Chapter 2 Variability in cereal grain composition and nutritional value: the importance of fibre

Large variation in the nutritional value of cereal grains, within and between animal types, is due predominantly to differences in digestive tract anatomy, site and extent of microbial fermentation, and amount and type of dietary fibre. Energy available from digestion is reduced by grain fibre in pigs, broilers and ruminants. The effects are greatest for broilers and least for ruminants, reflecting large differences in intestinal microbial populations. Individual grain samples are often better suited for digestion by one animal type than another. In pigs, the extent of starch digestion in the small intestine scales with the rate of amylase diffusion into grain particles, grain particle size and the time particles have for digestion, determined by residence time of digesta to the terminal ileum. Amylase diffusion rate is influenced by endosperm tissue and cell wall integrity, solubility of grain protein matrix, composition of starch and processing methods. Rate of passage of digesta to the terminal ileum increases as fibre content of the diet increases to similar to 15% neutral detergent fibre (NDF), then declines with additional fibre causing distention of the stomach. Diet hydration capacity, which is influenced by soluble fibre content, slows digesta passage rate once it exceeds similar to 1.2 g water/g dry matter. Undigested starch reaching the colon causes a decrease in passage rate through activation of the ileal/colonic brakes. Voluntary feed intake in monogastric animals is related to rate of passage to the terminal ileum for nutritionally balanced grain-based diets. Approximately 15% of energy in starch fermented in the large intestine is lost to the pig as heat, methane and microbial products excreted in faeces. Algorithms predicting effects of diet fibre, water-holding capacity and amylase diffusion rate can be used to predict threshold grain particle size, where all starch is digested by the terminal ileum, and the likely loss of energy through fermentation.