NEUTRAL DETERGENT FIBER DISAPPEARANCE AND GAS AND VOLATILE FATTY-ACIDPRODUCTION DURING THE IN-VITRO FERMENTATION OF 6 FORAGES

Samples of unfractionated forage and isolated NDF from six forages wer e fermented in vitro, and NDF disappearance and gas and VFA production were measured over time. Rates based on each of these data sets were calculated using a one-pool logistic model. The rates of NDF disappear ance and gas and VFA production did not differ within each forage. Gas and VFA production were linearly related to NDF digestion. Gas yield was .35 mL/mg (r(2) = .92) of NDF digested for the isolated NDF. The a mount of total VFA produced per milligram of NDF digested was more var iable than gas (r(2) = .72), with a slope of .01 mmol VFA/mg of NDF di gested. The relationship between gas and VFA production was linear (me an slope of 1.43 mmol gas/mmol VFA, r(2) = .69). The ratios of end pro ducts (gas and VFA) to NDF digestion and the ratio of acetate:propiona te were variable during the first 8 h of fermentation but changed litt le after this time. Changes in the acetate: propionate ratio explained 23% of the variation in gas produced per millimole of total VFA detec ted.