The use of naturally occurring and artificially applied n-alkanes as markers for estimation of short-term diet composition and intake in sheep

The potential use of faecal n-alkanes for estimation of intake and diet com position over periods of 1-2 days was assessed in two experiments. The aim was Co determine the accuracy with which intake and diet composition could be estimated by characterizing faeca.l excretion of n-alkanes following a d iscrete dose as opposed to steady state kinetics used in previous work. In the first experiment, 16 sheep were fed mixtures of spinach (rich in C-3 1-alkane) and cabbage (rich in C-29-alkane) in known proportions and amount s for two days. Artificial n-alkanes (C-28- and C-32- alkane) were dosed on four occasions during this time. Total intakes were controlled at 0.2, 0 3 , 0 4 or 0 5 kg dry matter (DM) per day and nominal amounts of spinach offe red (as a proportion of the total diet) were 0 00, 0.15, 0 30 and 0 45. Eac h sheep received a unique combination of intake and dietary proportions (fo ur intake rates x four proportions). Sheep were fed fresh grass (timothy, P hleum pratense) before and after feeding spinach and cabbage. Sequential re ctal grab samples of faeces were collected at regular intervals and total f aecal collections were carried out over 144 h from the start of the spinach /cabbage feeding period to obtain samples for n-alkane analysis. In the sec ond experiment, fresh grass was sprayed with two combinations of artificial n-alkanes (C-28- and C-32- or C-28- and C-36-alkane) and fed to 16 sheep o ver a 24 h period. Each sheep received one of four intake rates(0 8, 1 0, 1 2 or 1 4 kg DM/day) and, within intake rates, each sheep received one of f our different proportions of the herbage sprayed with the combinations of n -alkanes (0 2, 0 4, 0 6 or 0 8), in a similar fashion to the first experime nt. In order to estimate intake, C-26- and C-34- alkanes were dosed at the start of the feeding period. Faecal sampling procedures were the same as th ose in the first experiment. Different parameters of faecal excretion curves of dosed and natural n-alka nes were used to estimate dietary proportions and intake. Parameters tested included area under the excretion curve and curve maximum. Dietary proport ions were calculated using an iterative minimization procedure employing fa ecal and herbage n-alkane concentrations. Intakes were estimated using rati os of dosed:natural faecal n-alkanes. The best estimates of dietary proportions were obtained using faecal concen trations at a single point in time in both experiments (> 80 % variation ex plained for regressions of estimated v. actual proportions). Intake estimat es required the calculation of the: area under the excretion curve to obtai n acceptable estimates (70-90% variance explained for regressions of estima ted v. actual intakes in Expt 2). The experiments demonstrate that precise estimates of diet composition can be obtained using single faecal samples f ollowing consumption of simple herbage mixtures over 24-48 h. Intake may al so be estimated using this technique provided that a series of faecal sampl es are collected over 4-5 days following the period of ingestion.