Effects of different amounts of wheat bran and oat hulls on production of short chain fatty acids in the hindgut of pigs

Fermentation of soluble fibre in the large intestine of pigs favours beneficial microbiota, but can also reduce feed intake (FI) by stimulating the ‘intestinal brake’ (Black et al. 2009). Numerous studies (Black et al. 2009) have associated short chain fatty acids (SCFA) to ileal and colonic brakes in the gut, and increased transit-time and reduced FI. Measurement of end products of fermentation such as SCFA in faeces is a valid method to assess large intestinal fermentation activity (Bauer et al. 2004). The hypothesis tested was that fibre source alters the extent of hindgut fermentation.

Different amounts of an insoluble fibre, oat hulls, OH: 0, 2.5, 5, 10, 15 and 20% or a partially soluble fibre, wheat bran, WB: 0, 5, 10, 15, 25 and 35% were added to a highly digestible base diet containing maize starch and dextrose (67%) as the main energy source. Pigs were assigned to diets in a randomised block design with a minimum of five pigs on each diet. Pelleted diets were fed ad libitum to pigs housed individually with free access to water over 21 days and FI (Ratanpaul et al. 2017) was found to be higher with OH than with WB. At the end of d 7, 14 and 21 faeces from each pig were collected, stored in an air-tight container, and transferred to a freezer (−18°C) within 1 h. Short chain fatty acids from faeces were extracted with water and quantitatively determined by gas chromatography. The data were analysed using a linear mixed modelling approach using ASReml version 3 (VSN International, Hemel Hempstead, UK).

On d 7, 14 and 21 (Fig. 1), pigs fed OH diets showed no difference in amounts of SCFA produced, whereas WB on d 7 at 25% (216.91 mmol/L) was found to have produced over 22% more (P = 0.031) SCFA than any other diet. Wheat bran at 35% (165.81 mmol/L) produced 23% less SCFA than WB at 25%. Since the intake of fibre diets at 25% and 35% WB was similar (Ratanpaul et al. 2017), the most likely reason for the lower amount of SCFA produced at 35% WB was reduced transit-time in the large intestine at such a high proportion of WB (Wilfart et al. 2007). Overall, with the exception of WB at 10%, the amount of SCFA produced increased with increasing amount of WB up to 25% and then declined at WB 35% for d 7, 4 and 21. On d 21, WB diets were collectively found to have produced over 20% more (P = 0.009) SCFA than OH based or control (0% WB) diets.

Fig. 1.  Total SCFA (mmol/L of faecal water) in faeces from (a) OH diets and (b) WB diets.

It has been reported elsewhere using in vitro fermentation studies that WB is more fermentable than a variety of carbohydrate substrates including OH (Bauer et al. 2004). WB is likely to favour growth of healthy microbiota in the hindgut with possible activation of the ‘intestinal brake’ and a decrease in FI. OH may not trigger the ‘intestinal brake’ because of its low fermentability, when fed with a highly digestible base diet. Oat hulls have low or no fermentability when compared to WB diet. Thus, this study confirms that fibre source alters the extent of hindgut fermentation.