energy and lipid at 34 and 66 d of age. The results showed that, given diets containing 300C or above, both fatty and lean males regulate food intake so as to sustain a maximal rate of protein deposition. This maximal rate was greater in males than in females, and the sex difference was more marked in lean rats. Diets containing less than 300C did not permit maximal protein deposition and, in this instance, both sexes and phenotypes showed a similar reduction in food intake and protein deposition. The rate of deposition of body lipid did not appear to be controlled in either phenotype.

3. In Expt 2, fatty and lean rats were pair-fed diets 100C and 500C. Carcass composition at 66 d of age confirmed that obesity in the fatty rat was not due to hyperphagia but to an abnormal pattern of energy utilization between fat deposition (too much), and protein deposition and heat production (too little).

4. In Expt 3, fatty and lean, male and female rats were given diets 100C and 500C to slaughter at 66 d of age. The carcasses were analysed into different parts by weight, and according to protein and lipid contents of viscera, pelt and subcutaneous fat, and empty carcass. Fatty rats stored approximately 0.53 of their protein in the empty carcass, lean rats approximately 0.65.

5. The results confirm that food intake in the Zucker rat is intimately related to the capacity of the animals for protein deposition, but this capacity differs between sexes and between phenotypes, and the distribution of body protein in the fatty rat eating ad lib. is not that of a normal rat.

1. The activities of alkaline phosphatase (EC 3.1.3.1) and phytase (EC3.1.3.8) were similarly distributed in the small intestine of rats. Regional differences in activity were reflected by similar differences in the capacity of ligated intestinal segments to hydrolyse phytate in vivo. Activities were greatest in the duodenum and lowest in the terminal ileum.

2. Specific activities of both enzymes were tenfold greater in the brush border fraction of duodenal mucosa compared with entire mucosal homogenates.

3. Brush-border alkaline phosphatase and phytase activities required both magnesium and zinc ions for maximal activity.

4. Zn deficiency induced by feeding a diet low in Zn (0.5 mg Zn/kg) caused similar reductions in activity of both enzymes.

5. Zn deficiency induced by feeding diets marginally adequate in Zn (12 mg/kg) and phytate (10g/kg) caused reductions in alkaline phosphatase, phytase activities and phytate hydrolysis in vivo.

6. It is suggested that phytase activity is a manifestation of alkaline phosphatase and the significance of this in relation to phytate-induced Zn deficiency is discussed.

1. The effect of daily supplements of 20–30 mg inorganic iron as ferrous sulphate on the growth, activity and haematological status of preschool children was studied for 3.5, 7 and 12 months and compared to that of children who served as controls. All children were given their daily requirements of energy and protein. In addition, they received 5 μg cyanocobalamin and 200 μg folic acid.

2. Fe supplementation increased the haemoglobin, serum Fe and percentage saturation of transferrin and reduced the unsaturated Fe-binding capacity significantly compared to corresponding values for the controls.

3. Height and activity were unaffected by Fe supplements.

4. Of the children 45% had haemoglobin values below 110 g/l at the end of 7–12 months of Fe supplementation.

1. The effects of chronic ethanol administration on the metabolism of ethanol and acetaldehyde were studied in rats fed on a commercial diet containing an aldehyde dehydrogenase inhibitor, calcium cyanamide (calcium carbimide), as a contaminant in the calcinated bone-meal fraction of the diet.

2. Rats given an ethanol solution (150 ml/l) for 3 months and fed on a diet containing calcinated bone meal showed two times higher activity of the low-Km acetaldehyde dehydrogenase in the liver, 26 % higher rate of ethanol elimination, and two to three times lower acetaldehyde levels in blood during ethanol elimination compared with control rats pair-fed on the same diet.

3. The results obtained from the ethanol-treated rats were similar to those obtained in experiments on control rats fed on diets not containing calcinated bone meal.

4. Experiments performed in vitro and in vivo on the inhibition of the acetaldehyde metabolism by cyanamide suggested that the apparent effects of chronic ethanol intake were really caused by calcium cyanamide in the diet.

1. [35S]sulphate was used to obtain quantitative estimates of the transfer of sulphur between the blood, rumen and postruminal tract of four sheep given brome grass (Bromus inermis) pellets or lucerne (Medicago sativa) pellets at the rate of 33 or 66 g/h. Sodium sulphate (0–4 g S/d) was infused into the rumen or abomasum of sheep given brome grass during four periods of 19 d and was not infused into the sheep during a subsequent period in which lucerne was given. The flow of sulphide, sulphate, microbial S and non-microbial organic S from the abomasum was estimated using 103Ru and 51Cr.

2. The concentration of inorganic sulphate in serum was increased to maximum values of 35–46 mg S/l by infusion of sulphate into the rumen or abomasum. The rate of irreversible loss of serum sulphate and rumen sulphide was positively related to the amount of sulphate infused.

3. Reabsorption of sulphate by the kidney reached a maximum of 0.69–1.1 mmol sulphate/l glomerular filtrate.

4. The transfer of sulphate from blood to the rumen was related to the concentration of inorganic sulphate in serum, attaining maximum values of 133 (±13) mg S/d for sheep given brome grass plus sulphate, and 127–159 mg S/d for sheep given lucerne.

5. Bacteria derived 0.52–0.67 of organic S from rumen sulphide in sheep given brome grass, and approximately 0.45 of bacterial organic S was derived from sulphide for sheep given lucerne. Protozoa derived approximately 0.90 of organic S from bacteria.

6. It was estimated that endogenous organic S contributed 300–340 mg S/d to the rumen, and that 0.24–0.45 of S digested in the rumen was derived from endogenous sources.

1. Rats bred from vitamin B12-depleted dams were fed on a vitamin B12-deficient diet for 12–15 months and developed a severe vitamin B12 deficiency, as judged from methylmalonic acid excretion and tissue vitamin B12 levels at slaughter. Control rats were supplemented with vitamin B12 in the drinking-water.

2. Neurological signs were recorded after 7 months but the motor nerve conduction velocities remained normal. Neuropathological examination revealed mild changes in the peripheral nerves but no changes in the central nervous system.

3. The amounts of total lipids and phospholipids were normal, but in all examined tissues the proportions of pentadecanoate (C15 fatty acid) and heptadecanoate (C17 fatty acid) were considerably increased in vitamin B12 deficiency.

4. 3H2O was incorporated to the same extent into the fatty acids of nervous tissue from vitamin B12-deficient and control rats after 48 h. Less 3H was found in the liver fatty acids of the vitamin B12-deficient rats.

5. Neurological dysfunction can be demonstrated in the vitamin B12-deficient rat; the relation of the biochemical and neuropathological changes to the neurological signs needs further study.

1. Responses of hepatic glucose-6-phosphate dehydrogenase (EC 1.1.1.49; G6PD), malic enzyme (EC 1.1.1.40; ME), acetyl-CoA carboxylase (EC 6.4.1.2; ACAC), and fatty acid synthetase (FAS) were studied in male Wistar rats after a period of starvation and refeeding of diets containing 400 g glucose, or raw or cooked starches as the source of carbohydrate/kg. Starches fed included maize, potato, wheat, rice, and tapioca.

2. When compared to the responses of rats given the glucose-containing diet, rats given raw maize- or rice-starch-containing diets had a significantly lower ME response, and rats given raw potato starch had significantly lower responses of G6PD, ME, ACAC, and FAS. The enzyme responses of rats given cooked starches were similar to those of glucose-fed rats, except that rats given cooked wheat starch had significantly lower G6PD and ME responses than did glucose-fed rats.

3. When the enzyme responses to refeeding of the same starch source in either raw or cooked form were compared, it was found that (a) the FAS response was significantly higher to cooked than to raw maize starch, (b) the G6PD and ACAC responses were significantly higher to cooked than to raw tapioca starch, (c) the G6PD, ME, ACAC, and FAS responses were significantly higher to cooked than to raw potato starch.

4. The results suggest that reported differences in the lipogenic enzyme responses between simple sugars and starch may in some instances be magnified because of the use of uncooked starches in experimental diets.

1. A 30-week study has been made in growing ewe lambs of the effects of dietary supplementation with molybdenum, or Mo plus inorganic sulphate on the distribution of copper in their blood plasma and kidneys.

2. The addition of 25 mg Mo and 5 g SO42-/kg diet increased Cu concentrations in plasma and kidney and decreased those in liver. Plasma caeruloplasmin activities (ferroxidase; EC 1.16.3.1) were unaffected.

3. Subcellular fractionation of the kidney cortex and gel filtration of the plasma and kidney cortex cytosol showed that the increased Cu content of these tissues was associated with abnormalities in the distribution of Cu. It appeared that both Cu and Mo were associated with the same proteins.

4. Dietary supplementation with Mo alone (25 mg/kg) had no effect on plasma or kidney Cu concentrations, suggesting that S metabolism is involved in the formation of the abnormal Cu-binding proteins in plasma and kidney.

5. The results are discussed in relation to the induction of Mo-induced Cu-deficiency states and the possible involvement of thiomolybdates.

1. Digesta were collected from twenty-three pigs, initially of 30 kg live weight, and fitted with single Ash re-entrant cannulas in either the duodenum, jejunum or ileum. A further twenty-four pigs were used in a conventional digestibility trial.

2. The diets contained: barley, fine wheat offal, white fish meal, minerals and vitamins (diet BWF); starch, sucrose, maize oil, cellulose, minerals, vitamins and either groundnut (diet SSG) or casein (diet SSC).

3. The flow-rates of dry matter (dm), ash and water were measured hourly in the duodenum and jejunum, and every 6 h in the ileum during 24 h collection periods. Faeces were collected during 5 d periods.

4. Marked increases in the flow rates of each of the digesta components after feeding each diet were observed in the duodenum and jejunum, but not in the ileum. The total flow in 24 h periods was much lower in the ileum than at the other sites.

5. Values for the ratio, dm output:intake for dm outputs from the duodenal, jejunal or ileal cannulas, and in faeces, in 24 h periods were respectively 0.94, 0.81, 0.28 and 0.22 for diet BWF; 0.96, 0.76, 0.20 and 0.15 for diet SSG; 0.96, 0.73, 0.08 and 0.04 for diet SSC. The corresponding values for ash were: 1.47, 1.65, 0.74 and 0.53 for diet BWF; 1.28, 1.34, 0.59 and 0.51 for diet SSG; 1.63, 1.35, 0.50 and 0.26 for diet SSC. The corresponding values for water were: 3.49, 3.41, 0.88 and 0.18 for diet BWF, 2.75, 2.80, 0.80 and 0.08 for diet SSG and 2.61, 1.87, 0.23 and 0.01 for diet SSC.

1. The renewal of the intestinal mucosal epithelium has been investigated in germ-free and conventional chicks given a practical chick diet and a low- and high-fibre diet, using tritiated thymidine to label the nuclei of mucosal cells undergoing division.

2. Villus height, crypt depth and number of mitoses in the crypt were generally, although not always significantly, greater for conventional chicks than for germ-free chicks at all sites investigated in the intestine, and all became progressively lower from the upper intestine to the lower intestine.

3. There was a linear relationship between the height of the radioactively labelled mucosal cells on the villus and the period after injection, and the rates of epithelial cell migration were higher for conventional than for germ-free chicks, the relative differences being greater in the lower intestine than the upper intestine.

4. The inclusion of wheat bran as a source of dietary fibre had no effect on the rate of epithelial renewal, either in germ-free or conventional birds.

1. Methane production rates were estimated simultaneously in the rumen and caecum of sheep given 200, 400, 600, 800 and 1000 g lucerne (Medicago sativa) chaff/d using isotope dilution techniques. Estimates were also made of volatile fatty acid (VFA) production in the rumen at each level of feeding. In all studies three to four animals were used at each level of intake.

2. Production of VFA and of methane were both related to digestible energy (de) intake. Regression lines for both VFA production and methane production v.de intake had significant intercepts indicating an input of endogenous, fermentable organic matter into the rumen in excess of 50 g/d.

3. The values obtained for rates of methane production were compared with those calculated from stoicheiometric equations relating rates of methane and VFA production. Comparisons of methane production with that predicted from de intake were also made.

4. Balances for digestion of food determined for the rumen indicated that the energies in the end-products were more than 100% of the de intakes at low intakes of lucerne chaff. Correction for fermentation of apparent endogenous materials resulted in more realistic values. Endogenous materials appeared to make a significant contribution to VFA and methane production, particularly at low levels of intake.

1. Groups of three colostomized germ-free (GF) and conventional (CV) chickens aged 4 months were maintained for successive periods of 8 d on a diet containing 200 g casein/kg without and with sodium bicarbonate at the rate of 20 mmol/d and a nitrogen-free diet without and with NaHCO3 at 9 mmol/d. Urine and faeces were collected during the last 3 d of each period.

2. Total N, uric acid- and ammonia-N were determined in urine and total N in faeces. Amino acids were measured in hydrolysates of faeces collected during the periods when no NaHCO3 was included in the diets.

3. The CV birds excreted more N on the casein diets but less on the N-free diets than did their GF counterparts, the differences being mainly shown in the urine.

4. On both diets hydrolysates of the faeces of CV birds contained smaller amounts of amino acids. On the N-free diet the proportions (g/160 g N) of serine, proline and threonine were reduced, suggesting some conservation of endogenous N by micro-organisms, and the proportions of histidine, alanine, lysine and methionine increased, possibly through microbial synthesis; on the casein diet, proportions of most amino acids were less, probably because bacterial deamination had occurred.

5. Urinary excretion of total N, uric acid and ammonia was much greater on the casein than on the N-free diets. Inclusion of NaHCO3 caused a sharp fall in urinary ammonia on both diets and in both environments.

6. It was concluded that the level of dietary protein and the regulation of acid-base balance have more effect than microbial activity on the urinary ammonia excretion.

1. Pigs growing from 20 to 60 kg live weight were given diets based on barley, weatings and fish meal, or starch, sucrose and groundnut meal or starch, sucrose and casein. Seventeen pigs were fitted with single re-entrant cannulas in the duodenum (posterior to entry of bile and pancreatic ducts), jejunum or terminal ileum and twenty-four non-cannulated pigs were used in a conventional digestibility trial.

2. The amounts of calcium, phosphorus, magnesium, sodium and potassium passing through the reentrant cannulas and amounts excreted in the faeces were measured. These values were used to calculate the direction and extent of net movements of the five elements through the walls of the four parts of the digestive tract anterior to the collection sites.

3. The small intestine was the principal site of Ca and P absorption but there were differences between the diets in the relative importance of the regions anterior and posterior to the mid-jejunum.

4. Secretion of small amounts of Mg occurred in the anterior small intestine; the ileum and large intestine were the principal sites of net absorption.

5. There was a large net secretion of Na anterior to the duodenal cannulas and further secretion into the anterior small intestine with each diet. There were marked differences between diets in the amounts secreted but the ileal Na concentration was the same in each instance. Absorption occurred in the ileum and large intestine.

6. Secretion of small amounts of K was evident anterior to the duodenal cannulas and net absorption occurred in both parts of the small intestine with each diet.

1. Placental blood flow rate and calcium transfer rate were measured at 61 d of pregnancy in guinea-pigs carrying between one and eight foetuses.

2. Placental blood flow rate was significantly correlated with foetal weight. Ca transfer rate was related to placental size. Irrespective of litter size the mean amount of Ca transferred across a placenta was between 0.22 and 0.34 mg/h per g placental tissue.

3. It was concluded that there was a limit to the rate of transfer which was produced by a combination of limitations in placental blood flow rate, maternal plasma Ca concentration and placental tissue transfer capacity.

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.