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Wood and wood by-products

Useful reference: 388

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 percentages of the cell wall constituents of various plant materials on a dry matter basis are as follows:

								HEMI-
						CELLULOSE CELLULOSE LIGNIN						REF
Alfalfa (medium maturity).......							6.0		25.0		7.2	488
Orchard grass (medium maturity).							40.0		32.0		4.7	488
Rye straw.......................							27.2		34.0		14.2	426
Birchwood.......................							25.7		40.0		15.7	426
Sprucewood......................							20.9		46.0		24.1	426

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.