The second half of the XXth century has seen a sharp increase of the world pork (Sus scrofa) production, which reaches nowadays a herd of 964 million pigs (FAO, 2006). Intensification of the rearing techniques, breeding programs and genetic progresses has resulted in lower production costs. However, intensive production systems have caused nitrate leaching and phosphorus accumulation in the soils receiving pig manure. These systems also induced animal welfare concerns such as stereotypies in gestating sows and human health problems such as the development of a gut microflora resistant to antibiotics (Manero et al., 2006). All these issues seriously question the social and environmental sustainability of intensive pig production (Basset-Mens et al., 2005). During the last 15 years, different solutions have been proposed to cope with these problems. Efforts have been spent to formulate diets that better meet the pig’s requirements or contribute to reduce odour and pollutant excretion. In particular, attention is paid to dietary fibre (DF), for its capacity to reduce ammonia emission (Nahm, 2003; Aarnink et al., 2007) and to improve gut health (Williams et al., 2001; Montagne et al., 2003) and pig welfare (Meunier-Salaun, 1999; Courboulay et al., 2001).
Increasing fibrous ingredients provided by the food industry are now incorporated in rations for pigs, despite the negative impact of DF on performances due to lower digestibility of dietary energy and protein (Noblet et al., 2001) and fatter carcasses. The use of forages, rich in DF, is also envisaged in more extensive systems such as herbage in outdoor production systems (Rivera Ferre et al., 2001; Blair, 2007) or in tropical countries where alternative feeding systems are studied, developed and extended (Pérez, 1997; Leterme et al., 2007) because grains are needed for human consumption. The present review examines the influence of DF fermentation in the pig intestines on the digestive processes. Their consequences on pig protein and energy nutrition, health concerns and environmental issues in intensified and in more extensive tropical production systems are also discussed. Dietary fibre is defined as the plant polysaccharides that are resistant to digestive secretions and are potentially available for bacterial fermentation in the intestines of single-stomached animals. Resistant starch is also considered as a dietary fibre. The short-chain fatty acids released by bacteria contribute to the host energy supply and both regulate the composition of the flora and the growth of epithelial cells, especially in the case of butyrate. The bacterial growth supported by the fermentation induces a shift of N excretion from urine to faeces. Beside the fermentability, the physical properties of dietary fibre such as the water-holding capacity, the viscosity and the solubility influence the digestion, the satiety and the transit time. In relationship with the mechanisms of dietary fibre interaction with the digestive processes exposed in this review, the opportunities and treats of dietary fibre inclusion in swine rations for intensified and for more extensive tropical production systems are discussed. Dietary fibre is indeed a possible means to reduce nitrogen losses of production units and to improve pig intestinal health and animal welfare. Finally, the potential role of in vitro fermentation methods to investigate the fate of dietary fibre in the digestive system is discussed.