The most promising treatments for making wood more digestible are the following:
This method reduces the wood to microparticles. As particle size decreases, more cellulose is exposed. Carbohydrate digestibility in ball-milled wood approaches that of feed grain (70-80%).
These include treatment in aqueous solutions of alkali and vapour-phase treatment with sulphur dioxide. Red oak has been made 55% digestible by chemical treatment. After steeping in a solution of 15% sodium hydroxide, a poplar species with 5% dry matter digestibility in vitro had a digestibility of 50%. Alkali-treated aspen and birch sawdust have constituted up to 30% of ruminant rations with good results.
In some cases steaming has been a very effective method of increasing digestibility. Steamed aspen wood with a dry matter digestibility of 48% was used successfully as a substitute for hay in sheep rations.
The needles of conifers and the leaves of deciduous trees can be made suitable for animal feeds with little processing. Basically all that is required is heating at 210 C for a few minutes to drive off moisture and unpalatable essential oils, followed by milling. The major development of this process has taken place in the USSR, where about 100,000 tons of the product, called Muka, are fed to animals each year. Muka is somewhat similar to lucerne, being rich in cellulose, carotene and minerals and containing one half to two thirds as much protein. It is fed as a supplement to poultry, cattle, milking cows and pigs at 5-8% levels. The major impetus to the development of Muka in the USSR appears to have been the large-scale production of essential oils with Muka as a by-product.
Fungal degradation of lignin
This process is based on the ability of white-rot fungi to utilize lignin with minimum degradation of cellulose and hemicellulose. The product is a high cellulose-hemicellulose fibre which is a potential roughage for ruminants. Fungal degradation is slow compared to steaming or chemical treatment as it requires several weeks, but this is not necessarily a serious drawback. The process has not yet been developed for practical use, but the research in this area is intensive.