Feed efficiency data for growth selected and unselected and also stocks divergently selected for body weight were reviewed with special emphasis on changes immediately after hatching. Examination of feed efficiency differences between genetically diverse stocks from hatch to market age normally reveal only small differences in the efficiency of feed utilization. On the other hand, differences in feed intake appear to be the dominant factor in growth-related changes of these genetically diverse stocks. However, when data were segmented into periods which allowed investigation of differences at early ages, especially immediately after hatching, major differences in feed efficiency between genetically diverse groups were observed.

These data indicate that feed efficiency plays a more important role in explaining genetic differences in growth than indicated by previous investigations. The appearance of the greatest differences between genetically diverse stocks immediately after hatching indicates that early feed efficiency changes may allow for greater subsequent feed intake. Therefore, it appears that changes in early feed efficiency along with changes in feed intake both play major roles in explaining genetic differences in growth. The role of feed efficiency is apparently obscured by the maintenance requirements associated with large body sizes of selected stocks when measurements are obtained at older ages.

Throughout Asia and the Pacific region, inadequate supply of maize will limit further expansion of the poultry industries during the coming decade. In this context, the use of locally available energy feedstuffs merits increased attention. This review examines the potential feeding value and nutritional limitations of some carbohydrate sources, with special reference to Asia and the Pacific region. Some guidelines that may lead to better utilization of these feed resources are also discussed.

The non-starch polysaccharides (NSPs) of cereals exhibit anti-nutritive activity when present in broiler diets. The high levels of arabinoxylans (pentosans) in rye and β-glucans in barley are responsible for the poor nutritive value of these cereals. Australian wheats vary considerably in their nutritive value with some having very low apparent metabolizable energy (AME <13 MJ/kg) values. Recent studies have shown that the low AME wheat problem is probably due to the variation in the levels of the water-soluble wheat NSPs which are predominantly pentosans. When pentosans isolated from rye or wheat are added to broiler diets, dose-dependent depressions in nutrient digestion and growth occur. Several treatments have been shown to be effective in improving the nutritive value of cereals. Soaking the cereals in water or adding NSP-degrading enzymes decreases the anti-nutritive activity of the NSP. Addition of antibiotics to diets also has been shown to increase the nutritive value of diets with high levels of NSP suggesting that the action of these materials is, at least in part, mediated by the gut microflora.

Among poultry scientists there is an increasing interest in model building because models are useful in the investigation of the economic consequences of management decisions and in identifying gaps in current knowledge of the biological processes involved in production. In addition to empirical formulae giving the relationship between one or two dependent and one or more independent variables, more mechanistic models are needed to solve problems which involve more detailed ‘what if’ questions, in particular, those that take into account interactions between several factors influencing growth and require a fundamental knowledge of growth processes. The advantage of these models is that they can be used in a broader range of circumstances than empirical models. The development of such mechanistic models serves to identify several remaining gaps in our fundamental knowledge of the causal mechanisms of growth. The mechanistic models described in this review can be seen as a collection of several empirical models, each representing a part of the total biological growth process.

It can be concluded that in future a more quantitative exploration of the causal mechanisms in growth and their interactions with the environment will be needed for the development of a mechanistic, stochastic and dynamic model of growth in a broiler population.