The heat increments of mixtures of steam-volatile fatty acids in fasting sheep.

Kellner (1920)) in his calorimetric studies with cattle, showed that, as the fibre content of the ration increased, the net availability of its metabolizable energy fell. He attributed this fall to the additional physical work with consequent loss of heat in comminuting and digesting the more fibrous food. This cause may not be the sole contributor. Data in the literature, as summarized in Table I , suggest that the molar composition of the fatty-acid mixtures formed in the rumen differs from ration to ration, and that roughages may be associated with the formation of mixtures with higher proportions of acetic acid and lower proportions of propionic acid than those arising from concentrates. This table must, however, be interpreted with caution because of differences in technique adopted by the different workers. It is thus possible that the differences in utilization observed by Kellner reside in part in differences in the composition of the energy-yielding constituents absorbed from the gut when foods high and low in fibre content are given. I n this regard, it has been shown that acetic acid when given to fasting sheep is less efficiently utilized than either propionic or n-butyric acids; these in turn are less efficiently utilized than glucose (Armstrong & Blaxter, 1957). Table I. Percentage composition (on a molar basis) of the steam-volatile fatty acids found in the rumen liquor of cattle and sheep given diflerent rations m-equiv. of individual acid/roo m-equiv. total steam-volatile acid