Body Weight and Food Intake of Broiler and Leghorn Chicks

Age of market weight of broilers has been continuously reduced by approximately one day per generation during the past 40 50 years due to an average increase of 50 g/year of body weight at 42 days of age (Gyles, 1989). Although non-genetic factors such as improved feeding and management contributed to this growth rate increase, genotypic selection for growth seems to be the major cause (Chambers et al., 1981; Marks, 1979). This trend emphasizes the importance of the early growing period in today's broiler; increasing the proportion of the first week after hatch from 8% to 16% of the broiler's life span in the last 30 years. The decreasing growing period allows less time (which is reduced with each new generation) for compensation for any growth lag period, therefore any factor limiting maximal early growth delays marketing and reduces efficiency of production. The importance of growth rate during the first week of life is further emphasized by the significant high correlation between body weight at 42 and 6 days of age, although there is no correlation of 42 day weight with body weight at hatching. Growth curves of broiler and Leghorn chicks (Fig. i) and of different genetic stocks (Katanbaf et al., 1988) show a growth lag of several days after hatch .