In order to understand better some of the reasons for differences between forage plant species in digestibility, different parts of nine plant species in either milled or chopped (1 cm lengths) form were examined for in vitro digestibility and in milled form for neutral detergent fibre and lignin. The nine species were: Trifolium repens L., Medicago sativa L., Desmodium intortum (Mill.) Urb., Brassica napus L., Lolium perenne L., Festuca arundinacea Schreb., Chloris gayana Kunth, Cenchrus ciliaris L. and Zea mays L. In each case early harvesting was compared with later harvesting in each of two years. The plants were grown in spring–summer in a heated glasshouse.

The forage at the early harvest was, on average, 1–4% units more digestible in vitro than that at the later harvest and generally slightly lower in lignin and neutral detergent fibre content. However, the stems of Z. mays were higher in neutral detergent fibre at the early than at the later harvest.

The leaf sheaths of L. perenne and F. arundinacea were more digestible than the leaf blades. L. perenne was more digestible than F. arundinacea in both sheaths and blades. The sheaths and blades of C. gayana and C. ciliaris were less digestible and had a higher neutral detergent fibre content than those of L. perenne and F. arundinacea. The leaf blades, excluding the midribs, and the stems and leaf sheaths of Z. mays were all rather high in digestibility when milled and moderately low in neutral detergent fibre and lignin; the leaf blade midribs of Z. mays were less digestible and higher in neutral detergent fibre than the stems and similar to the stems in lignin content. The leaflets of T. repens had an appreciably lower neutral detergent fibre content than the stolons and petioles and a rather lower lignin content in dry matter and yet were, if anything, less digestible than the stolons and petioles. The stolons of T. repens were much more digestible than the stems of M. sativa and D. intortum. The digestibility of D. intortum was low in all the plant parts examined, leaflets, petioles and stems. In both D. intortum and B. napus, the leaflets or leaf blades were much lower than the stems in neutral detergent fibre and lignin and yet they were no more digestible than the stems when milled.

The digestibility of chopped leaflets and leaf blades was similar to that of milled leaflets and leaf blades, but chopping rather than milling reduced the digestibility of stems (particularly of those of Z. mays), petioles, the leaf blade midribs of Z. mays, and, to some extent, leaf sheaths.