Composition and rumen degradability of straw from three varieties of oilseed rape before and after alkali, hydrothermal and oxidative treatment

The composition and rumen degradability of straw from three varieties of oilseed rape Brassica napus L. (cv. Bienvenu, Rafal and Jet Neuf) grown under the same conditions were compared. Analysis accounted for greater than 97% of dry matter in each case. Although the composition of the three straws was broadly similar, Jet Neuf had a lower carbohydrate (67% of dry matter) and phenolic (8.4% of dry matter) content and a higher crude protein (9.4% of dry matter) content than Bienvenu (74.5, 10.0% and 5.4% respectively) or Rafal (73.9%, 10.1% and 3.9%, respectively). Values obtained for neutral and acid detergent fibre and acid detergent lignin were consistent with these differences. Additional information about the nature of the polysaccharides present in straw was obtained by methylation of whole straw samples. Degradability measurements in a sheep, made using the nylon bag technique, showed that the rate and extent of dry matter disappearance from Bienvenu and Rafal were virtually identical. Both straws were poorly degraded, showing only a 35% loss of dry matter after extended incubation. Jet Neuf had a higher degradability with 45% of dry matter lost after 72 h. Ammonia treatment had little effect on straw composition and no effect on its degradability. Sodium hydroxide solubilised between 10–20% of the total phenolics initially present in straw and improved the degradability of Bienvenu and Rafal by approximately 10 units but produced little improvement in the degradability of Jet Neuf. Autoclaving straw samples in water at 160°C and 200°C for 1 h resulted in a substantial solubilisation of dry matter. The degradability of the recovered insoluble residues, however, was not increased. Sulphur dioxide, ozone and alkaline hydrogen peroxide selectively degraded phenolic material without modifying the carbohydrate content of the straw. The degradability of ozone‐treated straw was substantially increased, whereas sulphur dioxide treatment had a lesser effect, and treatment with alkaline hydrogen peroxide was largely ineffective. Results obtained are discussed in relation to the structure of rape straw cell walls.