[Estimation of the net energy using volatile acids produced in the process of in-vitro digestion with ruminal fluid]

47 types of green feeds and roughages were subjected to an in-vitro fermentation with dilute ruminal fluid. The volatile fatty acids produced in this process were determined quantitatively in accordance with the method of digestibility estimation proposed by Tilley and Terry (1963). An average of 5 Mol FFS (FFS=volatile fatty acids) was found per kg of dried feed, a value is also reported in the literature. The ratios of acetate to propionate to n-butyrate to iso-butyrate to n-valerianate were 64.2 : 25.4 : 6.6 : 0.8 : 1.5 : 1.5. In this ratio, propionic acid predominated so that acetic acid and n-butyric acid were misrepresented compared with the data of in-vivo measurements made for the corresponding foodstuffs. Consequently, it is only within certain limits that values of FFS concentrations obtained in vitro may be used for estimating net energy data, disregardful of the fact that the FFS are the main source of metabolizable energy in ruminants. The reliability index for an estimation of Starch Equivalents and NEFr based on the above-mentioned method was found to be considerable lower (0.71 and 0.80) than that based on in-vitro digestibility measurements (0.86).