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Published online by Cambridge University Press: 27 February 2018
The fractional rate of digestion as compared with the rate of passage can determine the extent of digestion in the digestive tract and the amount of potentially digestible matter escaping digestion (Van Soest, 1982). The lower the rate of digestion or the higher the rate of passage, the more nutrients escape digestion. The effect of rate of passage is higher for food components with slow rates of digestion such as neutral-detergent fibre (NDF) (Mertens, 1985). Increasing feeding level is associated with higher rate of passage, lower residence time in the rumen and decreased digestibility of nutrients. Digestibility depression is considered in several energy evaluation systems (van Es et al., 1978; Moe, 1981; Agricultural and Food Research Council, 1993 etc.) although most of the systems use a standard percentage discounting energy value, regardless of foods or nutrient components. Studies (Tyrell and Moe, 1975) showed that digestibility depression is higher for cell wall than cell solubles, whilst El Khidir and Thomsen (1983) reported that digestibility depression of cell walls was responsible for proportionately 0-7 of the observed decrease in digestibility. Mertens (1985) reported that energy discount factors can be related to NDF content and total digestible nitrogen value of foods. Ørskov (1996) recommended prediction of food potential of both energy value and intake from degradation characteristics.