Submitted to: Journal of the Science of Food and Agriculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/21/1998
Publication Date: N/A
Citation: Interpretive Summary: Forages play an important role in providing nutrients to ruminants. The structural carbohydrate stored in cell walls is poorly digested by ruminants. Increasing the degradation of forage fiber would have a tremendous positive impact on dairy production by providing increased energy, improving protein utilization, and decreasing manure waste. Forage tissues, such as stems and leaves, are made up of different cell types whose walls are digested at different rates and extents by ruminants. To determine the relationship of wall composition to fiber digestion we isolated and analyzed individual cell types from sorghum stems. Greater than 75% of the fiber weight in stems was contained in cells with very thick walls that make up the vascular and support cells. The large central portion of the stem was made up of thin walled cells (pith) and small groups of vascular cells. The chemical make up of all cell wall types were similar except thicker walls contained more cellulose and slightly more lignin. Digestibility of the isolated walls did not follow previous observations in that the thin walls (pith cells) were no better and even slightly less degraded than the thick wall types. Cell wall degradation is affected by chemical composition, but also influenced by the way wall components interact or cross-link with each other. Minor changes in the chemical composition of grass walls that disrupt normal component cross- linking should result in increased digestion of structural carbohydrate and more energy to the animal.
Technical Abstract: Cell types were isolated from sorghum stems at two stages of development, anthesis and grain maturity, to study cell wall characteristics. Cell walls were isolated from epidermis (EPID), sclerenchyma (SCL), vascular bundle zone (VBZ), inner vascular bundles (IVB), and pith parenchyma cells (PITH) and analyzed for total carbohydrate, acid insoluble lignin, total uronosyls, neutral sugars, and hydroxycinnamic acids. In addition, walls from SCL, VBZ, IVB, and PITH were subjected to chemical fractionation to separate wall carbohydrate into polysaccharide groups. Wall characteristics were similar at both plant maturities. There were differences in lignin concentration, hydroxycinnamic acids, and carbohydrate composition among cell wall types. Lignin was lowest in the PITH walls (169 g/kg) and highest in SCL and EPID (ca 211 g/kg). Xylans were most abundant in IVB and PITH walls whereas cellulose was most abundant in VBZ and SCL walls with greater rsecondary wall formation. Pectic materials were most abundant in PITH walls. Releasable hydroxycinnamates were not as consistent among cell wall types. Total ferulates, including ester linked and releasable ether linked, tended to increase from PITH to SCL (8 to 15 g/kg CW) with an increase in the proportion etherified within wall matrices (PITH 51 %; SCL 66%). Total p-coumarates showed opposite trends with PITH walls having significantly more (35 g/kg CW) than VBZ or SCL (19 and 13g/kg CW). EPID walls contained the least pCA (6.5 g/kg CW). Except for hydroxycinnamates, compositional trends for different wall types would reflect changes from primary walls to increased amounts of secondary wall. Neutral sugar analysis of indigestible residues indicated similar carbohydrate compositions with xylose being less degradable than all other wall sugars.