Labelled waters of crystallization in gypsum to measure the intake by sheep of loose and compressed mineral supplements

Abstract

Tritiated water is a useful marker for determining the intake of a supplement by individual animals. The uniformity of mixing and the stability of tritiated water in dry supplements offered to grazing animals can be increased by incorporation of the tritium into the water of crystallization of an innocuous, non-deliquescent salt (e.g. gypsum). Laboratory and field tests in which labelled gypsum was mixed with salt (1 g/kg) indicated that the tritiated water was stable over long periods, up to 16 months, unless the salt was exposed to rain and absorbed sufficient water to dissolve appreciable quantities of the gypsum. In two field experiments there were large differences in consumption of salt both between individual sheep at any one sampling and between groups of sheep at different samples. The intake of granulated salt was consistently greater (720% in experiment 1 and 230% in experiment 2), and appeared more uniform, than the intake of the same material compressed into blocks. Problems from variations in intake between animals and over time are discussed. Further study is required to define the nature and extent of this variation and to explore means of reducing it.