K. L. Blaxter

Some observations on the digestibility of food by sheep and on related problems.

When, in nutritional studies, it is necessary to determine the losses of a particular nutrient in the faeces, two techniques are employed to ensure that the faeces collected are representative of the intake of food. The first consists of feeding a marker substance, such as carmine, immediately before and at the end of the experiment. This allows a separation of the faeces produced from the food given in the interval, and is the technique employed with non-ruminant species. With ruminants, however, the use of markers does not result in a clear separation of the faeces marking the beginning and end of the period. Consequently, the method employed to obtain accurate digestion coefficients with ruminants is to give the experimental ration in exact quantities for long periods, in order to ensure that a ‘steady state’ of faecal excretion is reached, and then to collect the faeces excreted during a measured interval of time. If refusal of food occurs on a particular day, the method is invalid, and so the amount of food given is usually kept small to avoid refusal. The low planes of nutrition which are thus employed are not desirable if results are to be applied in practical feeding (Schneider, 1954). Nor can the method be used where intake is voluntary or uncontrolled, because the faeces produced during a particular 24 h period are not simply referable to the food intake in some previous 24 h period. In the course of experiments designed to study the effect of methods of fodder preparation and nutritional plane on the energy metabolism of sheep, information was obtained on the passage of food through the digestive tract, its digestibility, and the diurnal variation in the amounts of dry matter and water excreted. This information is presented here, and the results have been examined in some detail in order to elucidate the problems involved in the determination of digestibility coefficients in ruminants. It has been found that the time relationships involved in the passage of food through the gut give rise to equations which allow the prediction of the errors of digestibility coefficients, the digestibility coefficients themselves and the diurnal pattern of faeces excretion. They also permit the estimation of the gut contents or ‘fill’ of the animal under a variety of feeding systems and show what precautions must be taken in the conduct of digestibility trials.

Environmental temperature, energy metabolism and heat regulation in sheep. I. Energy metabolism in closely clipped sheep

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 effect of the grinding and cubing process on the utilization of the energy of dried grass

1. Eighteen determinations of the energy retention of six sheep were made when they were given the same batch of dried grass in the form of chopped material or as cubes. The cubes were made following hammer milling to a medium and fine particle size. The fasting heat production of each sheep was also determined, following subsistence on a standard ration. 2. Agreement between determinations of energy retention calculated from the carbon and nitrogen retentions and from the energy exchange was good. The mean discrepancy was 4 Cal./24 hr. 3. There were no statistically significant differences in energy retention as between the three materials at either a low (600 g./24 hr.) or a high (1500 g./24 hr.) level of feeding. Calculations of net energy/100 Cal. of food ingested showed that higher values occurred at the lower level of feeding. Standard errors of the means were small, about ±3% of the determined values. Further analysis showed that no large differences in the net energy value of the materials would appear within the normal feeding range, but slight extrapolation of the data indicated that the cubes would be superior at high feeding levels. 4. Faecal losses of energy were considerably greater when cubes were given and methane losses were much smaller. Individual sheep which showed low methane losses also showed high faecal energy losses. Faecal losses of energy were smaller at the lower feeding level. Urine energy losses were unaffected by the amount or physical form of the food given. 5. Heat losses were greater at the higher nutritional level and were considerably less for cubes than for chopped material. Constancy of net energy value in this study thus involved compensation of high faecal energy losses by low losses of energy as heat and methane. 6. The determinations of the digestibility of the carbohydrate fractions of the grass showed that a fall in the digestibility of the structural components of the cell was the major factor causing increased faecal losses. The digestibility of intracellular constituents fell very much less. 7. It is shown that evaluation of the grasses in terms of metabolizable or digested energy does not place them in their correct physiological order of nutritive value, and that estimates of nutritive value using Kellners and other factors do not give their true nutritive worth. 8. It is pointed out that physical factors, which change the rate of passage of food through the gut, change the rate and nature of the microbial fermentation, and cause variation in the mechanical work involved in prehending, masticating and cudding food, are as important as the chemical composition of the food in determining its nutritive value.

The utilization of the energy of two mixtures of steam-volatile fatty acids by fattening sheep.

The energy of mixtures of steam-volatile fatty acids representative of those arising in the rumen when food undergoes microbial fermentation was utilized with high efficiency when given to fasting sheep, and was unaffected by the molar composition of the mixtures employed (Armstrong, Blaxter & Graham, 1957). Given as the sole source of energy, however, acetic acid was poorly utilized (Armstrong & Blaxter, 1957U). It was shown later that when the acids were added singly to rations which themselves permitted a retention of energy, the individual acids were utilized for lipogenesis with much lower efficiencies than in the fasting animal and marked differences in the utilization of the individual acids were found (Armstrong & Blaxter, 19573). It was therefore important to establish whether different mixtures of the steam-volatile acids are utilized with constant efficiency when they are given to fattening animals. The experiments described below were designed for this purpose.

The heat increments of mixtures of steam-volatile fatty acids in fasting sheep.

Kellner (1920)) in his calorimetric studies with cattle, showed that, as the fibre content of the ration increased, the net availability of its metabolizable energy fell. He attributed this fall to the additional physical work with consequent loss of heat in comminuting and digesting the more fibrous food. This cause may not be the sole contributor. Data in the literature, as summarized in Table I , suggest that the molar composition of the fatty-acid mixtures formed in the rumen differs from ration to ration, and that roughages may be associated with the formation of mixtures with higher proportions of acetic acid and lower proportions of propionic acid than those arising from concentrates. This table must, however, be interpreted with caution because of differences in technique adopted by the different workers. It is thus possible that the differences in utilization observed by Kellner reside in part in differences in the composition of the energy-yielding constituents absorbed from the gut when foods high and low in fibre content are given. I n this regard, it has been shown that acetic acid when given to fasting sheep is less efficiently utilized than either propionic or n-butyric acids; these in turn are less efficiently utilized than glucose (Armstrong & Blaxter, 1957). Table I. Percentage composition (on a molar basis) of the steam-volatile fatty acids found in the rumen liquor of cattle and sheep given diflerent rations m-equiv. of individual acid/roo m-equiv. total steam-volatile acid

Methods of Assessing the Energy Values of Foods for Ruminant Animals

Assessment of energy value of foods 131 of the same species? Two world wars have forced us to think of human beings and of farm animals as populations in a statistical sense. We count heads, assign ‘man values’ for energy requirements. Such a practice is essential for the administrator, but it does ignore individual differences. We have no evidence for variation in the efficiency of the fundamental biochemical processes with which individuals of the same species liberate energy from food. Yet human beings, as individuals, do differ in their instinctive demand for food. This difference is not necessarily correlated with the expenditure of energy. The obese human being within the nation is an inefficient individual. But the ox, fattening in his stall, is fulfilling his man-made destiny. The assessment of the energy value of human and animal foods cannot then be studied as a problem in pure chemistry. In due time biochemistry will elucidate the complexities of the processes at molecular and cellular level which determine the liberation of energy from food. But the final word is with the living animal itself which is a biological entity. And a human being is also a person.

Heat production and heat emission of two breeds of sheep.

1. Fifty-two experiments were made with two Cheviot and two Blackface wether sheep in which heat production and heat emission were determined at environmental temperatures of 8, 20 and 32° C. Initially the sheep were closely clipped to within 1–2 mm. of the skin and the fleece was then allowed to grow throughout the experiments. 2. At 32° C. fleece length had no effect on heat production. At 20° C. metabolism was elevated until fleece length exceeded 18 mm. At 8° C. metabolism was elevated until the fleece length exceeded 35–40 mm. No differences were found between the two breeds in their heat production at a particular temperature provided fleece length was identical. 3. The sensible loss of heat divided by the temperature gradient from the rectum to the environment (conductance) was linearly related to the logarithm of fleece length, both at environmental temperatures above and below the critical temperature. 4. No differences between the two breeds or between them and Down Cross sheep were found with respect to their conductances when devoid of fleece. The insulation provided by unit length of fleece was the same in all three breeds and crosses. The fleece of the Blackfaces grew at twice the rate of that of the Cheviots so that at a given time after shearing, the Blackfaces were more resistant to concold. 5. Studies of the losses of heat by the vaporization of water and of skin and fleece surface temperatures also showed no differences between breeds. 6. Analysis of the relation between heat concold, ductance and fleece length suggests that vasoconstriction and vasodilation border on all-or-none effects.

The effect of environmental conditions on food utilisation by sheep.

1. A series of calorimetric experiments was conducted with sheep which had fleeces ranging in thickness from 0·1 cm. to 12 cm. at environmental temperatures between 8 and 32° C. Heat production, heat loss by radiation, by convection and conduction, by vaporisation of water and due to warming food and water to body temperature were measured together with losses of energy in faeces, in urine and as methane. 2. The effects of a rise in environmental temperature on digestion of the food and on the loss of energy in urine or as methane resulted in a slight rise in the metabolisable energy of the ration by 6 Cal./° C. 3. Environmental temperature had a marked effect on heat production, particularly when the fleece was short. The critical temperature (i.e. the environmental temperature at which heat production was minimal) of the closely-clipped sheep varied from 24° C. at a high level of feeding to 38°C. at a sub-maintenance level of feeding. These critical temperatures are similar to that of naked, resting man but much higher than that of the pig when fed similarly. 4. As the fleece grew the critical temperature fell. Thus, on a maintenance level of feeding, a sheep with a fleece of 0·1 cm. had a critical temperature of 32° C.; when the fleece had grown to 2·5 cm. the critical temperature was 13° C. while with a 12 cm. fleece the critical temperature was 0° C. 5. Below the critical temperature heat losses increase more rapidly in sheep with light fleeces. Thus a heavy fleece not only depresses the critical temperature but also reduces the rate of increase of heat loss with falling temperature under sub-critical conditions. 6. At environmental temperatures well below the critical, the heat losses of the sheep per unit surface were identical. Under such conditions, when the whole of the metabolisable energy of the food is used to keep the animal warm, the criterion of ration adequacy is a high content of meta-bolisable energy in small bulk. 7. At environmental temperatures above 32° C. the heat production on a constant ration increased, the rise being greatest with the highest level of feeding. Consequently the net energy value of the food declined at these high environmental temperatures. 8. The calorimetric experiments were supplemented by two comparative feeding trials in which the effects of normal outdoor environmental conditions on the body weight of groups of Cheviot and Blackface sheep were measured. Control groups were kept indoors in heated pens. 9. During the mild winter of 1956-7 the out-wintered Blackface wethers i n full fleece did not loose any more weight than those fed the same rations indoors. 10. During the more severe winter of 1957-8, Cheviot, in-lamb ewes kept on a maintenance diet gained 2·3 lb.; those kept outside on the same ration lost 3·3 lb. With Blackface, in·lamb ewes the difference between the two groups was 0·3 lb. in favour of the indoor group. 11. The food utilisation of sheep is affected considerably by environmental conditions. With little fleece the critical temperature is high and even when in full fleece an effect of cold can be demonstrated under practical conditions.

The heat increment in fasting sheep of acetic acid partially neutralized with sodium hydroxide

In previous work Armstrong & Blaxter (1957) found that when acetic acid was given to fasting sheep as the sole source of energy it was inefficiently utilized in the sparing of body tissues from oxidation. Five experiments gave a mean heat increment of 40.9 kcal/Ioo kcal of acid metabolized, a value which greatly exceeded the heat increments found when propionic or n-butyric acids were given. A noticeable feature of the experiments with acetic acid was the marked acidosis that developed when the acid was administered, and it was suggested that the high increments of heat might be associated with accumulation of acid. Respiration-chamber experiments by Mollgaard & Thorbek (1941) with milking cows given A.I.V. silage were cited in this regard, for they showed that severe acidosis due to mineral acid increased the heat increment and that neutralization of the acid reduced it. This paper describes three experiments in which fasting sheep were given acetic acid partially neutralized with sodium hydroxide in an attempt to prevent acidosis ; the results are compared with those previously reported when acetic-acid solutions were given. A brief account of some of the findings has been published (Armstrong, Blaxter & Graham, 1958).