The response of wheat straw varieties to mild sulphur dioxide treatment

Straws of five wheat cultivars, including soft and hard and dwarf and semi-dwarf varieties, were treated with sulphur dioxide (SO₂), and the effect of treatment on monosaccharide composition and cell wall (CW) degradation by rumen microorganisms was studied. Despite agrobotanical differences, the monosaccharide profiles of the straw hydrolysates were similar. Glucose, xylose, uronic acids and arabinose comprised about 98% of the total monosaccharides and the ratio of glucose to xylose was generally 2:1. The major and general effect of SO₂ treatment was expressed in solubilization of the matrix polysaccharides, cellulose being hardly affected. More than 20% of the permanganate lignin was solubilized by SO₂ in four of the cultivars. The ‘Lakhish’ cultivar had the highest lignin content (13.3%), but its lignin proved resistent to SO₂ treatment. Organic matter digestibility in vitro (IVD) was maximally increased in four varieties (excluding the ‘Lakhish’ straw) to 77–84%, and the IVD of monosaccharides to 88–93%. The treatment effect on the ‘Lakhish’ variety was weaker, the IVD of organic matter and monosaccharides was only increased to 67 and 77%, respectively. Other straw batches of the ‘Lakhish’ variety have shown a strong response to SO₂ treatment similar to that found in the straw batches of the other four cultivars. The present study provides support to previous conclusions about the major biodegradation obstacles in wheat straw. The results of this study imply that mild SO₂ treatment is generally effective in increasing the degradability of wheat straw by rumen microorganisms.