In chickens and in animals in general, growth is influenced by genotype as well as by environmental factors including nutrition. Hormonal changes form, in part, the causal link between these interacting factors and the physiological processes leading to growth. It is well established that growth-related hormones (GH, SS) play different roles in different tissues. Several studies have investigated the nutritional and/or genetic effects on growth in chickens, leading to the general conclusion that the improvement of growth in modern broiler chickens is largely influenced by the genotype. In addition, it has been shown that the nutrition-determined regulation of tissues is also genotype-specific.
Besides growth, genotype-nutrition interactions also influence reproductive parameters (sexual maturation, egg production) and their endocrine control (hormone levels), apparently having long lasting (or epigenetic) effects for the entire lifespan. Especially for broiler breeders, these interactions are of importance in terms of a long-term solution for the broiler breeder paradox. Indeed, broiler breeders need the genetics for rapid growth and yet have to show a high rate of egg production, in spite of the long recognized negative relation between growth and reproductive fitness. Currently, restricted feeding is the only practical tool available to obtain an optimal equilibrium between growth and reproduction. This has the potential for the impairment of welfare (hunger, behaviour, health, viability). Recent studies suggest that modifying the timing of restriction programmes, the quantity/quality of the diet or the use of genotypes that tolerate ad libitum feeding may achieve a good balance between growth and reproductive performance on the one hand and breeder welfare on the other.