Dietary fibre for the newly weaned pig: Influences on pig performance, intestinal development and expression of experimental post-weaning colibacillosis and intestinal spirochaetosis

The diet fed in the immediate post-weaning phase influences the development of the intestinal tract and the establishment of microflora within the gut of the piglet. This development may prove beneficial to the long-term health of the pig or it may result in growth checks, diarrhoea and excessive growth of intestinal pathogens, depending on the diet fed and the stresses imposed upon the piglet. Post-weaning colibacillosis (PWC) is associated with excessive proliferation of enterotoxigenic Escherichia coli in the small intestine. These bacteria attach to enterocytes and release toxins that cause hypersecretory diarrhoea. Diarrhoea is accompanied by weight loss, anorexia, dehydration and eventually death if infection is severe. The pathogenesis of PWC is complex and multifactorial, and is heavily influenced by weaning-associated factors such as stress and the change in diet at weaning. The primary aim of this PhD study was to investigate the influence of dietary fibre on the intestinal and whole body development of healthy weaner pigs and pigs with experimental PWC. The latter animals were used to investigate the influence of fibre on the incidence of diarrhoea and the proliferation of pathogenic intestinal E. coli in experimentally infected pigs. Physiologically, the term dietary fibre includes any plant polysaccharides that are resistant to digestion in the small intestine, instead passing into the large intestine where they are fermented. The negative control diet used in all experiments contained cooked white rice as the main ingredient because it is very low in fibre content and is highly digestible. Sources of fibre were added into this diet to create the desired level and type of dietary fibre. In experiments 2, 3 and 4, a hydrolysed rice diet was also tested as a base rice ingredient. As different facilities were used for healthy and experimental infection experiments, Chapter 3 addressed the issue of food intake with regard to the design of experiments within the project. The remainder of the thesis was divided into sections based on the particular type of dietary fibre that was being targeted. Typically, pigs were killed 1-3 weeks after weaning, and intestinal and whole body measurements made. Dietary sources of primarily insoluble fibre were first examined, followed by sources of soluble fibre, and finally purified viscous soluble sources of fibre. Each of these areas was explored with regard to the intestinal and whole body development of the healthy pig before investigating how the type of fibre affected pigs with experimental PWC. As an extension of this work to other intestinal infections, the influence of dietary soluble viscous fibre on the pathogenesis of porcine intestinal spirochaetosis (PIS), a post-weaning large intestinal infection caused by the spirochaete Brachyspira pilosicoli, was also tested. In the healthy newly weaned pig there was rapid adaptation to the presence of increasing amounts of dietary fibre. Sources of insoluble fibre, particularly wheat and lupins, had the greatest effect on increasing the size of the large intestine and on increasing microbial fermentation, although the addition of soluble fibre (pearl barley) also was capable of significantly increasing large intestinal size and fermentative capacity compared to the base rice diet. As a result of feeding fibre sources, the carcase growth tended to suffer at the expense of intestinal growth, although all pigs expended energy into intestinal growth in the immediate post-weaning period. The ileal digestibility of nutrients at 10 days post-weaning was depressed by addition of fibre to the diet. The digestibility of crude protein and gross energy at the ileum decreased upon addition of insoluble fibre from lupins, and upon addition of resistant starch, whilst addition of pearl barley (soluble fibre) reduced the starch digestibility. The interaction of diet with the development and expression of PWC differed according to the diet fed. Inoculation with enterotoxigenic E. coli generally reduced the whole body and intestinal growth and intestinal fermentation of all pigs, although the pigs fed the cooked white rice diet suffered the least depression in growth and consistently had drier faeces throughout all trials. Raw wheat, as a primary dietary ingredient and source of insoluble fibre and resistant starch, did not significantly alter the proliferation of intestinal enterotoxigenic E. coli. However, extrusion of the wheat resulted in an increase in the proliferation of E. coli, as did hydrolysing the rice. Addition of pearl barley (soluble fibre source) to the cooked rice diet significantly increased the proliferation of enterotoxigenic E. coli as well as increasing the viscosity of the intestinal contents of healthy and experimentally infected pigs. Feeding a diet containing the viscous, purified polysaccharide carboxymethylcellulose (CMC) at 4% of the diet induced a natural infection of PWC without requiring experimental inoculation with enterotoxigenic E. coli. This effect was repeatable in the immediate post-weaning period but did not occur a few weeks after weaning. This study showed that dietary fibre affects the rate of development of the intestines and the pig’s ability to utilise dietary nutrients immediately after weaning. Although increasing the amount of dietary fibre was not always detrimental to the live weight growth of the pig, the experimentally infected pigs fed the cooked rice diet (low fibre) were the only infected animals to maintain positive growth rates. The expression of PWC was increased by the addition of soluble fibre and by increasing the viscosity of the intestinal contents. This work illustrates the complex interaction of diet with the microflora of the pig’s intestinal tract, and suggests that a highly digestible, low fibre diet may be most suitable for growth and prevention of intestinal disorders in the first one to two weeks following weaning.