The development of an in vitro system for predicting nutrient digestibility in feeds for pigs
The feed industry urgently requires an in vitro system for routine evaluation of the nutrient digestibility of a wide range of feedstuffs. The aim of this project was to ascertain the critical elements of the digestive system for the development of an in vitro simulation of protein and carbohydrate digestion to the terminal ileum and of carbohydrate fermentation in the hindgut. Standard ileal (using T-cannulated pigs) and whole gut digestibility data were obtained for soyabean, rape seed and sunflower seed meals, maize and wheat. Significant microbial activity was measured proximal to the caecum but an estimation of its effect on the availablility of nutrients was inconclusive. The in vitro system was developed by examining the digestive potential of individual enzymic stages of digestion and building up a simulation with the least number of elements necessary. Dialysis was used as a method of separating the products of digestion from the undigested remains. The crude protein digestibility of soyabean and rapeseed meal and the lysine digestibility of rapeseed meal were significantly affected by the molecular weight cut-off of the dialysis membrane. In the case of the cereals the presence of starch, variation in particle size and the fact that the storage protein was relatively inaccessible to enzyme action necessitated the inclusion of a high temperature amylase predigestion to the in vitro system. The measurement of crude protein digestibility after 12 hours dialysis (X) compared with apparent crude protein digestibility (Y) and true crude protein digestibility (Y) gave the linear regression equationsY = 0.022980 + 0.915562X (r = 0.878)Y = 0.0229635 + 0.701886X (r = 0.790) respectively. The prediction of essential amino acid digestibility was poor. The best correlation coefficients were obtained after 13 hours dialysis for apparent (r = 0.144) and true (r = 0.229) essential amino acid digestibility. Preliminary work is presented concerning the development of an assay to simulate hindgut fermentation.