Partition of excreted nitrogen, sulphur, and phosphorus between the faeces and urine of sheep being fed pasture

Abstract

Merino wethers in metabolism cages were fed a range of pasture samples at a level sufficient to maintain body weight nearly constant. The quantities of food eaten and of faeces and urine produced were recorded. The nitrogen, sulphur, and phosphorus contents of the feed and of the faeces and urine were determined. The phosphorus content of the faeces was further fractionated into organic and inorganic forms. Faecal excretion of nitrogen, sulphur, and organic phosphorus per unit of feed eaten was not significantly affected by the nitrogen, sulphur, or phosphorus content (respectively) of the feed eaten, nor by the level of feed intake. Average values were 0.835 g of nitrogen, 0.114 g of sulphur, and 0.059 g of organic phosphorus per 100 g of dry matter eaten. The remainder of the nitrogen and sulphur was excreted in the urine, and hence the proportion excreted in the urine depended on the nitrogen or sulphur content (respectively) of the feed. Most of the remainder of the phosphorus was excreted in the inorganic form in the faeces, and hence the proportion of the faecal phosphorus in inorganic form depended on the phosphorus content of the feed. However, these proportions also depended on the level of feed intake, especially if this was so low that the animal was in negative balance. Because the nitrogen / sulphur ratio of faeces (and also of wool) was smaller than that of the feed, the nitrogen/sulphur ratio of the urine tended to be larger than that of the feed. The sulphur and phosphorus contents of faeces, although correlated with digestibility, were inferior to nitrogen as an indicator of digestibility and were too highly correlated with nitrogen to be of much value as additional measures.