Fat deposition and growth performance in broiler chickens are diversely influenced by maize or wheat following dietary crude protein reductions plausibly involving insulin.

There is increasing interest in developing reduced-crude protein (CP) diets for broiler chickens because their commercial adoption would generate a diverse range of advantages that would enhance the sustainability of the chicken-meat industry. However, the development of reduced-CP broiler diets is proving to be not straightforward, particularly when dietary CP reductions exceed 30 g/kg. The capacity of broilers to accommodate dietary CP reductions when offered maize-based diets is superior to their counterparts offered wheat-based diets. Numerous factors could be contributing to this difference but have yet to be identified with certainty. Maize-based, reduced-CP diets characteristically support better weight gains and efficiencies of feed conversion than wheat-based diets, but this better growth performance is associated with increased fat deposition, monitored as heavier relative abdominal fat-pad weights. This is an intriguing dichotomy. Insulin is a powerful anabolic hormone in mammalian species capable of promoting fat deposition, protein accretion and growth, but the importance of insulin in avian species is usually dismissed. This is because broiler chickens are considered both hyperglycaemic and resistant to insulin. However, the likelihood is that young broiler chickens are more sensitive to insulin than is generally recognised and the anabolic properties of insulin may be contributing to the diverse responses observed between maize and wheat in the context of reduced-CP diets. Dietary CP reductions may trigger increased plasma ammonia concentrations and metabolic acidosis, but both factors can influence insulin secretion and insulin resistance. Maize has slower rates of starch digestion and glucose absorption than wheat and it has been suggested that this generates a more sustained insulin release resulting in increased weight gains and fat deposition. If so, this could be driving the differences generated by the feed grain selected as the basis of reduced-CP diets. The intention of this review is to explore this proposition because if the causal factors of the differences between maize and wheat can be identified the development and acceptance of reduced-CP broiler diets should be accelerated.