Effects of sulphate supplementation of untreated and alkali-treated wheat straws on ruminal fermentation and microbial protein synthesis in a semi-continuous fermentor

The effect of sulphate supplementation on the fermentation of untreated or alkali-treated wheat straw by mixed rumen microbes was studied in a semi-continuous fermentor where urea was the main nitrogen source. In addition to the detergent method, chromatography of neutral sugar derivatives was used to estimate cell wall degradation of the treated straw. Microbial protein synthesis was determined using the N-15 labelling technique. The most suitable nitrogen supply was determined with untreated straw and estimated to about 40 g N kg-1 of fermented organic matter. With both types of straw, sulphate supplementation improved microbial protein synthesis, all the degradative processes and particularly cellulolysis. The response to sulphate-addition was more intense with treated straw which provided more potential energy. It appears that even after treatment, the availability of sulphur contained in the straw was too low to satisfy rumen microbial sulphate-requirements, though total sulphur was theoretically sufficient for requirements. This study demonstrates the importance of a concomitant and appropriate nitrogen and sulphate supplementation to optimise straw fermentation.