Effect of species, plant part, and season of harvest on n-alkane concentrations in the cuticular wax of common rangeland grasses from southern Africa

Abstract

The use of plant alkane concentrations to measure diet composition of herbivores has been shown to be a reliable technique in animals grazing temperate, sown pastures that contain a relatively small number of plant species. There is potential to develop this technique for use with free-range animals foraging upon species-rich rangeland. In order for the technique to be effective, the alkane concentration patterns (ACP) of the component species of the diet must be distinct from one another. Common species of grasses from southern Africa were analysed for their alkane concentrations in order to evaluate the use of the alkane technique for measuring diet composition under complex rangeland conditions. The alkane profiles were determined in different plant parts from 40 grass species gathered during the wet season and 23 gathered during the dry season. Statistical analysis, using ANOVA, showed that there were highly significant differences (P < 0.001) in the C ₂₅, C ₂₇ , C ₂₉ C ₃₁ , C ₃₃ , and C ₃₅ alkane concentrations between flower head and stem during both the dry and wet seasons. Similar statistical differences were apparent in the C ₂₅ , C ₂₇ , C ₂₉ , and C ₃₁ alkane concentrations of leaf and stem during both seasons; differences in C ₃₃ and C ₃₅ concentrations were significant but at a lower level (P < 0.01 and P < 0.05, respectively). Differences in C ₂₅ , C ₂₇ , and C ₂₉ alkane concentrations between flower head and leaf were only significant (P < 0.001) during the wet season. Statistical differences (P < 0.001) between whole plant samples obtained in different seasons were due to changes in the proportion of flower head, leaf, and stem. Cluster analysis often showed less similarity between plant parts of the same species than between whole plant samples of different species. It was concluded that ACPs measured in the selected species were probably too similar and thus, plants could not be identified using the alkane technique. However, it was possible to use the alkane technique to determine the proportions of flower-head, leaf, and stem in the diet.