Urinary excretion of silica by grazing sheep

Abstract

Two series of field collections were undertaken to determine the excretion of silica, chloride, and various cations in grazing sheep, and the specific gravity and surface tension of their urine, and to assess the significance of these items in siliceous urinary calculus formation. Collections were made on four occasions in each series at intervals of 3 months, when urine was collected for three consecutive 24-hr periods from groups of 10 ewes. The concentration of urine fluctuated seasonally in both series, and was related inversely to urine volume and directly to environmental temperature. During summer and autumn the urinary silica concentration was significantly greater in an area 'affected' with urolithiasis than in the coastal plain, which is unaffected; and of the observations made, urine concentration was the only one which might account for the difference in incidence of fatal urolithiasis. In the second series, the mean urinary silica excretion in the affected area in September, December, March, and June was respectively 164, 95.3, 76.2, and 100 mg/ day, and the faecal excretion was 6.0, 14.6, 20.5, and 19.3 g/day, which suggests a greater availability of silica in green than in dry feed. Urine nitrogen ranged from 15.5 g/day in the flush of spring growth to 6.6 g/day in autumn. A hyperbolic relationship was found between urine silica concentration and urine volume, the point of inflexion occurring at a urine volume of about 600 ml/day. The significance of this observation and the excretion of a concentrated urine are discussed in relation to silica and mucoprotein precipitation.