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%.

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.

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