The objective of the current study was to investigate the possible use of alkenes (unsaturated hydrocarbons) of plant waxes as diet composition markers in sheep. Twenty-four sheep (30 kg live weight) were housed in individual pens (n=16, “pen” sheep) or in metabolism crates (n=8, “crate” sheep) for the total collection of faeces. Once daily, sheep were fed 720 g DM of a two-component diet consisting of a roughage source (chaffed perennial ryegrass) and a high-quality supplement (unpelleted sunflower meal, SFM) in different proportions, 7:1, 6:2, 5:3 and 4:4 (4 pen sheep and 2 crate sheep/treatment). SFM was labelled with a solution of beeswax to provide an alkane/ alkene profile different from the chaff. Diet composition was estimated from C27, C29, C31 and C33 alkene concentrations in diet and faeces (total faeces or rectal grab samples), using the least-squares package ‘EatWhat’. Estimates were conducted with or without correction of faecal alkane concentrations for incomplete faecal recovery, calculated from the alkene intakes and faecal outputs in the crate sheep.
Although the faecal alkene recoveries of alkenes were low, they did not differ between chain lengths for alkenes C27 to C33. Therefore in cage sheep, alkene-based estimates of SFM proportions, with or without faecal recovery correction, did not differ significantly from the proportions fed and were as accurate as earlier alkane-based estimates. In pen sheep, SFM proportions estimated from rectal grab samples were slightly but significantly over-estimated, regardless of whether or not recovery corrections were used. The over-estimates were not related to recovery corrections but to the small differences in concentrations between rectal grab samples from pen sheep and the total faeces from cage sheep. The lack of effect of recovery corrections does not mean they should be excluded from such calculations, as there may be situations in which there is an effect of carbon-chain length on alkene recovery. Use of recovery corrections also permits an estimate of whole-diet digestibility.
The present results indicate that alkenes could be useful additional markers to the alkanes for estimating the proportion of two dietary components in diets offered to sheep.