Feed intake and digestion responses in sheep to the addition of inorganic sulfur to a herbage diet of low sulfur content

Abstract

Young sheep were used in feed intake and digestion studies with a wheaten straw diet of low sulfur (S) content (0.71 g S kg-1 organic matter) fed without additional S (low S diet) or containing sodium sulfate (high S diet). With the high S diet, relative to the low S diet, (i) the levels of sulfide in rumen liquor were elevated, (ii) less time was spent in ruminating activities, (iii) rumen liquor volume tended to be lower (-l0%), (iv) indigestible markers were cleared more rapidly from the rumen (approx. +16%), (v) more organic matter (OM) and acid detergent fibre were digested in the stomach and in the alimentary tract as a whole (+8-16%), (vi) more non-ammonia nitrogen (NAN) flowed into the intestines (+13%) and this NAN had a higher content of sulfur amino acids (+ 17%), (vii) a greater quantity of NAN was digested in the intestines per unit of digestible OM intake (+ 12%), (viii) the concentrations of various amino acids in venous blood were substantially lower, and (ix) fungal activity in the rumen was higher. The number of bites performed on each rumination bolus was inversely related to fungal activity in the rumen, but the latter was not related to OM digestibility or to the sulfur amino acid content of the NAN flowing into the intestines. The OM digestibility was inversely related to the rate of clearance of markers from the rumen with the low S diet but not the high S diet. Voluntary feed consumption was similar with both diets, and in the low S diet it was inversely related to OM digestibility and positively related to marker clearance rate from the rumen. Digestible OM intake under conditions of ad libitum feeding was higher with the high S diet (+11%) and positively related to fungal activity in the rumen and to the sulfur amino acid content of the NAN flowing into the intestines. The data indicate that an inadequate amount of S in the low S diet impaired the metabolism of the rumen microbiota which in turn affected variables relating to digestion and metabolism. A quantitatively significant role for the anaerobic fungi in the structural degradation of fibre is suggested and attention is drawn to the need for studies on both feed consumption regulation with low S diets and the relation between herbage S status and the sulfur amino acid content of the rumen microbiota.