In‐vitro digestibility of kale (Brassica oleracea) secondary xylem and parenchyma cell walls and their polysaccharide components

Abstract

AbstractThe cell walls of forage kale (Brassica oleracea L cvs Maris Kestrel, Vulcan and Thousandhead) stems were separated into parenchyma and secondary xylem fractions of high and low digestibility respectively. Previous work had shown that the lignified secondary xylem cell walls were the main components of the plant indigestible to ruminants. The cell walls were subjected to in‐vitro digestion with fungal ‘cellulose’ and with rumen liquor. These two methods gave well correlated results except when secondary xylem cell walls were first delignified with chlorite, when their mean digestible organic matter in the dry matter (DOMD) was 41·9% with cellulase but 64·6% with rumen liquor. Chemical fractionation of the cellulase‐digested residues showed that all fractions of the parenchyma cell walls, except the hemicelluloses from cv Thousandhead, were of high digestibility. In the secondary xylem cell walls the α‐cellulose fraction had DOMD about 50%, the pectic fraction being more digestible and the hemicelluloses less digestible: but no one polysaccharide fraction in these cell walls was completely digested or untouched by digestion.