An in vitro study of the effects of supplementary dietary fibre in pig diets on forms of faecal N and P

Much of the N and P in pig slurry is in forms that can be readily lost to the environment via volatilisation, surface runoff and leaching with detrimental effects on the environment. This thesis investigated the effect of dietary fibre on the composition of pig faeces and explored the potential for its utilisation to valorise pig slurry. Specifically, it examined how increasing dietary fibre, based on its source and type, affects the forms of nitrogen and phosphorus in pig faeces using an in vitro model of the pig’s digestive tract. It was hypothesised promotion of fermentative activity in the hindgut of the pig via inclusion of dietary fibre would convert more available inorganic forms of N and P into less available organic forms.
The fermentative phase of the in vitro pig gut model was inoculated with pig faeces. Comparisons between the faecal N and P composition of gestating sows and effluent from the in vitro model ‘fed’ the same experimental diets as the gestating sows (control and sugar beet pulp supplemented diets) showed no significant interaction (P > 0.05) between source of waste (sows and model) and diet, indicating the model could be used reliably in further experiments.
Subsequent experiments showed that adding 5 % of soluble and insoluble dietary fibres, such as inulin or cellulose respectively, to pig diets significantly (P < 0.05) reduced total inorganic nitrogen (TIN) per gram of total nitrogen (TN) by an average of 50%. However, this reduction was not observed when 2% of calcium was added to these fibres. The inclusion of inulin in the diet significantly increased the ratio of inorganic phosphorus (Pi) to total phosphorus (TP) by 142% compared to the control diet (P < 0.05).
Then, an increase in fibre content from 15 to 27% was achieved by the inclusion of wheat bran, rice bran or soya hulls resulted in significant changes in the ratios of TIN to TN and Pi to TP, as soya hulls diet decreased by 54.34% the ratio of TIN to TN when compared to the control diet, while RB showed minimal changes in the ratio of TIN to TN and Pi to TP unlike the other fibre sources used in the experiment.
The experimental results showed that increasing soluble and insoluble fibre in the diets of growing pigs can favourably alter the forms of N and P excreted in an in vitro pig model. Soluble fibre, which is more fermentable, increased the availability of different inorganic and organic forms of N and P in the faeces compared to insoluble fibre. As a result, increasing levels of soluble fibre sources, such as inulin or sugar beet pulp, in pig diets may improve faecal quality and promote sustainable nutrient recycling.