The proportions of alkanes in plant cuticular waxes are influenced by sampling date, site and grazing pressure. Alkanes in organs of major wild forage grasses in Japan were evaluated to estimate the intake and diet composition of grazing animals. Samples of four grasses from a fixed site in native grassland and three grass species from different sites were collected from June to November, to test the temporal and spatial effects on alkane concentrations. Most alkane concentrations in Miscanthus sinensis Anderss, Pleioblastus chino (Franch. Et Savat.) Makino, Sasa nipponica and Zoysia japonica increased from June to July, and then decreased to mid November. Alkane concentrations in the Z. japonica sward used to test the effects of grazing pressure decreased from May to September. Although the sampling dates affected the alkane concentrations, the species effects accounted for 70–93% of the observed variation. Grazing pressure affected only the C29 (n-Nonacosane) alkane concentrations in dead aboveground materials of Z. japonica, but altered the ratios of C29[ratio ]C31 (n-Hentriacontane) in both dead and live Z. japonica. There were positive correlations between C27 (n-Heptacosane), C29 and C35 (n-Pentatriacontane) alkane concentrations in live leaves of M. sinensis and of C31 in live leaves of P. chino with growing season temperature and radiation. The concentrations of C25, C27 and C29 in dead leaves of M. sinensis and of C33 (n-Tritriacontane) and C35 in live Z. japonica were significantly correlated only with radiation. Alkane concentrations differed between the sampling sites, and considerable differences were observed between samples from Iwate and Kumamoto. Both live leaf and dead leaf samples from M. sinensis contained substantial amounts of C27, C29, C31, C33 and C35, and showed that alkane concentrations in stems were much lower during the whole growing season. Although concentrations of C29 were highest in Sasa nipponica (Makino) makino et Shibata, concentrations of C31 or C33 were highest in the other three grass species. Grazing pressures had little effect on alkane patterns, in which alkanes with odd-numbered carbon chains were predominant and most alkanes were of chain length C25 (n-Pentacosane) to C35 (n-Pentatriacontane).