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Introduction
The discipline of poultry nutrition has undergone remarkable changes during the past 50 years. Nutritional requirements have changed substantially due to dramatic changes in the genetics of the birds used (Havenstein et al., 2003), and due to improved nutritional knowledge. The structure of the diet has also undergone major changes during this period. The use of mash diets, often combined with whole cereals either mixed with the mash or fed separately, was the standard feeding method in the first half of the past century. Pelleting was however gaining grounds, not the least for turkey, where one major advantage, curiously enough, was fewer losses for turkeys raised at windy locations (Ewing, 1951). Although a fine grinding of the ingredients did not appear to be favoured in scholarly poultry nutrition textbooks, the beneficial effect of fine grinding on pellet quality, concurrent with an increasing interest in pelleting of diets for broilers, and the concern about feed segregation in mash diets, gradually made fine grinding of ingredients a dominating principle. Simultaneously, fibre-rich ingredients such as green fodders and oats became less used, and layers were increasingly raised in cages where no litter material was available. In recent years, a coarser structure of the diet has received renewed interest in the scientific community and in the industry, not the least stimulated by a commercial practice of using whole wheat in many European countries, and due to literature reporting beneficial effects of whole wheat on performance. The gizzard obviously plays a key role, and thus the issue of diet structure has resulted in a renewed interest in nutritional effects of diets which stimulate development and function of the gizzard. This review is an attempt to summarize some available literature on the interaction between diet structure and the gizzard.
Anatomy and function of the gizzard
The digestive tract of poultry has a number of distinctive traits compared to other domestic animals, such as the crop, caeca and cloaca. The adaptation to grinding food particles in the digestive tract through development of a very large mass of strongly myolinated smooth muscles immediately posterior to the glandular stomach, is another unique feature. The resulting specific organ - the gizzard - is not unique to poultry,...