The use of plant wax alkanes as marker substances in studies of the nutrition of herbivores: a review

Abstract

This review discusses the potential use of plant wax components, especially n-alkanes, as markers for estimating herbage intake, estimating the botanical composition of consumed herbage and studying digesta kinetics. Previous approaches to making these measurements are discussed briefly. Attention is drawn to the fact that current methods for estimating intake do not adequately allow for differences between individual animals. It is also suggested that the markers currently used to estimate botanical composition or study digesta kinetics are inadequate. The nature of the chemical constituents of plant waxes is briefly discussed and the concept of using alkanes to estimate intake is introduced. Particular emphasis is given to the fact that although the recovery of alkanes in faeces is not complete, intake can still be estimated using a pair of alkanes (one natural, one dosed) provided these have similar faecal recoveries. The accuracy of estimation of intake is discussed in terms of: obtaining a representative sample of herbage; alkane dosing and faecal sampling procedures; validity of the assumption of similar recoveries for the natural and dosed alkanes; sample preparation and analysis. Published comparisons of estimated and actual intakes are presented, with the conclusion that satisfactory results are obtained if intake is estimated using natural C33 alkane and dosed C32 alkane. The use of the different patterns of alkanes in herbage species, as a means of estimating botanical composition, is then discussed. Results are presented showing this can be done successfully with herbage mixtures or oesophageal extrusa. Procedures are then described for making the corrections for incomplete faecal alkane recovery, necessary to estimate the botanical composition of the herbage consumed by the free-grazing animal. This allows the quantification of the intake of individual plant species by individual animals, and it is suggested that this can be achieved without the need for oesophageally-fistulated (OF) animals. Differences in alkane levels between plant parts within a species are then discussed. It is suggested that these can lead to error in the estimation of intake, if OF animals should consume plant parts different from those consumed by the test animals. However, it is also suggested that differences in alkane levels between plant parts can be used to quantify the intake of these parts, in a manner analogous to the estimation of the intake of individual plant species. The usefulness of alkanes in studies of digesta kinetics is then discussed, principally in relation to the natural alkanes, which remain intimately associated with plant particles in the gut. It is suggested that natural alkanes could prove excellent markers for studies of particle breakdown and digesta flow. The preparation of natural 14C-labelled alkane, for use as a pulse dose in mean retention time studies, is also discussed.