Dry matter intake and eating rate of grass by dairy cows is restricted by internal, but not external water

The low dry matter (DM) content of fresh grass is often suggested as a factor restricting voluntary intake of ruminants. The objective of this study was to determine the roles of the external and internal water of fresh grass on its voluntary intake, feeding behaviour and characteristics of the rumen contents in dairy cow. In a first experiment, control grass was compared with the same grass dried for 4:30, 7:00 or 8:30 h, in order to establish the response of voluntary intake to the water content of grass. In a second experiment, four treatments were compared to isolate the respective effects of internal and external water: control fresh grass (C), grass dried for 5:00 h (D), C with addition of surface water by sprinkling (CW) and D with addition of surface water by sprinkling (DW). Each experiment was completed according to a Latin square design, with four lactating cows stall-fed ad libitum with perennial ryegrass. Drying led to an increase of the grass DM content from 107 to 162 g/kg in experiment 1 and from 218 to 302 g/kg in experiment 2. On an average, wetting the grass in experiment 2 lowered DM content from 260 to 196 g/kg. In both the experiments, the voluntary intake of fresh grass showed a strong negative correlation with its DM content. In experiment 1, the voluntary intake was increased, but not significantly (P=0.15), by 1.2 kg DM by drying. In experiment 2, the voluntary intake increased by 1.3 kg DM after drying of the grass, but was not modified by wetting. In both experiments, the rate of DM intake increased with drying (+30 and +10% in experiments 1 and 2, respectively), whereas the supply of external water in experiment 2 led to no modification of DM intake rate. The weight, structure and chemical composition of the rumen contents were unaffected either by drying or wetting the grass. Only the internal water of grass affected feeding behaviour and limited voluntary DM intake in dairy cows fed on fresh grass.