Thus, genetic selection might be more effective if animals are given a high plane of nutrition and suitable environmental conditions for optimal growth. In model systems, however, the responses to selection appear to be more complex, and genetic advances made under optimal conditions may not automatically be expressed if the animals are switched from optimal to suboptimal environmental growth restraints. One aspect of this problem is that physiological maturation may be delayed in a population whose growth is environmentally restrained. Growth rhythms Intrinsic seasonal rhythms in muscle growth are difficult to identify in farm animals that reach market weight in less than a year since intrinsic rhythms are difficult to isolate from seasonal reproductive cycles and seasonal cycles in the nutrient quality of animal feed. In laboratory animal experiments, intrinsic seasonal changes in rates of growth and cellular regeneration have been found to persist even in animals kept under constant conditions of temperature and day length.
Seasonal rhythms occur in milk and egg production.
In Scottish free-grazing sheep, the average empty body weight of ewes peaks in November and reaches a minimum at the end of April, with subcutaneous fat providing the primary energy reserve depleted during gestation. In poultry, light has a profound effect on aspects of animal behavior such as feeding and roosting that then secondarily affect growth rate. For example, poultry may be reared at a low intensity of light so that they expend less energy on muscular activity, but then they may start to use other clues, such as temperature or sounds of activity to adjust or entrain their circadian rhythm of activity. Chickens exhibit maximum growth if they area fed late in the evening, because then they have all night to work through the feed stored in their crop. But to do this, the chickens must learn how to anticipate the start of the period of darkness, and they may do this more readily if the light intensity is slowly decreased to simulate dusk.