Stem characteristics of two forage maize (Zea mays L.) cultivars varying in whole plant digestibility. II. Relation between in vitro rumen fermentation characteristics and anatomical and chemical features within a single internode

Internode 7 of the stem of two forage maize (Zea mays L.) cultivars was studied anatomically and chemically at anthesis and subjected to fermentation tests in rumen fluid, using an automated gas production system. For anatomical studies internode 7 was sectioned at the top, middle and base. For chemical analyses and fermentation studies, the internode was divided into five sections. The number of scle-renchyma layers increased from top to base of the internode, whereas cell wall thickness of the scle-renchyma decreased from top to base. The highest and lowest sections of the internode had a higher sugar content and lower levels of neutral detergent fibre, acid detergent fibre, and acid detergent lignin, and slightly higher levels of crude protein than the middle three sections. Also their hemicellulose and cellulose contents were different but the differences with the middle three sections were small. Maximum gas production from fermentation in rumen fluid was higher for cell wall material from the highest and lowest than from the middle three sections. This was mainly due to differences in chemical composition: the highest and lowest section had a higher lignin and a relatively high hemicellulose content compared with the middle three sections. Digestibility of the cultivar with the higher whole plant digestibility, Vitaro, exceeded that of the cultivar Volens for each of the sections examined within the selected internode. Secondary cell wall disappearance was only weakly correlated with lignin content or gas production. This suggests that other factors are important in the fermentation process. These may include anatomical factors influencing the accessibility of the tissues to micro-organisms and chemical and/or physical factors such as lignin composition, lignin localization within the cell wall and nature and extent of cross-linking lignin to other cell wall components.