1. Records of metabolic measurements on cattle by open-circuit indirect calorimetry have been examined to determine the practical significance in the calculation of heat production of including measurements of carbon dioxide and methane in addition to the measurement of oxygen. 2. It was found that the combined effects of CO, and CH, made only a small contribution to the total estimation of heat production which could be calculated with approximately k 1 % accuracy from analysis of 0, alone. In

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... all open-circuit systems for measuring respiratory gaseous exchange (face-masks, head-hoods or respiration chambers) it is convenient to compute metabolic rate (ni> from the directly measured quantities (flow-rate and gas concentrations). Using the energy factors recommended by Brouwer (1965) , the author derived an equation relating k? (kW) to exhaust air flow-rate ( VE; l/s; standard temperature and pressure of dry gas) and gas concentration differences (McLean, 1972). (1) where AFo2, AFCo2 and AFCH, are gas concentration differences (exhaust air minus inlet air) and hi is the rate of urinary nitrogen production (g/s). In a later publication (McLean & Watts, 1976), a differential term was included consisting of the volume of the apparatus, V (1) multiplied by the rates of change of gas concentrations (d(AE3/dt; /s) : k? = ( VE + Vd/dt) (-2O.47AFo2 + 0.73AFCo2 -6*46AF"*) -5.99N. (2) Over a prolonged period of successive measurements the differential term has no effect on the mean value of k? since I: Vd(AF)/dt, tends towards zero; the only effect of including it is to sharpen the speed of response of the calculated value of h 8 to changing gas concentrations. This is important in large respiration chambers where the fresh-air ventilation rate per min is small compared with the volume of the chamber, and gas concentrations may take many minutes or even hours to attain equilibrium levels. However, with face masks, where the dead space is small, inclusion of the differential term is often of little advantage. It was suggested (McLean, 1972) that, compared with the term involving oxygen, those involving carbon dioxide and methane were relatively small and approximately equal and opposite; but no direct evidence was available to support this. In the intervening period we have accumulated a sufficient bank of data from metabolic measurements on cattle to test the assertion. METHODS The results summarized here are taken from two series of experiments (McLean et al. 1983 a, 6 ) on cattle subjected to various fluctuating thermal environments. The animals received a balanced daily maintenance ration (160 g protein/kg dry matter; 18 g ration/kg https://www.cambridge.org/core/terms. https://doi.