The use of cumulative gas production technique to characterize changes in the fermentation characteristics of rumen contents following variable periods of starvation and grazing in dairy cows

The effect of the duration of grazing (experiment I) and starvation time an d placement in the rumen of inert bulk material before grazing (experiment 2), on the rumen content fermentability, teas investigated by means of meas uring cumulative gas production. In experiment 1, a comparison was made of four durations of grazing (2, 1.75, 2.50 and 3.25 h) after overnight starva tion. Rumen samples taken from the cows after 1 h of grazing had higher val ues of total accumulated gas with less (P < 0.05) time required to reach th e maximum fermentation rate than cows grazed for 3.25 h. Following grazing, a 7.75-h starvation period was imposed on the four treatments. The extent of fermentation was significantly lower (P < 0.01) after starvation than im mediately after grazing (49.7 v. 60.8% of incubated dry matter (DM), respec tively). Experiment 2 consisted of a factorial combination of two durations of starvation before grazing (16.5 (LS) and 2.5 (SS) h) with the presence or absence in the rumen of 12.5 kg of a synthetic indigestible material. Be fore grazing the total accumulated gas production was less (P < 0.05) for t he LS than for the SS cows. After the grazing session, the total gas of rum en samples from the LS cows was significantly higher (P < 0.05) than for th e SS cows. This was in agreement with the observed higher DM intake during grazing and DM rumen pools after grazing in LS cow. For both starvation periods, the p resence of inert rumen bulk Zed to a higher total gas, a shorter half-time and less DM left unfermented. The measurement of fermentation kinetics by c umulative gas production was suitable to defect changes in rumen content fe rmentation patterns due to the clearance of material from the rumen (effect of starvation) or DM intake during the grazing sessions.