Effect of allowed grazing time, inert rumen bulk and length of starvation before grazing on the weight, composition and fermentative end-products of the rumen contents of lactating dairy cows

The effect of the length of the allowed grazing time (Experiment 1), length of starvation time and placement in the rumen of inert bulk material before grazing (Experiment 2) on liquid and particulate rumen pool sizes, composition and fermentability was investigated, In Experiment 1, four lengths of allowed grazing time (1.00, 1.75, 2.50 and 3.25 h) after overnight starvation were compared. The allowed grazing time had no significant effect on total and liquid rumen pool sizes after grazing but did affect (P < 0.05) dry-matter (DM) and organic matter (OM) rumen pool sizes. The nonsignificant differences between the volatile fatty acid (VFA) rumen pool sizes before and after 1 h of grazing may indicate a delay in the availability of the more rapidly fermentable substrate for the microorganisms. The total VFA rumen pool sizes increased significantly (P < 0.01) with the allowed grazing time, which suggests that these fermentation products may be involved in the control of the grazing time in later stages during the day. Experiment 2 consisted of a factorial combination of two lengths of starvation before grazing (16.5 and 2.5 h) and the presence or absence in the rumen of 12.5 kg of a synthetic indigestible material. The duration of starvation before grazing did not affect significantly the particulate, ammonia and VFA rumen pool sizes after grazing except for propionic acid, which was reduced (P < 0.05) by the longer starvation time. The inclusion of inert bulk material in the rumen before grazing significantly reduced (P < 0.05) the total, liquid, DM, OM and ammonia rumen pool sizes but not the VFA rumen pool sizes after grazing. High levels of ammonia as well as total rumen contents may be involved in the control of the grazing time in this experiment.