Experiments were conducted to examine the difference in intake between hay and silage made from lucerne (Medicago sativa) and whether these differences could be accounted for in terms of physical rumen characteristics. Voluntary intake was lowest on lucerne hay ensiled with molasses (16% on a DM basis) and highest on lucerne hay and lucerne ensiled with molasses and formic acid. There was a tendency for the OM content in the rumen (Z) to increase with increasing OM intake (I). Differences in the rate constant for fermentation and outflow of fermentable OM (ᵧ₁ + ᵧ₀) and rate constant for outflow of unfermentable OM (ᵧ₂) were small except for the lucerne hay diet.