Review article: The alkali treatment of straws

The Beckmann method of alkali treatment consists of soaking straw in dilute alkali solutions for 24 hours and then washing it with clean water. Straw digestibility is increased from about 40 to about 70%. While this process has been known for 50 years, it has not been much used because treatment costs are too high. Also, it cannot be industrialised. The spray process, in which the straw is wetted with an alkali solution (Wilson and Pigden, 1964) is an improvement from both points of view, but poses nutritional problems since straw is not washed after treatment. So great is the potential, however, that within the last seven years nearly 100 research reports have been published on the production and use of spray-treated straw. These reports are reviewed in this article. Straws are poorly digested by ruminants because of their high cell-wall content. Alkali treatment disrupts the cell-wall by dissolving hemicellulose, lignin and silica, by hydrolysing uronic and acetic acid esters and by swelling cellulose. Digestibility in vitro increases from about 40 to about 80% with 10 g NaOH100 g straw. Equally large increases are not, however, obtained in vivo because of unreacted alkali and/or high sodium concentration. Several hypotheses concerning the depression of digestibility in vivo are reviewed. In general 3–6 g NaOH100 g straw is the optimum. For maximum effectiveness the volume of the alkali solution must be between 50 and 200 ml100 g straw. The usefulness of neutralising unreacted alkali has not yet been determined. Crop and industrial residues with lower initial digestibility than straw (e.g. paddy hulls, bagasse and some types of sawdust) are usually still too poor after treatment (digestibility 30–50%) to be useful feeds. Pressure and temperature increase the effectiveness of alkali, but add to the cost of treatment. The pelleting of treated straws probably increases the effectiveness of alkali treatment. The length of time treated straw is allowed to “cure” before being fed does not affect its digestibility. Chemicals other than NaOH, like chlorine, ammonia and peroxides, are also effective in treating straws but are more expensive and/or more difficult to apply. Animal feeding experiments with sprayed straw have shown its utility for livestock normally fed poor straw diets, in high concentrate diets for growing, finishing and milking stock, and as an extender of silage. The factors affecting the degree of improvement to be expected in digestibility, growth and production from the treatment of straw need to be identified and studied. In spite of its high pH and Na content, sprayed straw has not been found to cause any health problems in livestock when treatment is in the range of 3–8 g NaOH/100 g straw. The extra sodium is excreted in the urine. Water intake and urine volume increase. Milk composition is unaffected. Several factories producing alkali spray-treated straw-based diets in pelleted form are already in operation in Europe. The process is briefly described.