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In animal production, it is essential to know the dynamics of the digestive processes and their relationship with feed consumption and composition to optimize the animals' performance.
The fibre fraction of the feed has a significant impact on digestive processes. Traditionally associated with a reduction in the energy density of diets and the digestibility of nutrients, fibre is being "rediscovered" due to its multiple benefits in pig feeding. In this sense, the proportion of fibre in the feed is no longer necessarily synonymous with lower quality. The positive effects of fibre on sow's satiety, intestinal health, and reduction of ammonia (NH3) emissions from slurry are well known. Furthermore, in a framework of improving sustainability and reducing costs in livestock, the inclusion of fibrous agroindustrial byproducts is increasingly common in feeds for all ages. This practice can also achieve a reduction of the carbon footprint of pork by incorporating by-products in feed, in substitution of high-grade raw materials. However, the specific effects of the different types of fibre on voluntary consumption, digestion and metabolism can be very variable. Here are some of these benefits and their relationship to the type of fibre.
At the physiological level, different sources of fibre influence key aspects of digestion, such as the food retention time in the gastrointestinal tract. This is determined, mainly, by the speed and frequency of gastric emptying and the efficiency of intestine peristaltic contractions (Black et al., 2009). Initially, an increase in retention time allows a longer contact time between food enzymes and other digestion components, facilitating their digestion and absorption. Traditionally, it is considered that the inclusion of indigestible fibre (bran and other lignocellulosic raw materials) in the diet increases the transit rate. However, some studies do not show differences in the transit rate or even reductions in transit rate at the level of the large intestine (Wenk, 2001, Urriola and Stein, 2010) when including indigestible fibre, which suggests that these effects may depend on the feed, its level of inclusion or other factors. On the other hand, the inclusion of soluble fibre (sugar beet, citrus...) reduces the transit speed, although it does not have any benefits on the digestion and absorption of nutrients. This reduction in transit velocity is mainly due to a delay in gastric emptying due to an increase in the viscosity of the digesta (Guerin et al., 2000) which, in turn, modifies the distribution of the food at the intragastric level. (Figure 1).
Figure 1. Images of gastric emptying in pigs fed a diet based on starch (a), beet pulp (b) and wheat bran at 10, 60 and 120 minutes post-ingestion (Source: Guerin et al., 2001 ).
Although soluble fibre initially exerts a negative effect on nutrient digestibility, it is the ingredient of choice when a positive effect on satiety in pregnant sows is sought. The reason is that this type of fibre is able to maintain the absorption of nutrients for longer periods of time and stimulate the production of volatile fatty acids (VFA) which, besides being a source of energy for the animal, are capable of influencing peptide release such as ghrelin, type 1 glucagon and YY peptide, associated with energy and lipid metabolism (Leeuw et al., 2008; Sánchez et al., 2012).
At the intestinal health level, fibre inclusion is capable of improving the absorption of nutrients, promoting the proliferation of beneficial bacteria and preventing colonization by opportunistic pathogenic bacteria. This prebiotic effect is key nowadays, in the current scenario of reducing the use of antibiotics. The effects on microbiota and health may be different depending on the type of fibre (Table 1). As an example, the inclusion of soluble fibre sources, such as resistant starch and pectins, promotes the proliferation of bifidobacteria and Lactobacillus, and reduces the enterobacteria count (Bikker et al., 2006; Cerisuelo et al., 2015; Fohuse et al., 2015). On the other hand, insoluble fibre could stimulate the growth of cellulolytic bacteria such as Ruminococcus (Bindelle et al., 2010). These effects are based on the fact that the VFA (mainly butyric) from fermentation are able to promote the proliferation and absorption capacity of the intestinal epithelium and modify the microbial growth. However, at high concentrations, soluble fibre can have negative effects on intestinal health, due to the increase in the viscosity of the digesta. Therefore, the best strategy to promote intestinal health seems to be the combination of different types of fibre in the feed (Agyekum and Nyachoti, 2017).
Table 1. Summary of studies assessing the effect of fibre type on productive performance and intestinal health in pigs (Adapted from Agyekum and Nyachoti, 2017)
| Phase | Challenge | Type of non-starch polysaccharides | Response | Reference | |
| Productive performance | Intestinal health | ||||
| Weaning | E. coli | Soluble | ADG Reduction | Higher PWD1 incidence and higher pH | Hopwood et al. (2004) |
| Weaning | E. coli | Soluble | No | Higher PWD1 incidence, no differences in pH | Montagne et al. (2004) |
| Grow/Finish | Lawsonia intracelularis | Insoluble | No | No differences in severity / extent of intestinal lesions and episodes of diarrhoea | Whitney et al. (2006) |
| Grow/Finish | Swine dysentery | Soluble | No | Lower PWD1 incidence | Thomsen et al. (2007) |
| Weaning | No | Insoluble | No | Lower PWD incidence, greater faeces consistency | Kim et al. (2008) |
| Weaning | E. coli | Soluble | No | Lower PWD1 incidence , higher ratio Lactobacillus: coliforms | Welloc et al. (2008) |
| Weaning | No | Insoluble | No | No differences in PWD, increased production of short chain fatty acids, reduction of E. coli and coliforms | Molist et al. (2001) |
| Weaning | E. coli | Insoluble | No | Lower incidence of PWD and E. coli, greater microbiological diversity | Molist et al. (2010) |
| Grow/Finish | No | Soluble | No | Enterobacteria reduction | Smith et al. (2011) |
1PWD: post weaning diarrhoea
The inclusion of fibre in the diet also has environmental benefits, such as the reduction of NH3 emissions from the slurry. Numerous studies suggest that the inclusion of ingredients rich in fermentable (soluble) fibre such as beet pulp, fruit pulp or soybean hulls, increases the proportion of excreted nitrogen in faeces in the form of microbial protein (organic N), thus reducing the amount of N that is eliminated by urine (N mineral) and, consequently, NH3 emissions (Portejoie et al., 2004; Jarret et al., 2011). Recent studies, however, indicate that the inclusion of insoluble fibre in the feed may also lead to a reduction in NH3 emissions, although the control mechanisms would be different (Beccaccia et al., 2015).
In a framework of economic efficiency, sustainability and health, the management of fibrous diets in pig feeding constitutes a new challenge and, at the same time, an opportunity to take advantage of its benefits in swine nutrition. However, these must be accompanied by the development of strategies that improve their digestibility.
