The n-alkane technique does not accurately predict the nutrient digestibility of mixed diets fed to horses

This study was conducted to determine the fecal recovery rate of n-alkanes and compare the precision and accuracy of the n-alkane technique and the conventional total fecal collection method in estimating the apparent nutrient digestibility of mixed hay diets fed to horses. Six castrated male adult horses (361 +/- 12 kg body weight) were fed diets composed of different proportions of Tifton 85 bermudagrass (Cynodon dactylon) hay and alfalfa (Medicago sativa) hay in a 6 x 6 Latin square design. Treatments were as follows: T-100, 100% Tifton hay; T80, 80% Tifton hay and 20% alfalfa hay; T-60, 60% Tifton hay and 40% alfalfa hay; T-40, 40% Tifton hay and 60% alfalfa hay; T-20, 20% Tifton hay and 80% alfalfa hay; and T-0,T- 100% alfalfa hay. The n-alkane detected at the greatest (P < 0.05) mean concentration in feed was n-hexatriacontane (C36H74), followed by n-hexacosane (C26H54), n-heptacosane (C27H56), n-nonacosane (C29H60), n-hentriacontane (C31H64), n-tritriacontane (C33H68), and n-pentatriacontane (C35H72). A strong correlation (r = 0.91, P < 0.001) was found between carbon chain length and fecal recovery. The n-Hentriacontane, n-tritriacontane, n-pentatriacontane, and n-hexatriacontane had the greatest fecal recovery (mean = 0.91, P < 0.05) and were therefore used to estimate nutrient digestibility. None of the n-alkanes provided consistent results. Fecal recovery was incomplete, and fecal recovery rates above 100% were obtained regardless of the proportion of Tifton and alfalfa hay in diets. Dry matter, crude protein, neutral detergent fiber, acid detergent fiber, and mineral matter digestibility estimated using n-alkanes differed (P < 0.05) from digestibility values obtained by the reference method (total fecal collection). The simplest explanation for these results would be that horses preferred one type of hay over the other and had an uneven intake of each diet. Because Tifton and alfalfa hay differ greatly in composition, their uneven intake makes estimation of nutrient digestibility by fecal recovery of n-alkanes unreliable. Even after correction for fecal recovery, n-alkanes were inaccurate markers of nutrient digestibility.