In order to learn more about some of the reasons for differences between forage plant species in nutritive value, plant parts of three contrasting species, Onobrychis viciifolia Scop., Spergula arvensis L. and Lolium multiflorum Lam., were examined. In vitro digestibility was recorded in milled and in chopped (1 cm lengths) material, neutral detergent fibre (NDF) and lignin were recorded in milled material and the proportions of thick-walled, thin-walled and epidermal cells and the thickness of the cell walls were recorded in cross-sections. Early harvesting was compared with later harvesting in each of two years. The plants were grown in a heated glasshouse in spring-summer.

In O. viciifolia the petioles had much higher concentrations of NDF and lignin, a higher proportion of thick-walled cells and generally thicker cell walls than the leaflets, but the petioles were, if anything, slightly more digestible than the leaflets. The leaflets of O. viciifolia had cell walls of similar thickness to those of Trifolium repens and Medicago sativa. The leaves of S. arvensis had generally thin cell walls and a high proportion (0·84) of thin-walled cells. The stems of S. arvensis had high concentrations of NDF (633 g/kg) and lignin (94 g/kg) and a high proportion of thick-walled cells, but the walls of these cells were not particularly thick (1·4 μm). The leaflets and petioles of O. viciifolia and the stems of S. arvensis were much less digestible than the leaf blades or leaf sheaths of L. multiflorum. L. multiflorum leaf blades and leaf sheaths were low in lignin (16–18 g/kg) and had rather thin cell walls; the sheaths were lower in NDF than the blades and tended to be more digestible than the blades, despite thicker epidermal cell walls.