Silica metabolism of the Merino sheep

Abstract

Diets of wheat or oat grain with or without chaff, mixtures of all three, and a diet of chaff, oats, and bran with and without lucerne chaff were fed to sheep for the purpose of determining silica metabolism on diets approximating those in the field. These diets supplied from 0.08 to 16.6 g silica per day.

Silica digestibilities and balances showed marked positive and negative values, which were considered to be due to variable retention in, or passage of particulate silica from, the rumen.

The mean urinary silica excretion on wheat was 22 and 25 mg/day and oats 126 and 163 mg/day for two levels of feeding. Values for wheat plus chaff and oats plus chaff were respectively 190 and 224 mg/day and for mixed diets from 169 to 205 mg/day. No relationship between total urinary silica excretion and apparent absorption of silica was observed, but excretion seemed dependent on silica intake up to a level of about 8 g/day. Beyond this intake, urinary excretion plateaued at approximately 200 mg/day.

Urine volumes on wheat grain diets were greater than on the corresponding oat grain diets, and these in turn were greater than on the mixed diets. The combined effects of urine volume and total excretion of silica resulted in urinary silica concentrations of 30 and 32 µg/ml on wheat grain diets, 255 and 336 µg/ml on oat grains, 266 µg/ml on wheat plus chaff, 419 µg/ml on oats plus chaff, and 421–728 µg/ml on mixed diets.

A hyperbolic relationship between silica concentration and volume of urine, similar to one obtained in field studies, and also one between specific gravity and volume of urine were demonstrated.

These findings are discussed in relation to the absorption and excretion of silica and the formation of siliceous urinary calculi.