Selection for Rapid Growth Rate in Chickens

SINCE the advent of the broiler indus-try more than 20 years ago, poultry breeders have been selecting for rapid growth rate in broiler-strain chickens. They have made much progress. How-ever, it has been difficult to separate genetic progress from that resulting from advances in nutrition for improvement of growth rate. Jaap, Smith and Goodman (1962) used a randombred control popula-tion to measure the genetic change which occurred in eight-week body weight of broilers produced commercially between 1955 and 1959. During this period the breeders were estimated to have increased the eight-week weight of broilers by approximately J lb. This \ lb. was almost a 20 percent gain achieved in four genera-tions. Naturally, poultry breeders are wondering whether they may be ap-proaching the upper limits for rapid creases in growth rate in present day populations. The implications of genetical theory (Lerner, 1958; Falconer, 1960) are not clear. Therefore, several additionals elec-tion experiments were initiated to explore the mechanisms by which growth rate to eight weeks of age might be increased and to expand our search for responses cor-related with genetic gain in growth rate. Negative genetic correlations between reproductive fitness and increased growth rate could be one mechanism which might reduce rate of progress toward more rapid growth. The following report considers only the response to selection as measured by randombred controls. SYNTHETIC STRAINS The seven synthetic strains used in the selection experiments are briefly described in Table 1. The last two columns of Table 1 show the accumulated inbreeding and the last generation for each of the strains included in this report. All but 6 of the 80 parents from which strain A originated were from Commercial Crossbred A (Jaap, Smith and Goodman, 1962). Six of its 40 dams were from com-mercial Crossbred B cited in that report. The results of the first four generations of growth selection in strain G were reported previously by Jaap, Smith and Goodman TABLE 1.—Description of synthetic strain