N. McC. Graham

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.

Simulation of the effects of rumen function on the flow of nutrients from the stomach of sheep: Part 1—Description of a computer program

Abstract A computer program to simulate rumen function in sheep is described. It predicts the degradation of dietary components within the rumen, products of fermentation, microbial growth yields and the flow of protein and other materials from the rumen. Input information includes: feed intake; modulus of fineness of the diet, chemical composition of the diet in terms of β-hexose, α-hexose, soluble carbohydrates, true protein, non-protein nitrogen, total fatty acids, inorganic sulphur and ash; potential degradability of β-hexose and protein; factors indicating the fractional reduction in maximum rate of degradation of β-hexose, α-hexose and protein due to chemical and physical properties of the diet; and time spent feeding and ruminating. Inflow to the rumen of protein, non-protein nitrogen and inorganic sulphur in saliva is predicted, and the amount of each salivary and dietary substrate degraded in the rumen, flowing intact from the rumen or accumulating within the rumen is assessed. Adenosine triphosphate (ATP) released during fermentation is used for maintenance of the existing microbial mass. If ATP release is insufficient a portion of the microbial mass is catabolised, otherwise microbial growth can occur and some degraded substrates are incorporated directly into microbial protoplasm. Possible limits to microbial growth from an insufficiency of amino acids, ammonia, total nitrogen or inorganic sulphur are assessed, and the change in rumen microbial population is determined from the difference between growth and outflow of micro-organisms. Predictions from the program are shown to be stable and to respond appropriately to changes in intake and composition of the diet.

Growth in sheep. I. The chemical composition of the body

Fifteen sheep were fed ad libitum from 2 days to 27 months of age, and another 15 sheep were each fed exactly half the average amount consumed by the first group, age for age. The body composition of each sheep (water, fat, protein, energy) was estimated from tritiated water space on 13 occasions during this period. To describe the course of growth in individual sheep in terms of the relationships between the various body components and body weight, a model was set up in which 4 phases of growth were distinguished, viz. the milk-feeding phase, the period of rumen development, and a prefattening followed by a fattening ruminant phase. Each phase was represented by a linear equation. Except for phase 1, mean composition within each phase differed significantly between well-fed animals and those which had been given a restricted diet. Individual animals differed in the body weight at which the final phase commenced; the average weight was ca. 31 kg. Fat storage was zero or negative during the main period of rumen development; otherwise the fat and therefore energy content of weight gain increased from phase to phase. The protein and water content of gain was high in phases 1 and 2 and decreased subsequently. Calculations based on data in the literature indicated that, in phase 4, the composition of weight loss was the same as that of weight gain. It is also suggested that the body weight at which this fattening phase commences is related to mature weight, with animals of large ultimate size starting to fatten at heavier body weights than those of small ultimate size. The application of the results to the determination of nutrient requirements is discussed.

Simulation of growth and production in sheep-model 1: A computer program to estimate energy and nitrogen utilisation, body composition and empty liveweight change, day by day for sheep of any age

Abstract A computer program based on empirical relationships is described. It predicts daily energy and nitrogen utilisation repetitively for sheep of any age, before, during and after weaning; provision is also made for pregnancy, lactation and cold stress. Input information includes: intake, protein content and digestibility of the diet; age, empty body weight, fat content and feeding activity of the sheep; ambient temperature and wind speed; times of shearing and mating. Metabolisable energy from milk and/or dry feed is estimated and energy requirements for maintenance, including the cost of feeding activities and homeostasis in the cold, are deducted to obtain energy balance. The amount of amino acid nitrogen absorbed from the small intestine is estimated, and nitrogen balance in body tissues and wool is calculated from this, allowing for body weight and net energy intake. Potential wool growth is calculated from nitrogen and energy intakes, and potential conceptus growth or milk production is estimated primarily from stage of pregnancy or lactation. The use of nitrogen and energy for these products is assessed and balances of energy and nitrogen in body tissues are then obtained by difference. If achievement of the potential rates of production in pregnant or lactating animals would cause excessive loss of energy or nitrogen from body tissues, production of wool and conceptus or milk is reduced sufficiently to avoid this problem. Gain or loss of body fat and protein, and hence change of empty live weight, are finally derived and the animal parameters are incremented before proceeding to calculation for the next day. Evidence is presented that the model is stable in predicting lifetime performance, and that predictions of growth curves, body composition and various nutritional parameters are reasonably accurate in a variety of circumstances.

Change of skeletal dimensions during growth in sheep: the effect of nutrition

Measurements of skeletal size were made at 2–3-month intervals on 30 Border Leicester × Merino castrate male (wether) sheep between 2 and 27 months of age. Fifteen sheep were fed ad libitum on a high-quality diet and the other 15 half the average amount consumed by the first group, age for age. The ad libitum group grew faster and were larger in all body dimensions on each occasion, except for leg length at 27 months which showed no statistical difference between groups. When the groups were compared over the live-weight range common to both (16–44 kg) the unrationed animals were consistently wider at the shoulders but smaller in leg and chest dimensions. The relationship between each body component and age is described by a Mitscherlich equation and the relationship with live weight by a linear equation in which both variables are log transformed. Separate relationships were determined for each sheep and tested for differences within and between groups.

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.

Longitudinal studies of body composition during growth in male, female and castrate male sheep of two breeds with different wool growing capabilities

Dorset Horn and Corriedale ewes (breeds with low and high wool growing capabilities respectively) with single lambs were fed ad libitum on a pelleted roughage-concentrate diet. Following weaning at 6–7 weeks of age (live weight (LW) ca. 15 kg), ten entire male, ten female and ten castrate male lambs of each breed were housed in single pens and fed the same diet ad libitum . Body composition (fat, protein, water, ash and energy) was estimated from tritiated water space at 5 kg LW increments from 10 to 55 kg. Fleece weight was also estimated on each occasion. The relationship between the weight of body components and fleece-free fasted live weight ( W ) for each sheep was described by a two-phase piecewise linear model, the slope in each phase representing the average composition of weight gain. In the first phase, the composition of weight gain did not differ between breeds and contained similar amounts of fat and protein (14%), 66% water and 5% ash. Within breeds females tended to be fattest. There were significant differences due to breed and sex in all body components in the second (fattening) phase. The fat content of gain was lower in Dorset Horn ewes and wethers (45 and 51% respectively) than in corresponding Corriedales (64 and 58%) with the males being even lower (44 and 41% respectively). Dorset Horns entered the fattening phase at a significantly higher W (26 kg) than Corriedales (23 kg), there being no sex difference within breeds. The combined effect of lower slope and higher weight at the commencement of the phase change in the Dorset Horns was to produce a considerable breed difference in fatness in the second phase of growth in ewes and wethers. Possible physiological reasons for the differences are discussed.

Physiological state of sheep before and during fattening

Various physiological characteristics of castrate male sheep fed ad libitum were studied between 1986 and 1987 in Australia. The sheep were between 20 and 45 kg live weight (LW) before and during fattening. Sequential data on body composition, estimated from TOH space (starting at 10 kg), established that the weight gain contained 19% fat and 13% protein below c. 26 kg LW and 55% fat and 10% protein above. For a given feed intake, the rate of fat gain was constant but the rate of body protein gain was 45% lower above 26 kg LW