Toxicology of Sulphur and Nitrate in Ruminants

Abstract

Sulfur, in either its inorganic or organic forms is essential in the diets of ruminants. Many studies have shown that ruminants require no more than 0 .2 % dry matter total sulfate sulfur in their rations to satisfy rumen microbial protein synthesis and tissue sulfur needs. In most instances ruminants receive adequate quantities of sulfur from a balanced ration, and supplementing the element is not necessary. Total sulfate intake that exceeds 0.3% dry matter has potential to decrease feed intake and cause either direct or indirect toxicities. Sulfur Metabolism Sulfur and its compounds are very rapidly reduced in the rumen to sulfide [sulfate (S 0 4) » s u lf ite (S 0 3) » s u lf id e (S2)]. The optimum pH for this reduction is around 6.5. It is the sulfide that is readily absorbed across the rumen wall. Sulfates are not absorbed in the rumen but are absorbed to some extent (30-40%) in the intestine. Sulfur containing amino acids (methionine and cysteine) is also reduced to sulfide in the rumen, and therefore contributes to the total sulfides available for absorption. Rumen microorganisms incorporate sulfide in protein synthesis, but when in excess, sulfide inhibits cellulose digestion by these organisms. Adequate dietary sulfur also enhances non-protein nitrogen (urea) utilization in the rumen. Once absorbed sulfur is incorporated in sulfur containing amino acids (cysteine, cystine, cystathionine, and methionine). Deficiency of sulfur and therefore the S-containing amino acids results in abnormal keratinization of hooves, hair and wool. The latter may contain up to 4% sulfur. Muscle and brain contain 0.25% and 0.5% sulfur respectively. Vitamins, biotin and thiamine, essential in lipid and energy metabolism also require sulfur. Sulfur is also im portant in the thiosulfate (S 2 0 3) binding and detoxification system of cells. Insulin, glutathione, and hemaglobin incorporate sulfur in their structures, as does chondroitin sulfate, an important component of cartilage. Animals therefore, deficient in sulfur generally show signs of poor growth and wasting