Estimation of feed intake and apparent digestibility of equines and cattle grazing on heathland vegetation communities using the n-alkane markers

The application of n-alkanes as faecal markers to estimate feed intake and apparent digestibility (DMD_ap) of equines and cattle was studied. Additionally, the effect of using different data on diet composition, known proportions of the diet components (DC1) and those estimated using the alkane markers (DC2), on the accuracy of intake and DMD_ap estimates was evaluated. Six mature horses, divided in two groups of three animals (H1 and H2), and three adult non-lactating cows of Asturiana de los Valles breed (C) were housed in individual stalls. H1 and C groups were fed on a diet composed of Lolium perenne L. (70%) and heather (30%) and H2 received L. perenne (40%), heather (30%) and Ulex gallii Planchon (30%). The dietary component heather represented the field proportions of different plant species of heathland, namely Erica umbellata L., Erica cinerea L. and Calluna vulgaris L., at this experimental period. All animals received a daily dose of paper pellets containing C₂₄, C₃₂ and C₃₆ n-alkanes as external markers with the purpose of using different n-alkane pairs of adjacent chain length for feed intake estimations. The results indicated that a period of 3 and 5 days was sufficient for these external markers to reach a steady concentration in faeces of cattle and equines, respectively. In contrast to the results obtained in cattle, the alkane faecal recovery in equines was unrelated to the carbon chain length. Diet composition only affected the faecal recovery of the alkanes C₂₄ (P < 0.05), C₃₁ (P < 0.05), C₃₂ (P < 0.05) and C₃₆ (P < 0.01) in the faeces of the equines, suggesting a different dispersion of the synthetic n-alkanes in the digesta. In equines, DMD_ap estimates were not affected by the n-alkane (C₂₇, C₂₉, C₃₁ and C₃₃) used in the calculations, contrasting with the significant (P < 0.001) effect observed in cattle. In both animal species, the data on diet composition (DC1 or DC2) used in the calculations did not affect DMD_ap estimates. Feed intake estimates were affected by the alkane pair used in the calculations in H1 (P < 0.05), H2 (P < 0.001) and C (P < 0.001). The data on diet composition used in the intake calculations affected the resultant estimates in H1 (P < 0.05) but not in H2 and C. The differences from the known intake values were lower when using C₃₁:C₃₂ alkane pair, overestimating intake in only an average of 4.5, 13.0 and 1.3% in H1, H2 and C, respectively, using DC1 or DC2. The results obtained in this study confirm the accuracy of the n-alkane markers to estimate simultaneously feed intake, apparent digestibility and diet composition of equines and cattle grazing these type vegetation communities.