In order to understand more about the nature of forage particles before they enter the rumen and about the degradation of cell walls in the rumen, particles of different plant parts produced by sheep when eating each of eight plant species were described in terms of volume and exposed surface area and in terms of numbers of cells and volume and surface area of cell wall. Estimates were made of the extent to which the walls would be degraded in the rumen. The eight species were: Trifolium repens L., Medicago sativa L., Desmodium intortum (Mill.) Urb., 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 a heated glasshouse. The plant parts which were chewed to the smallest particle size during eating, and which produced particles with the largest proportion of total cell wall area exposed on the outside of the particle (6±10%), were the legume lea¯ets and the grass leaf blades and sheaths. The proportion of total cell wall area exposed on the outside of chewed particles derived from stems was only 3±4%. The number of thick-walled cells per mm$ of chewed particle was greatest in particles derived from the leaf blades and stems of C. ciliaris, the leaf blades and leaf sheaths of L. perenne and the stems of C. gayana. The average number of thick-walled cells per particle was 860000 in particles derived from the stems of C. gayana and C. ciliaris and 260000 in particles derived from L. perenne leaf blades and sheaths. The volume and surface area of walls of thin-walled cells per mm$ were greatest in particles derived from legume lea¯ets. The total surface area of walls of epidermal cells per mm$ was greatest in particles derived from lea¯ets and leaf blades and least in those derived from stems. The estimated percentage of wall thickness of thick-walled cells which would be degraded in the rumen was relatively high in the case of leaf blades, sheaths and stems of C. ciliaris, leaf blades of Z. mays, leaf sheaths of L. perenne and F. arundinacea and stems of C. gayana, relatively low in the case of petioles of T. repens and stems of D. intortum andM. sativa and close to zero in legume lea¯ets. The estimated percentage of wall thickness which would be degraded did not seem to be related to the concentration of lignin in the walls, either in grasses or in legumes.

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