Net Availability Of Metabolizable Energy Research Articles

Energy and protein value of lupin seed as a production ration or as a supplement for sheep fed chaffed wheaten hay

Lupin seed was evaluated as a supplement to cereal hay and as the sole component of a production ration, by measuring energy, nitrogen (N), and carbon balances in 4-6 adult wether sheep. Seven diets of different combinations of lupin seed and chaffed wheaten hay were fed in sequence to 4 sheep. The hay was offered as a restricted ration of 800 g/day with and without 100 g lupin seed/day, and ad libitum with and without 2 levels of lupin seed (100, 400 g/day). In addition, lupin seed alone was offered at near maintenance and ad libitum to 6 sheep. The dry matter (DM) content of the wheaten hay contained 7.7 g N/kg DM and 572 g cell wall organic matter (CWOM), and its gross energy (GE) value was 18.1 MJ/kg DM. The GE was 49% digestible, and its metabolisable energy (ME) value was estimated at 6.4 MJ/kg DM at ad libitum intake levels. Corresponding values for the lupin seed fed alone were 49 g N, 199 g CWOM, 20.1 MJ GE, 86% digestible energy, and 15.7 MJ ME/kg. Ad libitum intakes of wheaten hay were below that required to sustain energy maintenance, but total DM intake was increased substantially with 100 g/day of lupin seed supplement. At higher levels of supplementation some substitution did occur, although the response was variable. Methane production was reduced by increasing the level of lupin seed in the ration, but ME and heat (both expressed in relation to DE) and energy balance (expressed in relation to ME) could each be effectively described by single rectilinear relationships, suggesting that the energy value of lupin seed was similar if used as a production ration or as a supplement. Energy values of lupin seed fed alone compared favourably with published values for other leguminous seeds and some cereal grains. Net availability of ME for gain was estimated to be 61%. Urinary N loss, in terms of apparently digested N or available energy, was very high when lupin seed was fed alone, indicating that protein retention was low compared with legume forages of similar N and cereal grain (e.g. oats). Therefore, although leguminous seeds have rely high levels of protein and obvious potential as energy and N supplements, it may not be valid to presume they are ipso facto important sources of amino acids for ruminants. High energy availability and a low potential for protein deposition would imply a significant diversion of energy into fat with such diets.

Energy exchanges of broilers, selected for fatness and leanness, on diets with and without a repartitioning agent

1. Two experiments were undertaken for 3 or 5 d in respiration chambers, on two experimental lines of broiler chickens (aged 25-38 d) selected for leanness and fatness. Diets were without or with 0.4 mg cimaterol per kg. 2. The lean line with sexes combined (experiment 1), or with females only, had a significantly greater heat production than the fat line. Net availability of metabolisable energy for gain (kg) was 0.54 for the lean birds and and 0.84 for the fat birds. 3. Cimaterol did not have an effect on any of the variables examined.

Calorimetric measurements made on rats during repeated periods of weight gain and weight loss

1. 1. Rats in respiration chambers were grown for 32 days and their weight reduced by 27–40% on three occasions and realimentated. After the first cycle rats lost body weight more rapidly and regained that weight more quickly than previously. 2. 2. The initial growth phase took 32 days compared with only 8 days in the final period. Each gram of weight gain was associated with 13–14 kJ of dietary net energy during the last two periods of realimentation compared with 19–20 kJ during the first period. 3. 3. Calorimetric measurements showed that although maintenance energy requirement increased during the periods of growth, mean net availability of metabolizable energy was 0.91 compared to 0.60 during weight loss. 4. 4. During the final period much of the weight gain was in the form of lean (67%) but the majority of energy retained was as fat (67–70%).

Energy and protein values of four forages, including a comparison between tropical and temperate species

Artificially dried grass of 2 tropical species (Setaria spacelata and Digitaria decurnbens) from which most of the stem had been removed, and a temperate grass (Loliurn perenne) grown under the same subtropical conditions, were compared for energy and protein value. Data for a temperate clover hay (Trifolium resupinatum) that was tested concurrently are included because they provide useful contrasts. Each was evaluated by measuring energy, nitrogen (N) and carbon balances in 4 adult sheep at 2 levels of intake (ad libitum and near energy maintenance) and during fasting. The 3 grasses had similar leaf content (76-83%) and the clover much less (60%). The tropical forages had similar chemical composition but they contained less crude protein (10%) than the ryegrass (L. perenne) (1 5%) and more cell wall constituents (63-66% v. 58%). The clover had the most crude protein (19%) and the least cell walls (38%); it also was highest in carbohydrates, pectin and heat of combustion. Voluntary intakes of the setaria, pangola (D. decumbens) and ryegrass were similar and 20-30% lower than the intake of clover. Digestibilities of dry matter (DM), organic matter (OM) and energy were similar for the grasses and 8 percentage units lower than for the clover whereas digestibility of crude protein differed between the tropical and temperate species, the latter having digestibilities 12-15 units greater. Digestibility of cell walls did not vary. The metabolisable energy (ME) values of the grasses at maintenance were 8.3-8.5 MJ/kg DM while that of clover was 10.1; at maximum intakes, corresponding ME values were 7.9 and 10.0 MJ/kg DM. Net availability of ME for gain was 0.42 for the tropical grasses and the persian clover and 0.36 for the ryegrass. By contrast, the tropical forages supported higher gains of N relative to N supply than did the temperate forages because urinary N loss was not affected by intake of the tropicals; these differences in N gain were drastically reduced when gain was expressed in relation to ME. These results do not support the contention that forages of tropical species have a lower energy value per kg than temperate forages of similar composition. Indeed, the tropical species tested here appear to have had an ideal balance of energy and protein insofar as efficiency of use of digestible N was close to 100% and their energy values were commensurate with their chemical composition.

Energy values of whole lucerne (Medicago sativa) and of its stem and leaf fractions in immature and fully grown sheep

Chopped lucerne hay (Medicago sativa) and a stem fraction derived from it were fed to two adult and two immature wethers ad libitum and at a level near maintenance. Energy, nitrogen, and carbon balances were measured during feeding and fasting. The hay contained 17% crude protein and 46% cell wall constituents (dry matter basis) and the stem, which was 53% of the total, contained 10% crude protein and 64% cell wall. Voluntary dry matter consumption rates of the hay (per kg 3/4) were 103 and 145 g/day by the adults and immatures, respectively; the corresponding values for the stem were 73 and 100 g/day. Maximum daily energy balances were 290-3 16 kJ/kg3/4 for the hay and approximately maintenance for the stem. With both ad libitum and restricted feeding, energy digestibility was higher for the hay (56- 63%) than for the stem fraction (45-51%). The metabolizable fraction of digestible energy was 78% at the low and 82% at the high level of feeding and tended to be greater with the stem than with the hay. At the lower feed intake, metabolizable energy was about 10 and 8 MJ/kg organic matter for whole lucerne and stem respectively. Net availability of metabolizable energy was 64 and 49% for maintenance and gain on the hay, compared with 53 and 34% on the stem. As estimated by difference, the energy values of leaf were: digestible energy, 76%; metabolizable energy, 77% of digestible energy or 12.4 MJ/kg organic matter; net availability of metabolizable energy, 78% for maintenance and 60% for gain. All these figures are for the adult sheep; the immature animals gave values that were lower to various degrees. Consideration of the present results together with published data for other samples of lucerne suggests that the use of equations based on study of grasses to predict the energy values of lucerne is likely to introduce significant bias. Equations for this limited set of data on lucerne are given, gross energy being related to crude protein content, metabolizable energy to crude fibre and net availability of metabolizable energy to metabolizable energy content.

Energy and nitrogen metabolism of broilers selected over ten generations for increased growth rate, food consumption and conversion of food to gain.

Energy and nitrogen (N) metabolism were studied in 6-week-old male birds taken from 4 lines of chickens selected for 10 generations for increased weight gain (line W), increased food consumption (line F), increased conversion of food to gain (line E) or at random (controls, line C). Calorimetric measurements were made 8 times on each line while fed ad libitum in large open-circuit respiration chambers for 3 d, and 11 to 13 times without food in smaller closed-circuit respiration chambers for 24 h. The F line ate 60% more food, produced 90% more excreta and 34% more heat and retained 80% more energy and 35% more N in their bodies than lines E and C. Line W was intermediate. When differences in body weight were taken into account, the E and W lines had lower heat production than the C line, while the F line ate 40% more food, produced 30% more heat and retained 70% more energy and 30% more N than the E line. In lines W, F, E and C respectively, the mean metabolisability of dietary energy (%) was 69.4, 62.9, 70.1 and 67.8; the fasting heat production (mean +/- SE) was 481 +/- 9, 569 +/- 10, 485 +/- 9, and 508 +/- 9 kJ/kgW d; the net availability of metabolisable energy (NAME) was 0.68 +/- 0.05, 0.76 +/- 0.04, 0.85 +/- 0.06 and 0.73 +/- 0.04; the estimated daily maintenance energy requirements were 671 +/- 15,866 +/- 14,701 +/- 13, and 742 +/- 11 kJ ME/kgW; and the proportion of N retained per unit increase in N intake was 0.38 +/- 0.08, 0.50 +/- 0.06, 0.56 +/- 0.10 and 0.53 +/- 0.06. The contribution of line differences in the above traits to large line differences in efficiency of food utilisation is discussed.

Calorimetric studies on selected lines of Japanese quail

Calorimetric measurements were made on groups of Japanese quail selected for increased body weight at either 20 or 35 days on either a high- or low-nutrient density diet. For the majority of biological parameters measured, the selected lines outperformed the unselected controls. Weight gain, metabolizable energy (ME) intake, food conversion efficiency, and energy retention were greater for the selected than for the control line. The major difference between the two groups was the higher mean food consumption of the selected lines. Quail selected at 35 days on the high nutrient density diet ate more food, gained more weight and retained more energy and body fat than those selected at the same age on the low nutrient density diet. Net availability of ME for growth was the same (0.69) for the selected and unselected lines; it was 0.66 for protein deposition and 0.83 for fat deposition. Maintenance energy requirement for eight quail was 1.2 MJ ME (kg wt)-1 day-1. Retention of dietary nitrogen was 0.61 of nitrogen intake. There was no difference between the lines in starvation heat production. However, birds fed ad libitum prior to starvation had a 12% higher heat production than those restricted to 70% of ad libitum feed.

Influence of diet and microbial activity in the digestive tract on digestibility, and nitrogen and energy metabolism in rats and pigs

1. Balance trials with respiration measurements were performed with twelve rats and twelve pigs given either low- or high-crude-fibre diets. There were six collection periods with the rats over a live-weight range of 86-264 g and three collection periods with the pigs over a live-weight range of 30-55 kg. Measurements were made on the influence of microbial activity in the digestive tract on digestibility and nitrogen and energy metabolism. Dietary inclusion of the antibiotic Nebacitin was the method used to reduce the microbial population. 2. The microbial activity in the hind-gut (mumol ATP/g air-dry contents) of antibiotic-treated rats was reduced to approximately one-tenth of that of untreated rats. 3. Live-weight gain was not significantly affected in either species by a reduction in the microbial activity, in spite of a reduction in dry matter digestibility in animals with reduced microflora. 4. For rats on low-crude-fibre diets, a reduction in microflora reduced digestibility of all nutrients and energy and metabolizability of digestible energy by approximately 5.4%. All differences were highly significant. On high-crude-fibre diets the decrease was approximately 5.9%. In pigs on both crude fibre levels, the digestibility was also influenced by the level of microflora, but the pattern was somewhat different from that obtained with rats, with the Nebacitin treatment increasing the digestibility of N slightly, and the digestibility of fat markedly. 5. Retained N in rats reached a maximum when the rats were approximately 60 d old and thereafter decreased with increasing age. However, for pigs daily N retention increased with age. The retained N: digested N value decreased linearly with age in the rats, but varied little with age over the range (104-146 d) studied in the pigs. 6. The metabolizability of gross energy (metabolizable energy (ME): gross energy) was significantly reduced with an increase in crude fibre level and by the addition of Nebacitin. 7. Retained energy (RE) in relation to ME (RE:ME), was not significantly affected either by level of microbial activity or by crude fibre. 8. The ratio, RE as fat (RF): RE as protein (RP) increased as the animals grew. In the rat experiment there was a tendency for RP to be higher for animals with normal microflora than for animals with reduced microflora for both crude fibre levels. 9. With rats, the regression analyses indicated that the energy requirement for maintenance could be influenced by both the level of microbial activity in the digestive tract and by the level of fibre in the diet. The net availability of ME for maintenance and growth by rats averaged 0.72 for all treatments. 10. The net availability of ME for growth in the pigs averaged 0.65 for all treatments.

Effects of food intake regulation on the energy metabolism of laying hens

1. The effects of dietary energy restriction on the energy metabolism of post‐peak‐of‐lay hens of two hybrid layer strains were studied by indirect calorimetry. 2. Starving and resting rates of heat production (SHP and RHP) were measured, over 1‐d periods, at intervals during a 25‐week period in which the experimental birds were individually restricted to 80% of their previous energy intake ad libitum. 3. In both strains mean RHP per bird was about 7% lower in the restricted birds than in controls fed ad libitum, but when RHP was expressed in terms of metabolic body size (kg0.75) the two groups did not differ. 4. Mean SHP per bird was about 18% lower in the restricted birds of both strains than in the corresponding controls; the decrease in SHP per kg0.75 was 12%. 5. Heat increment of feeding and calculated maintenance energy were higher, and net availability of metabolisable energy for maintenance and production was lower, in the restricted than in the control groups. 6. Gross efficiencies of egg production, in terms of both mass and energy, increased in the restricted birds. 7. Live‐weight and total carcass energy after 25 weeks of restriction were respectively about 15% and 30% lower in the restricted groups of both strains than in the groups fed ad libitum.

Effects of temperature treatments on the energy and nitrogen metabolism of fed chickens.

1. Calorimetric measurements were made for 5 d on individual broiler chickens (1 kg) fed ad libitum and acclimated or unacclimated to six temperatures from 2 to 35 °C (constant), or alternated between these temperatures and 22 °G (alternated). For the four treatments, heat production was related to temperatures and the resultant curves were significantly different. 2. Food intake, heat production and maintenance energy requirement all increased linearly with decreasing temperature. Metabolisability of the diet was only 13.8 kJ/g at 35 °C, compared with a mean value for all treatments of 14.1 kJ/g. 3. Energy retention and nitrogen (N) retention (g/d, or % dietary N) were maximal at 22 and 16 °C, however the amount of energy deposited as protein remained relatively constant below 30 °C. 4. Net availability of metabolisable energy was calculated in two ways: by calculating the increase in heat production of fed birds above their starvation values giving a mean value of 0.82; this was similar to the mean regression coefficient which included starvation data, and related ME intake and energy retention; but without these data availabilities ranged from 0.45 at 35 °C to 1.0 in the cold. 5. Acclimation or alternating of temperature had very few significant effects, however there were temperature x acclimation effects on N retention and heat production. Similarly alternating temperature significantly increased food intake and heat production at high temperatures, but decreased metabolisability of the diet to 13.7 kJ/g at 35 °C from an overall mean of 14.1 kJ/g.

Observations on the effect of environmental temperature and environment at moult on the heat production and energy requirements of hens and cockerels of a White Leghorn strain

SUMMARYThe heat production of hens and cockerels of a White Leghorn strain (‘H & N’) was measured after acclimation to environmental temperatures of 16, 23, 27 and 33 °C. No difference in fasting heat production was found between 16 and 23 °C for well-feathered hens in the first few months after moult though there were substantial reductions at 27 °C and above. Both hens and cockerels showed a trend of increasing net availability of metabolizable energy with increasing environmental temperature though this was non-significant. There were significant reductions between 16 and 27 °C in the metabolizable energy requirements for maintenance for both sexes.Results are also given for the heat production of laying hens exposed to short daylight periods and high or low environmental temperatures. Although the mean fasting heat production of the high-temperature group was greater, the difference was not statistically significant.

The effects of daily and weekly feeding regimes on the efficiency of energy utilization by sheep given all-wheat diets

The metabolizable energy (ME) content of an all-wheat ration was similar whether given to sheep at a rate of 300 g each day or at 2100 g once each week. However, the net availability of ME was 79 % in the former group compared with 70 % in the latter, owing to the significantly (P < 0.05) higher heat production of sheep given feed once weekly. It is suggested that the increased heat production resulted from fluctuations between storage of energy and oxidation of tissue reserves associated with the extended feeding interval.

The utilization of the energy of artificially dried grass prepared in different ways

SUMMARYSheep were given artificially dried grass either in the long form or as pellets, and their energy retention measured. Experiments were made both at the maintenance level of feeding and when feed was offered ad libitum. An analysis of the effects of preceding and current daily feed consumption on heat production was made and both exact and approximate methods of estimating the mean food intake to which a determination of heat production should be referred, are given. Pelleting the dried grass increased voluntary intake of dry matter by 27%, and the intake above maintenance by 59% The metabolizable energy of the grass was depressed by pelleting but the net availability of metabolizable energy for production was 52% for pellets and 40% for long material. The percentage of the gross energy supplied above maintenance which was retained was 28% greater for pellets than for long dried grass. The results show that energy retention of the sheep was almost doubled when the grass was pelleted.

Balances of energy and matter in growing sheep at several ages, body weights, and planes of nutrition

Energy, carbon, and nitrogen balances in young sheep (aged 5, 10, and 14 months) given a roughage-concentrate mixture were measured in closed-circuit respiration apparatus. Animals of diverse weight but the same age and vice versa were studied at levels of feeding between half maintenance and ad libitum. Digestibility decreased by 7 units between the lowest and highest feeding level but was not shown to depend on weight or age. Methane production was lower at 5 months of age than subsequently and, in consequence, metabolizable energy decreased from 3.8 kcal/g digestible organic matter at 5 months of age to 3.6 kcal/g at 10 and 14 months. Net availability of metabolizable energy was measured directly in 10-month-old sheep only, and was then 52 and 62% in animals of 44 and 28 kg respectively. Otherwise, storage of metabolizable energy was not found to vary with weight or age. The young sheep had higher maintenance requirements and therefore lower overall efficiency than adults would have, but had higher net availability of metabolizable energy. Neither appetite nor efficiency was depressed by obesity in 14-month sheep. The fraction of energy balance accounted for by protein did not depend on age but varied inversely with body weight. Two obese animals continued to store a large fraction of energy in fat.

The energy requirement for maintenance and production of laying hens.

Heat production was recorded by open-circuit respiration calorimetry. In the first 3 experiments heat production after 24 h without feed by birds laying at about 80% production was measured. Mean daily fasting heat production of 12 White Leghorn hens of 1.68 kg bodyweight caged individually, 12 White Leghorn hens of 1.68 kg caged in pairs and 10 Australorp x Rhode Island Red hens of 2.4 kg caged in pairs were 97.0, 98.0 and 90.6 kcal per kg W0.75. The overall relation between heat production (H) and W0.75 was H=85.2 W0.75+16.6. The relation between daily energy retention (Y) in kcal per kg W0.75 and intake of metabolisable energy (ME) (X) in kcal per kg W0.75, established with 37 White Leghorn hens at peak and end of production, after 12 months, was Y= 0.642 X-65.7. The net availability of ME for egg production was 64.2+or- 5.5% and the daily maintenance requirement was 102+or-7.6 kcal per kg W0.75. The result was applied to a laying trial with 1020 hens in batteries and 300 on deep litter to get a measure of ME used for maintenance. After subtraction of ME used for production the calculated mean for maintenance was 131 and 138kcal per kg W0.75 for hens in batteries and on deep litter. ME for maintenance was greater in winter.-D. W. F. S. (Abstract retrieved from CAB Abstracts by CABI’s permission)

The net energy value of artificially dried subterranean clover harvested before flowering

Subterranean clover (Trifolium subterraneum L. cv. Clare) was harvested before flower emergence and dried rapidly without heating. Each of four adult sheep was given the dried clover at rates of 200, 600, 1000, and 1400 g/day in four successive fortnights and was finally fed ad libitum for 1 month. Energy, nitrogen, and carbon balances and body weight changes were measured. The fasting energy losses of the sheep were determined at the end of the experiment. The hay contained approximately 27% crude protein, 14% crude fibre, and 3% lignin (dry matter basis) and was eaten avidly by the sheep. Voluntary consumption ranged from 1400 to 2100 g dry matter/day, and maximum balances were 2080 kcal and 17 g nitrogen/day. Digestibility of energy was 78% at the lowest level of feeding and 74% at the highest. Cell wall and cell contents were respectively 74 and 82% digestible. Energy losses in methane were unusually low (7–11% of digestible energy) whereas losses in urine were unusually high (12–21%). The net result was that metabolizable energy was a lower fraction of digestible energy than for most forages, e.g, it was c. 77% at maintenance instead of the normal 82%. Net availability of metabolizable energy was 84% between fasting and half maintenance, 69% between fasting and maintenance, and 54% above maintenance. The value of this clover per gram was in keeping with its chemical composition and digestibility; its exceptional value as a feed was due to the large amounts of digestible energy which the sheep took in when fed ad libitum.

The net energy value of three subtropical forages

Energy and nitrogen balance experiments with three subtropical forages, a legume and two grasses, are reported. Each forage was given to four sheep at several levels of feeding. One of the forages had a low nitrogen content (4% crude protein) and was supplemented with urea and molasses containing 7 g nitrogen and 56 kcal/day. This caused voluntary consumption to increase by 50% and digestibility of all chemical constituents to increase by 6–12 units. Utilization of metabolizable energy for maintenance also improved by approximately 10%. Digestible energy (42–62%) was predicted quite accurately from chemical composition by means of published equations which refer to forages grown in temperate climates. Metabolizable energy was close to 82% of digestible energy, as found with non-tropical forages. Net availability of metabolizable energy for maintenance ranged from a very low 60% to 80%. It was predicted more accurately from percentage metabolizable energy than from digestible crude fibre: the reverse applied to net availability for production (40–50%).

Energetic efficiency of fattening sheep. II. Effects of under-nutrition

Balances of energy, carbon, and nitrogen were determined with five castrate sheep at several levels of feeding and at fasting, when the sheep were in lean condition and when they were very thin; measurements had been made previously when they were fat. While the animals were on a constant low ration to cause loss of condition, the observations were continued under both thermoneutral and cold conditions, and glucose and ketone levels in the blood were also measured. The digestibility and metabolizability of the gross energy of the food were not affected by body condition. However, as the sheep lost weight, the net availability of metabolizable energy at each level of food intake declined. The change of efficiency was not apparent when comparisons were made at equivalent levels of energy storage. In the weight range 21–59 kg, the fasting heat production varied directly with body weight at a rate of 10 kcall24 hr/kg. Prediction of fasting metabolism from body weight is discussed. Changes of energy utilization were largely accounted for by changes of fat metabolism. When the sheep were emaciated, oxidation of protein during fasting was somewhat greater than when they were fat, but nitrogen supplied by the food was used more efficiently. Under cold stress, the heat production of the sheep increased to a high level, which did not vary with body weight until homeostasis failed when the animals were extremely emaciated. The apparent digestibility of the food (at the lowest intake) decreased by 0.47 unit/°C fall of ambient temperature. While the sheep were on the submaintenance ration, the levels of blood glucose and ketones remained at the higher end of the "normal" range under thermoneutral conditions and were somewhat elevated under cold conditions. Hypoglycaemia occurred only when homeostasis failed.

Utilization by fattening sheep of the energy and nitrogen in fresh herbage and in hay made from it

Balances of energy, carbon, and nitrogen were measured with fresh herbage cut at 28 days' regrowth, and with hay made from it. Measurements were made on four wether sheep at fasting and at four levels of feeding. The herbage contained paspalum grass and white clover in proportions ranging from 0.6 : 1 to 1.2 : 1, with dry matter between 11 and 17%. It contained (dry matter basis) : 10% ash, 3% soluble sugars, 25% cellulose, 23% crude protein, and 9% lignin; crude fibre was 22% and nitrogen-free extractives 42%. The hay had the same composition. Digestibility of organic matter was 75% for the fresh material and 73% for the hay; digestible energy values were 73 and 70% respectively. The difference between fresh herbage and hay was due entirely to a decrease in the digestibility of protein, from 82% to 73%, in drying. Efficiency of utilization of digested protein was not affected. Digestibility of both materials decreased by 2 to 4 units between the lowest and highest feeding levels. One other difference was found, viz. the ratio methane production/kcal digestible energy was 1–2% greater with the hay. Metabolizable energy was 82–84% of digestible energy for both materials. Heat production bore a linear relation to metabolizable energy above maintenance. Net availability of metabolizable energy was 75% between fasting and maintenance, and 52% above maintenance, for both the fresh and dry food. These quantities were predictable to within 8% from metabolizable energy or fibre digestibility; the normally used starch equivalent factors underestimated the correct values by some 25%.

Energetic efficiency of fattening sheep. I. Utilization of low-fibre and high-fibre food mixtures

The energy, carbon, and nitrogen exchanges of nine castrate male sheep in moderately fat condition were determined with the aid of closed-circuit indirect calorimetry. Five of the sheep were kept on a diet containing equal parts of chopped lucerne hay and chopped wheaten hay (mixture A). The other four were given a pelleted 5:4:1 mixture of lucerne hay, maize meal, and peanut meal (mixture B). Each mixture was given at five different rates and each sheep was fasted on two occasions. Digestible energy averaged 62% for mixture A and 76% for mixture B, irrespective of feeding level. Of this, 10% was lost as methane and 5 to 13%, depending on level of feeding, in the urine, leaving on the average 81% metabolizable. Thus metabolizable energy amounted to 51 and 62% of the gross energy intake with mixtures A and B respectively, while net energy was 89 and 97% of the metabolizable energy intake at the lowest level of feeding and 61 and 69% at the highest. At any given level of metabolizable energy, mixture B provided 30% more digestible nitrogen than mixture A, but, allowing for differences between sheep in nitrogen economy, any additional energy obtained from mixture B was stored in fat. Consideration of the present results, along with data from earlier experiments with fattening sheep and cattle, showed that the net availability of metabolizable energy, both for maintenance and fattening, decreases regularly as the quantity of digestible fibre increases. Net energy could be estimated more accurately from this relation than by use of the commonly used factors of Kellner.