Estimating compartmental and total tract apparent digestibility in horses using internal and external markers

Partitioning of apparent digestibility is helpful to assess the nutritive value of feedstuffs and to identify possible risks for digestive disorders. In horses, information on partial digestibility is less available and address less on carbohydrate fractions. It is suggested that large parts of water-soluble carbohydrates (WSC) are degraded already in the stomach, which elevates the risk for mucosal ulceration. Our objective was to investigate apparent digestibility of dry matter (DM), proximate nutrients, and WSC along the digestive tract using internal and external markers. Twelve adult horses were adapted for 20 d to meadow hay (1.5 kg/100 kg body weight per day) and crushed oats (1.2 g starch/kg body weight per day), which were offered 2 times a day. Six horses on the treatment group also received 0.15 g of fructooligosaccharides + inulin (via Jerusalem artichoke meal) per kg body weight per day. The other 6 horses on the control group received corncob meal without grains. The tested markers were 4 N acid insoluble ash (MA), acid detergent lignin (ADL), n-heptacosane, n-nonacosane, n-hentriacontane, and n-tritriacontane. Additionally, Cr2O3 (approximately 2.8 g/bolus) and TiO2 (approximately 2.5 g/bolus) were provided as a bolus 2 times a day. The horses were euthanized at d 21 approximately 1 h after the morning meal. Samples of digests were collected from the stomach (pars nonglandularis and pars glandularis), cecum, ventral and dorsal colon ascendens, and colon transversum. Feces were sampled at 5 d ante mortem. Compartmental and total tract apparent digestibility was calculated for each horse. The stomach seemed to be considerably involved in nutrient degradation with particular great disappearance of simple sugars, starch, and fructans (apparent digestibility was up to 78, 74, and 56%). No WSC were measured in the cecum. In the hindgut, apparent digestibility increased from the cecum to the colon transversum (e.g., for DM: 0.49-0.79 with AIA, and 0.44-0.56 with n-tritriacontane; for crude protein: 0.51-0.82 with AIA, and 0.47-0.63 with ntritriacontane; and for neutral detergent fiber: 0.43-0.76 with AIA, and 0.37-0.50 with n-tritriacontane; P < 0.05). Supplying of fructooligosaccharides and inulin had no effect on apparent digestibility. Differences among apparent digestibility estimates did mainly belong to the gut compartments (P < 0.05) and the markers (P < 0.001). For all nutrients, apparent digestibility estimates did differ less among the plant alkanes, but they had a vast variation among the horses when based on Cr2O3 or TiO2. The extensive disappearance of WSC, including starch, in the stomach should be considered for dietary composition and risk assessment concerning stomach ulceration. Parts of WSC possibly had not been degraded in the stomach, but rapidly flowed with the liquid phase into the intestine. The probable underestimation of apparent digestibility by ADL confirms the known instability of ADL during the digestive process. Acid insoluble ash probably overestimated apparent digestibility along the hindgut, especially for the fiber fractions. Thus, apparent digestibility seemed to be best estimated by cell wall-associated plant alkanes. The estimations made by Cr2O3 and TiO2 were only accurate for the total tract, considering mealtime feeding of mixed diets.