Animal Feed Science and Technology, 23 (1989) 141-167 141 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Approach to the Energetic Importance of Fibre Digestion in Pigs. I. Importance of Fermentation in the Overall Energy Supply N.A. DIERICK, I.J. VERVAEKE, D.I. DEMEYER and J.A. DECUYPERE Research Centre for Nutrition, Animal Science and Meat Technology (R.U.G.), Proefhoevestraat 10, B-9230 Melle (Belgium) ABSTRACT Dierick, N.A., Vervaeke, I.J., Demeyer, D.I. and Decuypere, J.A., 1989. Approach to the energetic importance of fibre digestion in pigs. I. Importance of fermentation in the overall energy supply. Anim. Feed Sci. Technol., 23: 141-167. Much research has been focussed on fibre in pig nutrition during recent years. In order to ap- proach the energetic importance of fibre digestion as an integrated part of the overall energy supply in pigs, several aspects of fibre assimilation in pigs are examined in this paper. Firstly, the physiological influence of fibre on the enzymatic pre-caecal and microbial post-ileal digestion of nutrients is reviewed: ingestion, secretion, absorption and transit time. Secondly, the digestibility of fibre in pigs under different feeding and performance conditions is examined: source and level of fibre, feeding level, treatment, age, liveweight and physiological state of the animal, environment (climate), and the site in the gastrointestinal tract. Thirdly, the supply and utilization of energy from hindgut fermentation (volatile fatty acids, VFA) in terms of overall energy metabolism and requirements in pigs are approached through several techniques: slaughter techniques, cannulated pigs, arterio-venous differences in VFA con- centration, in vitro fermentation rates in caecum and colon contents, and isotope dilution tech- niques. From this literature study, it appeared that VFA may make only a small, but significant, contribution to the total energy available to growing-finishingpigs under practical conditions; the mean supply of net energy (NE) from VFA to the net energy for maintenance (NEro) or basal metabolism (BM) amounts to about 15%. The importance of VFA production for energy reten- tion may be greater, however, for non-lactating mature sows and breeding boars. Finally, measurements of energy supply from hind gut fermentation, through respiration and balance trials, are given for some fibre sources. The effect of fibre on performance and carcass traits of swine are also briefly reviewed. The conclusions emphasize that correct evaluation of the importance of VFA production to the energy supply of the animal can only be made through respiration and balance trials or carcass analysis because only these approaches take into account the depressive effect of fibre on nutrient (protein, amino acids, minerals) absorption in the small intestine, and the amount and nutritional value of the VFA formed. Fermentation should, therefore, be evaluated not as an isolated part, but as an integrated part of the overall energy-supplying system of the pig, otherwise the energetic importance of fermentation may be greatly overestimated. 0377-8401/89/$03.50 © 1989 Elsevier Science Publishers B.V.