A trial was carriedout to evaluate the change in digestion of tropical grasses leaf and stem tissueswith age. The grasses were signalgrass (Brachiaria decumbens), molassesgrass(Melinis minutiflora) and Tifton 85 bermudagrass (Cynodon sp).The 7th leaf of signalgrass and molassesgrass and the 11thleaf of bermudagrass were sampled by the time of their complete expansion (liguleexposure) and 20 days later. Segment of stem just below the sampled leaf wasalso sampled. Transversal sections of leaf and stem were examined at the microscopebefore and after rumen in vitro incubation. The degree of digestion ofdifferent leaf and stem tissues, as well as the cell wall thickness of stemsclerenchyma were estimated. Leaf blade and young stems showed larger digestedareas. Stem tissues with thick and lignified cell wall, the parenchyma bundlesheath, the sclerenchyma, the xylem and the epidermis were not digested. Tissueswith thin non-lignified cell wall, the mesophyll, the phloem and the parenchymawere completely digested. Molassesgrass and signalgrass leaves sampled 20 daysafter expansion had lower mesophyll digestion in relation to their recentlyexpanded leaves. Similarly, molassesgrass stem parenchyma digestion decreasedas plant aged. Regardless of age, leaf epidermis was only partially digested.Stem sclerenchyma cell wall showed percentage reduction in thickness from 7to 37%, after in vitro incubation. The rate of reduction in thicknessvaried from 0.007 to 0.018 µm/h.
