Prediction of the amount of methane produced by ruminants

Abstract

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.Understandably, several attempts have been made to predict the losses of energy as methane by cattle and sheep from knowledge of the amount and type of the food they ingest (Kriss, 1930; Bratzler & Forbes, 1940; Swift, Bratzler, James, Tillman & Meek, 1948; Axelsson, 1949).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