Evaluation of n-alkanes and their carbon isotope enrichments (δ13C) as diet composition markers

Plant cuticular n-alkanes have been successfully used as markers to estimate diet composition and intake of grazing herbivores. However, additional markers may be required under grazing conditions in botanically diverse vegetation. This study was conducted to describe the n-alkane profiles and the carbon isotope enrichment of n-alkanes of common plant species from the Mid Rift Valley rangelands of Ethiopia, and evaluate their potential use as nutritional markers. A total of 23 plant species were collected and analysed for long-chain n-alkanes ranging from heptacosane to hexatriacontane (C-27 to C-36), as well as their carbon isotopic ratio (C-13/C-12). The analysis was conducted by gas chromatography/combustion isotope ratio mass spectrometry following saponification, extraction and purification. The isotopic composition of the n-alkanes is reported in the delta notation (delta C-13) relative to the Vienna Pee Dee Belemnite standard. The dominant n-alkanes in the species were C-31 (mean+/-s.d., 283+/-246 mg/kg dry matter) and C-33 (149+/-98 mg/kg dry matter). The carbon isotopic enrichment of the n-alkanes ranged from -19.37 parts per thousand to -37.40 parts per thousand. Principal component analysis was used to examine interspecies differences based on n-alkane profiles and the carbon isotopic enrichments of individual n-alkanes. Large variability among the pasture species was observed. The first three principal components explained most of the interspecies variances. Comparison of the principal component scores using orthogonal procrustes rotation indicated that about 0.84 of the interspecies variances explained by the two types of data sets were independent of each other, suggesting that the use of a combination of the two markers can improve diet composition estimations. It was concluded that, while the n-alkane profile of the pasture species remains a useful marker for use in the study region, the delta C-13 values of n-alkanes can provide additional information in discriminating diet components of grazing animals.