Effects of length of grazing session, rumen fill and starvation time before grazing on dry-matter intake, ingestive behaviour and dry-matter rumen pool sizes of grazing lactating dairy cows

The effects of the length of allowed grazing time (Experiment 1) and length of starvation period before grazing (Experiment 2) on dry-matter (DM) intake, ingestive behaviour and changes in DM rumen pool sizes during the first grazing bout were investigated in lactating Holstein-Friesian dairy cows. In Experiment 1, four lengths of allowed grazing time (1, 1.75, 2.50 and 3.25 h) after overnight starvation were compared. Increasing the length of the allowed grazing time significantly increased DM intake (P < 0.01), the proportion of time spent actively eating (P < 0.01) and DM rumen pool size after grazing (P < 0.05). However, DM rumen pool size after grazing was smaller than that measured immediately before the start of starvation (P < 0.01). Bite mass during the first hour of grazing was greater than in the following grazing time. Experiment 2 consisted of a factorial combination of two durations of starvation before grazing (16.5 h and 2.5 h) and the presence or absence in the numen of 12.5 kg of a synthetic indigestible material. DM intake, as well as grazing time, were greater after a starvation period of 16.5 h and were reduced by the presence in the rumen of indigestible material (P < 0.01). The interaction between factors on grazing time, although not significant (P < 0.06), supports the idea of a combination of signals controlling meal size under grazing conditions. Bite mass was not significantly modified by the period of starvation. DM rumen pool size after grazing was reduced by the placement in the rumen of synthetic indigestible material (P < 0.05) and tended to be lower in cows with the larger period of starvation (P < 0.058). In both experiments, bite rate declined as the grazing session progressed, but bite rate was not affected by treatments.