Current rationing systems for sheep and cattle aim to balance a deficit in a basal roughage diet by giving group-fed animals a fixed amount of supplementary food. Assumptions are made that both the intake of basal diet and supplementary feed are average values. Coefficients of variation in individual intake of supplementary feeds of 16-36% have however been observed (Foot and Russel, 1973; Foot et al, 1973) and this variation may be larger (67-107%) where supplements are available as feed blocks (Kendall et al, 1983; Ducker et al, 1981).
Recent work on the selection of feed ingredients by sheep (Kyriazakis and Oldham, 1993) and the effect of physiological factors such as parasitism on diet selection (Kyriazakis et al, 1994) suggest that there may be biological mechanisms behind this variation. Estimation of intake of supplements has been difficult in the past, particularly at pasture using chronic oxide and N-alkane indigestible marker systems owing to the need for complete faecal collection procedures and handling procedures that disrupt grazing. A promising new method using lithium as a marker has been developed in Australia (Nolan et al, 1994). This work was undertaken to evaluate the lithium technique for use under UK conditions to elucidate causes of variation in supplement intake of sheep.