Wood

Wood contains a high percentage of potentially digestible carbohydrates, but when fed in the form of untreated sawdust or chips it is largely indigestible, even by ruminants. The structural components of wood - lignin, cellulose and hemicellulose - form a close physical and chemical complex called lignocellulose. Lignin gives plants strength and rigidity. The content of lignin varies from about 2% in immature forages to about 15% in mature forages, whereas in wood the percentage is somewhat higher. It is completely indigestible and also lowers the digestibility of cellulose and hemicellulose by acting as a physical barrier to cellulose-splitting enzymes. Hemicellulose consists of digestible polysaccharides constructed mainly or 5-carbon sugars. The sugar xylose is the commonest component of hemicellulose in forages. The digestibility of hemicellulose varies from 45-90%, depending on the sugars it is composed of. Cellulose is usually the most abundant polysaccharide of the lignocellulose complex and consists of 6-carbon sugar glucose. Pure cellulose is fully digestible by ruminants. The lignocellulose complex accounts for most of the gross energy in common forages and wood. The mechanism by which lignin affects digestibility is complex. Rye straw has nearly the same lignin content as birchwood, but rye straw is far more digestible. Hence the lignin content in itself is not a reliable yardstick for measuring digestibility. Wood species differ widely in lignin content, but as a rule the wood of conifers contains more lignin than deciduous or broad-leaf trees.
Numerous feeding trials and laboratory experiments have shown that the nutrients in untreated wood are essentially unavailable to farm animals with the exception of a few less lignified hardwood species. The new concept of feeding cattle on high-grain rations has increased the possibility of using wood residues like sawdust and chips as the roughage component. Experiments have shown that sawdust is an effective roughage substitute when it constitutes up to 15% of the total ration. Cattle compensate for the lower energy of sawdust-diluted feed with higher intake. Some sawdusts - poplar is an outstanding example - are partly digestible by cattle. The in vitro digestibility of spruce sawdust is nil, of oak sawdust 5% and of poplar sawdust 30%.
Comparatively mild treatments can markedly increase the digestibility of wood from certain species by exposing the cellulose from the protecting lignin to render it more accessible to attack by cellulose-splitting enzymes. In some woods the cellulose is partly exposed by openings in the lignin which can be widened by swelling. In other woods - for instance, white oak - these openings are plugged and chemical treatments are of little value.