The stable carbon isotope technique has been widely used to infer the dietary ecology of a range of animal species; however calibration of the technique with animals fed known diets is essential for accurate back-calculation of dietary preferences. The aim of this study was to identify suitable samples and back-calculation methods to predict short-term (2 to 3 week) dietary selection by sheep among plants with C-3 and C-4 photosynthetic pathways. Variation in integration time of dietary carbon into plasma and faeces: diet-tissue discrimination of carbon isotopes (fractionation) and the importance of accounting for the digestible or indigestible components of the diet was investigated. The results indicate that faecal and rumen samples provided the most accurate prediction of short term dietary changes in sheep selecting between C-3 and C-4 plants. The most accurate back-calculation method for these samples used delta C-13 of the C-3 and C-4 plants and accounted for both diet-tissue discrimination and differences in the indigestibility between the C-3 and C-4 forage. For faecal samples, the organic matter content of the diet originating from C-4 plants could be predicted with a mean error as low as 2.7%. Wool and plasma samples were not conducive to predicting proportion of C4 forage in the diet within 18 days after a change in diet; however plasma could be used to discriminate between animals fed 100% C-3 and C-4 diets after 3 days. The delta C-13 technique provides a valuable tool for researchers when designing pastures for dual environmental and production purposes. An understanding of what sheep select allows for development of appropriate grazing management strategies to optimise productivity and/or persistence of target species. (C) 2008 Elsevier B.V. All rights reserved.