Wheat and barley in pig and poultry diets: Effects of physical processing and enzymes; Assessment of nutritive value

Abstract

Knowledge of the effects of processing on the nutritive value of cereals is of direct relevance to the animal feed industry. Recognition of the importance of optimal combinations of mechanical and thermal inputs in grain processing and the identification of appropriate combinations of process and enzyme supplement are likely to lead to developments in systems of feed preparation which can be incorporated directly into industrial practice.

In examining the nutritional consequences of processing the primary focus was on digestibility and the secondary focus on growth performance. The rationale for this was that processing would alter the chemical and physical structures of the grain and thereby increase or decrease the amounts of nutrients released in digestion and that these changes would be the primary cause of any alterations in growth performance. For the most part this supposition proved correct. However, the effects observed differed in magnitude according to the nutrient examined. Furthermore, some changes associated with heat processing were detected, the effects of which were not confined to the gastrointestinal tract but could also affect post-absorptive metabolism. In particular, the effect of thermal energy on protein was found to have opposing effects: on the one hand, increased digestion and absorption due to the disruption of cellular structures; on the other, heat damage to protein, with a reduction in the availability of lysine.

There were other effects of processing which went beyond alterations in digestibility. The most significant general effect of processes involving heat treatment was the increased solubilisation of non-starch polysaccharides leading to an increase in the viscosity of the diet, and of digesta. These changes led not only to reduced digestibility, but (in chicks) to reductions in feed intake and growth rate as well. In the more extreme cases these changes were accompanied by sticky beaks and sticky droppings. The addition of supplementary enzymes generally had the effect of at least partially reversing the deleterious effects of heat treatment and at best enhancing the recovery of nutrients to an extent not achieved by either processing or enzyme addition alone.

In the final part of the project four methods of processing were compared and the interaction between processing and enzyme supplementation examined in whole diets based on a mixture of wheat and barley. This confirmed in practical diets what had previously been shown with single cereals alone.

The project has highlighted the need for further and more detailed investigation of the alterations in grain structure and chemical composition resulting from mechanical and thermal processing, in particular the effects on the cell walls of the aleurone layer.