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

The positional distributions of fatty acids in triglycerides from cows' milk, cows' milk enriched in linoleic acid by dietary means, and two commercial infant formulae fortified with vegetable fats have been determined for comparison with human milk. The results are discussed in terms of the digestibility of the fats and of the requirements of the human infant for linoleic acid.

Most calorimetric experiments to determine the value of feeds as energy sources are made with mature animals. The question arises whether the energy values of feeds determined with them apply equally to young growing animals, but there is little information to answer it. Ritzman & Colovos (1943) determined the energy retention of cattle throughout their growing period. Their results were recomputed to find whether the efficiency with which the metabolizable energy consumed in excess of their maintenance needs was used by growing animals to promote energy retention differed from that to be expected from independent experiments made with adult cattle. Excellent agreement between the observations of Ritzman & Colovos and expectations based on results obtained with adult animals was noted when the animals were almost full-grown, but with animals weighing 200 kg. or less agreement was poor, younger animals apparently being more efficient than older ones. Such an effect of age on efficiency might be accounted for by a change with age in the ratio of protein to fat deposited in the body, for it seems likely that protein synthesis from absorbed amino acids is a more efficient process than is the synthesis of fat from the oa;o-acids formed from them.The evidence of Ritzman & Colovos, (1943) experiments that young animals are more efficient in using metabolizable energy for body synthesis than are old ones is, however, indirect: the diets given to the animals changed with their age and general equations had to be used to calculate maintenance needs and expected efficiencies above maintenance. The experiments described below were undertaken to provide more direct evidence about the effect of age on the efficiency of energy utilization by feeding the same diet to cattle of different EXPERIMENTAL Animals. Seven Ayrshire steers numbered 1-7 were used. They were reared on whole cows' milk, weaned when 7 weeks old and thereafter given a * Present address: Rowett Research Institute, Bucksbum, Aberdeen. diet of hay and a commercial cattle-rearing pellet in amounts sufficient to result in steady growth. The animals were castrated as soon as the testes had descended.Experiments. Five experiments, each with three steers, were made. Exps. 1, 2 and 3 were made with steers 1, 2 and 3, commencing when they were 31, 46 and 81 weeks of age. Exp. 4 was made with steers 4, 5 and 6, commencing when they were 15 weeks of age, and Exp. 5 with steers 4, 5 and 7 when they were 35 weeks of age.Each experiment began by each animal being fasted for 4 days to determine its fasting energy metabolism. The steers were then given the experimental diet in increasing amounts for 1 week, or until such time that a normal intake of feed was re-established. Each animal was then given three different amounts of the diet in successive 3-week periods, the sequence of treatment for the three animals in an experiment conforming to the rows of a latin square. During the last week of the periods energy and N metabolism were measured. The amounts of feed given varied...

I . The results of over 500 determinations of the heat of combustion of the urine produced by cattle and sheep have been analysed statistically. 2. The analytical errors for nitrogen, carbon and heat of combustion were k 0.54, f 1'4 and ? 2.2 %. The error attached to an estimate of the heat of combustion of the urine produced by an individual sheep in 4 days was & 10 %. 3. At the maintenance level of feeding, the heat of combustion of the urine ( U kcal/ 100 kcal food) was related to the crude protein content of the diet (P%) by the equation U = 0.25P+ 1.6, with a residual standard deviation of To.88 kcal/Ioo kcal. 4. Regression analysis of the relation between the heat of combustion of urine and its N content showed significant differences with diet. The heat of combustion of the urine of sheep was 9.7 kcal/g C and of cattle 10.3 kcal/g C, and did not vary with diet. 5. It is shown that the variation in the heat of combustion of urine/g N and its relative constancy/g C arises largely from variation, from diet to diet, in the proportion of the N excreted as hippurate. The metabolizable energy of a feed is its heat of combustion less the heat of combustion of the faeces, urine and combustible gas produced when it is eaten. In many trials with ruminants designed to provide an estimate of the nutritive value of feeds, the measurements made are limited to determinations of the heat of combustion of the feed and the faeces and neither methane nor urine energy are determined. In some trials, however, the nitrogen content of the urine is determined. Methods are available which enable the methane produced when different diets are given to ruminants in different amounts to be predicted with reasonable precision from knowledge of the faecal loss of energy (Blaxter & Clapperton, 1965) and, if methods were available for estimating the loss of urine energy from commonly measured attributes of the feed, then metabolizable energy could be estimated from the results of trials more simple than complete calorimetric ones.Understandably many attempts have been made to estimate the heat of combustion of ruminant urine from its more easily measured N content. The earliest of these consisted of assigning a constant calorific value to the urine/g N although most of the workers concerned realized that this ratio varied with diet (see Armsby, 1908). More recently the heat of combustion of urine has been related to its N content by linear regression methods, which, since they rarely have intercepts of zero, implies that the heat of combustion of urine changes with its N content. Thus Paladines, Reid, Van Niekerk & Bensadoun (1964) Other less satisfactory approaches have been made. Thus both Elliot & Loosli (1959) with cattle and Street, Butcher & Harris (1964) with sheep and cattle have related the heat of combustion of urine/unit weight to its N content/unit weight. Both found positive intercepts for their regressions. This approach implies that the heat of combustion of the urine produced by an animal in a day is greater when it is s...

Although many investigations have been made, notably by Brody and his associates (Brody, 1945), of the energy cost and energetic efficiency of muscular work in the larger farm animals, horses, mules and cattle, no systematic studies have been made with sheep and goats. It was to try to fill this gap that this investigation was begun.The net efficiency of muscular work is defined as the ratio of the work done to the energy expended in doing it. Work is defined as force x distance, and can be expressed in units of energy by use of the mechanical equivalent of heat, i.e. I kg m work = 2-34 cal. It is impossible to assess in any simple way how much external work is done by an animal walking horizontally and, therefore, it is not possible to measure the efficiency of an animal for horizontal walking. Measurements made with animals walking on gradients, however, permit an assessment of efficiency because the energy expended in raising the body can be equated with its gain in potential energy. T o estimate the energetic efficiency of muscular work thus involves measurement of energy expenditure when the animal walks on the level and on gradients. E X P E R I M E N T A LAnimals. Two Cheviot wether sheep, each weighing approximately 90 lb, were used. One of them (sheep H) was well accustomed to walking on a treadmill having been used in previous experiments and the other (sheep J) was trained to walk over a period of I week before the experiment began. Both sheep were used to living in cages and respiration chambers.Diets. Each sheep was given a ration of artificially dried grass finely ground and pelleted; it contained 1 5 . 1 yo crude protein on a dry-matter basis. Two amounts were given to each sheep. One, referred to as the high level, was computed to be sufficient to allow the animal to retain energy even when working at the highest rate; the other, referred to as the low level, was computed to result in loss of energy from the body even when the sheep was at rest. Each of the rations was offered for 14 days before the experiment began. Food was given twice daily at 5 pm and 5.30 am. No refusal of food occurred during the experiments. The experiments with the two amounts of food were duplicated with each sheep and the amounts of food given and the order in which they were offered are shown in Table I