Reduced ruminai degradation of ryegrass caused by sulphur limitation

Abstract

AbstractItalian ryegrass was grown at a site low in plant‐available sulphur, with either 40 kg S ha−1 (S(+)) or no sulphur (S(−)) added. At the second cut after 8 weeks' regrowth, the S(−) grass had a lower concentration of sulphur while that of nitrogen was greater than in the S(+) grass. A greater proportion of the total nitrogen was recovered as amides in the S(−) compared with the S(+) grass. The S(−) grass also had a different morphology from the S(+) grass, with a reduced proportion of dry matter present as leaves and more found as flowers. The degradability of grass was determined by nylon‐bag measurements in the rumen of sheep fed either S(+) or S(−) grass. The S(−) grass had greater dry matter and cellulose degradabilities than the S(+) grass, irrespective of the sulphur status of the rumen. Continuously feeding the S(−) forage apparently reduced the ability of the rumen microbes to degrade both S(+) and S(−) grass. Amino acid analysis of grass residues recovered after rumen incubation showed that although the sulphur status of the grass influenced the loss of aspartic acid (present predominantly as asparagine), decreasing the sulphur status of the rumen had a greater effect—decreasing the loss of arginine, tyrosine, glycine, alanine, leucine, cyst(e)ine and methionine. Changes in the sulphur status of the rumen had no effect on the numbers of total viable or cellulolytic bacteria, but changing the diet to the S(−) grass increased the numbers of cellulolytic fungi.