Another source of greenhouse gases.
GAS PRODUCTION
The gas mixture found in the rumen consists largely of carbon dioxide and methane. There is some nitrogen, and traces of oxygen may be present, particularly as the length of time after feeding increases. Little or no hydrogen is present, except when feeding resumes after a fast, when large amounts of it appear for a few days in place of methane. Methane is formed by reduction of carbon dioxide by methanogenic bacteria. Hydrogen, formate, and succinate are hydrogen donors for this reaction, and this may be why they are absent from the rumen, or present only in small concentrations, although they are all metabolic products of rumen bacteria. Between 30 and 40 percent of the total gas present in the rumen of cattle is methane, but carbon dioxide can vary from 20 to 65 percent in cattle fed once in 24 hours. Usually carbon dioxide forms about 60 percent of the gas present in animals that are fed ad libitum; it arises (1) from the fermentation of carbohydrat!
es, (2) from the deamination of amino acids, (3) from saliva bicarbonate as a result of neutralization of fatty acids formed during fermentation, and (4) by exchange across the rumen epithelium during the absorption of fatty acids.
"In spite of the complexity of the carbohydrates ingested, the results of their fermentation are simple mixtures of VFAs with carbon dioxide. The fermentation of purified cellulose, starch, or glucose in vitro by mixed rumen bacteria produces acetic and propionic acids in about equal proportions, but in the rumen acetic acid usually represents 60-70 percent, propionic acid 15-20 percent, and butyric acid 10-15 percent of the fatty acid mixture in animals fed hay or other roughages. Branched-chain isomers of C4 fatty acids, together with straight- and branched-chain C5 fatty acids, are usually present in small quantities; these fatty acids are associated with the deamination of amino acids and presumably are derived from their carbon skeletons.
