Metabolizable energy is defined as the heat of combustion of a feed less the heat of combustion of the faeces, urine and gases which are produced when it is eaten. The losses of energy in faeces and urine can be determined easily in sheep and cattle kept in metabolism cages, but to determine the energy they lose as combustible gas, that is as methane, involves quantitative measurement of the gaseous exchange and the use of much more complex and expensive equipment. During the course of calorimetric experiments at this Institute many thousands of determinations of methane production by sheep and cattle during 24 h periods have been made. These observations have been used to extend an earlier analysis of the relation between methane production and the type and amount of the diet (Blaxter, 1961). M E T H O D SMethane production was determined using closed-circuit respiration equipment (Wainman & Blaxter, 1958a, b) in which the CO, and H,O produced were absorbed continuously, the 0, consumed replaced continuously and the CH, produced allowed to accumulate. From the volume of the chamber, the estimated volume of the animal, the barometric pressure, and the air temperature and humidity, the total volume of gas in the chamber at s.t.p. was calculated at the beginning and end of each day. CH, concentration was determined in samples of gas taken at these times and CH, production estimated as the difference between the final and initial volumes of CH, at s.t.p. The calorific value of I 1. CH, was taken to be 9.44 kcal throughout. In May 1964 this factor was changed, as the result of a recommendation by Professor Brouwer, to 9.45 kcal (see Brouwer, 1965).Gas analysis for CH, until 1959 was by the Haldane method, in which the gas sample was burned in a gas pipette, and the total contraction in volume and the CO, production were determined. After 1959, CH, was determined in gas samples, after removal of water vapour and of CO,, by burning the CH, and measuring by thermal conductivity methods the amount of CO, formed. The instrument used (Cambridge Instrument Company) was calibrated each day using standard CH,-free and CH,-containing gas mixtures, stored in cylinders. Hydrogen, which on rare occasions arises in metabolic experiments with ruminants

1. A series of experiments with adult sheep were carried out in an attempt to place on a quantitative basis the generalisation that the voluntary food intake of ruminants increases with the quality of the fodder they are given.2. Methods of determining voluntary intake free of subjective bias were developed. It was shown that voluntary intake varied with a fractional power of body weight close to 0·734. The length of time necessary to establish stable intakes was 12–15 days and the number of times fresh food was offered each day had no apparent effect on intake.3. It was found that voluntary intake of long fodders was related to the apparent digestibility of their energy, increasing rapidly as digestibility increased from 38% to 70% and thereafter more slowly.4. The giving of concentrated food resulted in a drop in the voluntary intake of fodder. With high quality hay 100 g. concentrates replaced 100 g. hay. With poor quality hay, 100 g. concentrates replaced 47 g. of hay.5. The passage of three widely different hays through the gut was measured and the poorest passed most slowly. Calculations showed that the dry matter content of gut contents was the same for all three materials irrespective of their quality.6. It was shown that an increase in digestibility of 10 units in the range 40–60% resulted in considerable increases in the total amount of energy apparently digested and in equivalent increases in daily gain.7. The digested energy consumed/day/kg. W0·734 (E) can be related to voluntary intake (I) g./day/kg. W0·734 by the equation:—E=4·(I—31)which provides a rapid and easy method of assessing fodder quality under conditions of ad libitum supply.8. The results are discussed and it is shown that under ad libitum feeding conditions an increase in the nutritive value of unit feed from 50 to 55, i.e. by 10%, increases body gain by 100%.

No abstract

I .Nine experiments, each with one of six sheep with cannulated rumens given a constant diet of dried grass, were made in which oleic, linoleic or linolenic acid was infused into the rumen and energy and lipid metabolism were measured. One experiment was made in which palmitic acid was given. 2. Judged by changes in the composition of isolated fatty acids, the unsaturated fatty acids were hydrogenated in the rumen. An increase in the excretion of lipid in the faeces occurred when the unsaturated acids were given. The heat of combustion of the faeces increased by 12.6 3.0 kcal/Ioo kcal fatty acid, of which 94 yo was accounted for by the additional lipid. 3. Methane production fell when the unsaturated fatty acids were infused, the decreases being 13.8 k 1.6 kcal CH,/IOO kcal oleic acid, 142 f 1-5 kcal CH,/IOO kcal linoIeic acid and 16.4+ 1.3 kcal CH,/IOO kcal Iinolenic acid. The introduction of a double bond into an n-alkyl acid was calculated to reduce methane production by 0.24 f 0.09 moles/ mole double bond. 4. Because the depression of methane production on infusing the fatty acids exceeded the increase in the heat of combustion of the faeces, the metabolizable energy of the fatty acids was 104.1 k5.3 % of their heat of combustion. 5. The efficiencies with which the fatty acids were used to promote energy retention were 74.6 5 7 % for oleic acid, 79.2 f 2.0 % for linoleic acid and 82.5 f 3.0 yo for linolenic acid. These efficiencies agreed with those noted in experiments by others with rats, horses and pigs given glycerides, but were higher than those noted by others when glycerides were added to the diets of ruminants.A part of the methane produced by micro-organisms in the digestive tract of ruminants arises from the reduction of carbon dioxide. This reduction accompanies the oxidation of formic acid in the rumen, indeed formic acid when added to rumen contents in vitro, or given to sheep leads to the production of methane, I mole formic acid giving rise to 0.25 moles CH, (Carroll & Hungate, 1955 ; Vercoe & Blaxter, 1965). Since the CO, reduced is identical with the CO, pool of the rumen (Williams, Hoernicke, Waldo, Flatt & Allison, 1963) it is possible that hydrogen acceptors other than CO, added to the rumen might reduce methane production. Accordingly, linolenic acid was given to a sheep by intraruminal infusion and it was found that the methane production of the sheep fell markedly. The fall in methane production, however, was considerably greater than that expected even assuming that all three double bonds of the linolenic acid had been hydrogenated. This paper deals with the primary observations made with linolenic acid and with similar experiments in which oleic and linoleic acid were given to sheep. E X P E R I M E N T A LAnimals. Six castrated male sheep each with a permanent cannula inserted in the rumen were used as experimental animals.Food and fatty acids. Artificially dried grass was given as the only solid food. The amounts given were either 900 or 1000 g daily in two meals and in any one exp...

1. Six diets in which the percentage of hay varied from 0 to 100% and the percentage of flaked maize varied conversely from 100 to 0% were each given to three sheep and three steers in a series of 72 calorimetric experiments. In addition the fasting metabolism of each animal was determined on two occasions.2. The loss of energy in the faeces increased linearly with the percentage of flaked maize in the diet, but the loss of N in the faeces did not, the loss being proportionately smaller for diets containing more than 60 % maize.

1. The energy exchange of two sheep closely clipped at weekly intervals was determined at three feeding levels and seven environmental temperatures, using a respiration apparatus in which radiant temperature was equal to ambient temperature. All measurements were made under conditions in which the animal was in equilibrium with its environment and heat storage was zero.2. Body weight and fleece growth were both markedly reduced at the lowest feeding level. Weight losses were most marked at the lowest temperatures.3. The energy lost in faeces decreased slightly as environmental temperature increased from 8 to 38° C. Urine energy losses also fell. Losses of energy as methane were maximal in the temperature range 23–28° C. As a result of these changes, the metabolizable energy of food increased with environmental temperature by 7 Cal./24 hr./° C.4. The environmental temperature of the sheep at which their heat production was minimal, i.e. the ‘critical’ temperature was 39–40° C. for the lowest feeding level, 33° C. for the medium feeding level and 24–27° C. for the highest feeding level.

The normal process of methane production in ruminants is described and it is pointed out that about 8 % of the energy of the diet is inevitably lost to the animal as methane. Experiments are described which show that fatty acids and other alkyl compounds added to the diet or infused into the rumen reduce methane production by, in some instances, as much as two-thirds. Although methanogenesis is markedly depressed, with some compounds there is little concomitant depression of the digestion of cellulose or indeed of the non-lipid organic constituents of the diet. The implications of these findings are discussed.The ability of herbivorous mammals to obtain energy from the cellulose and hemicelluloses of plants is due to the activity of the microbial populations of their digestive tracts. Micro-organisms in the gut degrade cellulosic materials with the formation of characteristic end products, notably steamvolatile fatty acids with 1-5 carbon atoms, some dicarboxylic and tricarboxylic acids, carbon dioxide, methane and on occasions hydrogen. The microbial populations increase at the expense of the free energy of these degradations, synthesising large amounts of nucleic acids and proteins. Many of the organisms are swept on from the sites where the fermentation takes place to be digested by enzymes secreted by the host or to be expelled in the faeces. In addition, heat is generated.Generally speaking, the biologically useful energy which a herbivore obtains from its normal diet is a much smaller proportion of its total energy than is the proportion obtained by omnivores subsisting on diets rich in oligosaccharides which can be hydrolysed without microbial assistance. The reasons for this difference are, firstly, microbial degradation of cellulose is a slow process and rarely goes to completion: much of the cellulose is excreted in the faeces unchanged, particularly if the cellulose is lignified. Secondly, the end products of microbial digestion which are absorbed are relatively small molecules, and the energy required for their activation before they can enter the cycles of intermediary metabolism are a high proportion of the free energy they eventually provide.1 To activate one mole of acetic acid by forming acetyl-coenzyme A, involves the expenditure of 2 moles of high-energy bonds of adenosine triphosphate: to activate one mole of glucose involves but one. Thirdly, a proportion of the energy derived from the microbial fermentation is diverted to synthesis of compounds by the microorganisms which are subsequently lost to the animal. Thus purines synthesised by bacteria are converted after absorption to allantoin and excreted in the urine. Lastly, a considerable part of the energy of the cellulose which is degraded by microorganisms is lost to the animal as heat and as methane.This paper is concerned entirely with the extent of the loss of energy by herbivores as methane, and gives an account of investigations we have made with a view to its control in ruminant animals (Czerkawski et ~1 .~) ) . Methane production of ...

The object of these experiments was to examine the effect of environmental temperature on the energy exchange of the calf during its early life and, in particular, to determine the environmental temperature below which heat production increases in response to increasing cold. This latter temperature is called the critical temperature. The range of temperature between the critical temperature and that higher environmental temperature when heat production increases is called the thermoneutral zone. There is evidence that the mortality and morbidity of calves is greater in winter than in summer and is greater in the more northern and hence colder parts of Britain (Withers, 1952). This evidence suggests, but does not prove, that cold conditions in calfhouses are undesirable. Most farmers concur in their opinion that calves require a warm environment in early life, but no quantitative information about the conditions optimal for their growth and well-being is available. E X P E R I M E N T A LAnimals andfood. Four Ayrshire bull calves (A, B, C and D) were used as experimental animals. They were given either 4 1. or 6 1. cow's whole milk each day in two meals. The milk was always given at body temperature. Experimental measurements began on or shortly after the 2nd day of life and continued for about 4 weeks.Sequence of experiments. Each calf was kept throughout the experiments in a respiration chamber, and measurements of metabolism were made at environmental temperatures of about 3', 13' and 23'. Initially, measurements were restricted to periods of 12 h because the calves had not learned to drink from a bucket and the chamber had to be opened to feed the calf. Once the calves had learned to drink, 24 h periods of measurements were used with the exception that with calf A all experiments were of 12 h duration. T h e details of all experiments are given in Table I . I n addition, on six occasions with calves C and D, heat production was measured at hourly intervals during the cooling of the chamber, over periods of 8-9 h, from temperatures of from over 30' to 3'. With the same two calves, rectal, skin surface and hair surface temperatures were measured throughout all experiments. The losses of energy in the faeces and the urine of calves B and D were measured when they were given 4 1. and when they were given 6 1. milk and the heat of combustion of the milk