EFFECTS OF LENGTH OF GRAZING SESSION, RUMEN FILL AND STARVATION TIME BEFORE GRAZING ON DRY-MATTER INTAKE, INGESTIVE BEHAVIOR AND DRY-MATTERRUMEN POOL SIZES OF GRAZING LACTATING DAIRY-COWS

The effects of the length of allowed grazing time (Experiment 1) and l ength of starvation period before grazing (Experiment 2) on dry-matter (DM) intake, ingestive behaviour and changes in DM rumen pool sizes d uring the first grazing bout were investigated in lactating Holstein-F riesian dairy cows. In Experiment 1, four lengths of allowed grazing t ime (1, 1.75, 2.50 and 3.25 h) after overnight starvation were compare d. Increasing the length of the allowed grazing time significantly inc reased DM intake (P < 0.01), the proportion of time spent actively eat ing (P < 0.01) and DM rumen pool size after grazing (P < 0.05). Howeve r, DM rumen pool size after grazing was smaller than that measured imm ediately before the start of starvation (P < 0.01). Bite mass during t he first hour of grazing was greater than in the following grazing tim e. 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 ab sence in the numen of 12.5 kg of a synthetic indigestible material. DM intake, as well as grazing time, were greater after a starvation peri od of 16.5 h and were reduced by the presence in the rumen of indigest ible material (P < 0.01). The interaction between factors on grazing t ime, although not significant (P < 0.06), supports the idea of a combi nation of signals controlling meal size under grazing conditions. Bite mass was not significantly modified by the period of starvation. DM r umen 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 expe riments, bite rate declined as the grazing session progressed, but bit e rate was not affected by treatments.