Synthesis of microbial protein in the rumen. IV. The influence of the level and form of dietary sulphur

Abstract

When sheep were fed on a diet supplying 0.61 g sulphur per day, 82 g microbial protein was produced daily in the rumen. Raising the sulphur intake to 1.95 g/day increased protein production to 94 g/day (P .: 0.05), but there was no further increase when the intake was raised to 3.42 g/day. Protein production was not influenced by the s form of the supplementary sulphur. Sulphur as cystine resulted in higher rumen fluid sulphide levels than sulphur as inorganic sulphate. Nevertheless, measurable amounts of free cystine flowed out of the rumen when cystine was given. This may explain the higher nitrogen balances supported by cystine than by sulphate. Sulphur balances were always positive, and there were no differences between treatments. At least 0.44 g sulphur was recycled daily when the basal die twas given. This accounted for 42% of the total sulphur flowing out of the rumen. The importance of sulphur recycling is discussed.