1. Introduction
Genetic selection to enhance egg production increases susceptibility
of laying hens to environmental temperatures. It is well
known that optimal temperature for laying hens is between 18
and 22 °C, with a relative humidity between 50% and 75% during
the laying period. Under lower or higher temperatures than optimum,
the cost of thermoregulation is near the maximum sustainable
threshold of energy mobilization and therefore less energy
will be available for egg production (Blem, 2000). There is
considerable evidence that body weight, feed intake,