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A F.NIRS calibration equation was developed for predicting the 13C ratio in faeces, and therefore the proportions of C4 tropical grasses to C3 non-grass plants (primarily forbs, legumes and browses) selected by grazing cattle. This calibration equation was used to measure seasonal profiles of dietary non-grass intake for a NE Qld speargrass site with and without introduced pasture legume, and for a NW Qld Mitchell grass site. In addition, results are presented for cattle grazing savannahs in the monsoonal tropics of central, northern Australia. Non-grass can form a substantial component of the diet even where there is ample grass on offer and may contribute substantially to diet protein content during the dry season.