1. The characteristics of digestion, passage and rumen fill of three hays: early- and late-cut perennial ryegrass (Loliurn perenne cv. Endura) and white clover hay (Trifolium repens cv. Blanca and Pronitro) were studied using six rumen-cannulated sheep fed at a restricted level of intake (18 g dry matter (DM)/kg live weight (LW) per d), in a two 3 x 3 Latin square design.
2. Voluntary intake of the same diets was measured using a further six non-cannulated sheep in a similar design.
3. Rate of digestion of the three hays was measured using dacron bags and the rates of digestion of DM and neutral-detergent fibre (NDF) for clover hay were significantly (P < 0.05 and P < 0.001 respectively) faster than those for the two grass hays whose rates did not differ. Rates of passage, determined using chromium-mordanted hay, did not differ between treatments.
4. Rumen pool sizes of DM, organic matter and fibre were generally greatest for the late-cut grass hay and lowest for the clover hay, while voluntary intake was highest (P < 0.001) for the clover hay (36.6 g DM/kg LW per d) and lowest for the late-cut grass hay (24.7 g/kg LW per d).
5. The net rate of removal of indigestible fibre from the rumen appeared to vary within the day, with maximal disappearance occurring during eating, followed by a lag phase between 5 and 10 h after feeding, with a second increase in rate between 10 and 24 h post-feeding.