Fibre-rich ingredients differing in physicochemical properties modulate digesta transit and digestion kinetics in pigs

Physicochemical properties of fibres can affect digestive processes such as digesta transit, thereby affecting nutrient digestion kinetics. We evaluated the effects of (1) insoluble fibres differing in cell wall composition and physicochemical properties; wheat straw (WS), softwood flour (WF), and sunflower seed hulls (SF), and (2) addition of pectin to coarse insoluble fibres from straw (WSP), on the mean retention time (MRT) of digesta fractions and nutrient digestion throughout the gastrointestinal tract (GIT) of pigs. Twenty-four boars (49.1 ± 2.96 kg) were allocated to one of four diets, containing either 138 g/kg of WF, 144 g/kg of SF, or 150 g/kg of WS without or with the addition of 100 g/kg high-methylated pectin. The apparent total tract digestibility of nutrients was evaluated by quantitative faecal collection. After feeding diets supplemented with tracers, pigs were dissected following a frequent feeding procedure to approach steady-state passage of digesta, and assess MRT of liquids (Yb-EDTA), fine solids (TiO₂), and insoluble fibrous particles (Chromium mordanted fibres) throughout the GIT. Unlike coarse WS, no differences in pH nor DM were observed between the proximal and distal stomach for WSP, and fine WF and SF (P > 0.05). Compared with WS, WF and SF reduced the MRT of fine solids (-01:51 to 01:03 h:min; P ≤ 0.05), and fibrous particles (-03:14 to 02:23 h:min; P ≤ 0.05) in the proximal and distal stomach, decreasing their separation, especially in the distal stomach (-01:30 to 01:20 h:min; P ≤ 0.05). Addition of pectin to WS reduced separation between fine solids and liquids in the distal stomach (-43 min; P ≤ 0.05), increased starch digestibility in the mid-small intestine (SI; +9%-units; P ≤ 0.05) but not at the end of the SI, and reduced total tract digestibility of nitrogen (-8%-units; P ≤ 0.05), reflecting a lower ileal digestibility (-8%-units). In the large intestine (LI), coarse WS reduced the MRT of digesta phases compared with fine WF and SF (-17:38 to 08:08 h:min). Regardless of origin, fermentation of insoluble fibres was poor (≤19%), while complete fermentation of pectin did not impact the fermentation of WS. In conclusion, fine WF and SF accelerated the emptying of solids and suppressed regional differences in pH in the stomach, and delayed digesta transit in the LI, compared with coarse straw. Pectin addition to coarse straw reduced gastric sieving between fine solids and liquids, and increased starch digestibility in the proximal SI.