Validation of the n-alkane and NIRS techniques to estimate intake, digestibility and diet composition in sheep fed mixed lucerne: ryegrass diets

An experiment was carried out with sixteen non-pregnant, non-lactating adult Rasa Aragonesa ewes fed different proportions of lucerne and ryegrass hays to validate the use of n-alkanes and NIRS methods to estimate diet composition, intake and digestibility. The effect of two different faecal sampling procedures (daily pool (S1) or rectal spot samples (S2)) was also tested. The faecal concentration of n-alkanes was affected by the sampling procedure, although differences were lower than 12%. Faecal recoveries were not affected by diet, except in the case of C23 (P < 0.0001), C28 (P = 0.0264) and C33 (P = 0.0241), and followed a curvilinear pattern with alkane chain length. Recovery was complete in the case of C29, C31 and C33, which countersigns the usefulness of these internal markers for digestibility and digestive flow studies (especially C31, given its high concentration in most plants). The faecal recoveries of C31 and C32 were not similar, the assumption of identity affecting in an important way the reliability of intake estimates. The NIRS methodology appeared as a promising alternative for predicting diet composition (R2 of the cross-validation = 0.98) and faecal concentration of most natural alkanes, although that was not the case for dosed alkanes. In all cases, the prediction equations obtained from the spectra of the faecal spot samples were better than those obtained from daily pool samples.