Prediction of the amount of methane produced by ruminants.

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).