The application of calorimetry to the study of ruminal fermentation in vitro

Techniques evolved for the study of heat production during the metabolism of pure cultures of microorganisms have been adapted for the study of certain aspects of the behaviour of the mixed cultures responsible for fermentation within the rumen. This approach allows whole untreated rumen contents, collected via fistulae, to be studied. The instrument is sensitive to very small changes in the overall evolution of heat, and so is capable of indicating overall changes in metabolic activity. In the samples studied in this series of experiments, the rates of heat and gas (carbon dioxide plus methane) production varied directly with the solids content of the sample. At 24 hr after feeding, the heat produced by ruminal fermentation in the sheep used was calculated to be between 480 and 1360 cal/hr. Ruminal fermentation was not affected by 240 mmoles of volatile fatty acid per litre, but was markedly inhibited by the same level of sodium ion. Lactate ion too appeared to have a specific inhibitory effect. The solid portion of rumen contents was found to possess much greater cellobiose-fermenting activity than did the fluid portion, and lactate accumulated as a transitory intermediate during cellobiose fermentation by whole contents. Small changes in pH had little effect on the course of fermentation in samples collected 16 hr after feeding. These findings and possibilities for the use of calorimetry in the study of complex biological systems are discussed.