Vertebrate animals lack endogenous enzymes to break down insoluble plant fiber, and therefore are dependent on symbiotic gut microbes to utilize this abundant food source. Although chemically higher in fiber, the efficacy of fiber particles in some commercial feeds, physically reduced in size during processing, has not been quantified. A concept developed for dairy cattle, physically effective fiber (peNDF) considers fiber physical characteristics that influence mastication and the biphasic nature of rumen contents (Mertens, 1997). The importance of describing fiber’s chemical and physical effects in nonruminant herbivores has also been identified (NRC, 2007). Tortoises provide unique insights into digestive processes influencing fiber particle length, because unlike mammalian herbivores, they do not reduce food particle size via mastication. We offered 16 leopard tortoises a diet supplemented with Miscanthus grass differing in physical length (1.18 – 4 mm and 8 – 19 mm). Individuals were allocated to treatment based on body weight. Total food intake and fecal output was quantified. We hypothesized that increased particle length of plant fiber would increase digesta retention, prolonging the plant fiber exposure to microbial fermentation and thus resulting in increased digestibility.
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