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1. Groups of adult colostimized chickens were given diets with and without dietary fibre in the form of bagasse or wheat straw. The fibrous materials were analysed for their contents of cellulose, hemicellulose and lignin. The digestible energies (DE) and metabolizable energies (ME) of these diets were measured by a balance method. 2.Groups of germ-free and conventional chicks aged 4 weeks were given diets with and without penicillin or with and without graded levels of wheat straw as a source of fibre and the ME of these diets were determined.3. The incorporation of the forms of dietary fibre tested reduced the DE and the ME of the diets in adult conventional chickens and the ME in young germ-free and conventional chicks. Both DE and ME were reduced in proportions that suggested that the dietary fibre present was acting largely as an inactive diluent of the dietary energy and was not affecting the absorption of other nutrients. In the young germ-free and conventional chicks, there was a linear relationship between the proportion of wheat straw in one diet and the reduction in ME.4. Dietary penicillin did not alter the ME of the diet in either the gem-free or the conventional environments. 5. The reduction of ME with incorporation of wheat straw was less in conventional than in gem-free chicks. It is suggested that this may be due to the ability of chicks with a gut flora to obtain a small amount of energy from wheat straw.Until recently dietary fibre has not been considered a necessary ingredient in the food of simple-stomached animals. Non-ruminant species are not thought to obtain nutrients directly from fibre although those with caeca may derive energy from fatty acids produced by fermentation. Relatively little is known of the effects of dietary fibre on the digestion and absorption of other nutrients. It has been shown that wheat bran accelerates intestinal transport in man (Payler et al. 1975) and the suggestion was made that fibre may act as a restriction on energy intake and mitigate against obesity (Heaton, 1973). In view of the increased rate of passage induced by dietary fibre, its inclusion in a diet may be expected to decrease the availability of nutrients by reducing the period of exposure of the food to digestive enzymes and the absorptive surfaces. Moreover, mechanical damage by the abrasive fibre could also affect nutrient uptake. It is, however, possible that the hydrophilic properties of fibre could lead to increased digesta volume and easier access by the enzyme.The aim of the work reported here was to investigate the effects of incorporating fibre in the diet on energy digestion and absorption in the chick. In view of the known ability of bacteria to digest cellulose, consideration was also given to a possible interaction between gut flora and the fibre in the diet. M A T E R I A L S A N D M E T H O D S ChicksRhode Island Red x Light Sussex cross chicks were used throughout; they were hatched from eggs from a specified pathogen-free flock maintained at this Institute. For Expt 1 the birds w...

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