important contribution to this objective in that it provided a stable 

 base for other research. 



Mass selection was effective in increasing 10-week body weights and 

 breast angle measurements in meat type birds. Crossbreeding was effec- 

 tive in improving reproductive performance in broiler stocks but growth 

 rate to 10 weeks of age suffered in the process. Recurrent selection 

 showed no advantage over closed flock selection in broiler stocks. 



In the absence of a significant amount of non-additive genetic varia- 

 tion in 8-week body weight, conventional breeding methods should be 

 effective in improving the trait. The production of genetic variation by 

 the irradiation of sperm was not a useful adjunct to selective breeding 

 for the improvement of ten-week body weight. 



B. GENETIC CORRELATION AND SELECTION RESPONSE 



The second objective of NE-6 was to study genetic correlations 

 among economic traits, especially egg production and growth rate. 



Poultry breeders have long been aware that selection for superiority 

 of one economic trait may result in correlated responses in other traits. 

 The association between body weight and egg weight demonstrates a 

 positive correlated response. On the other hand, selection for egg pro- 

 duction was often accompanied by reduced growth rate. Genetic corre- 

 lations were estimated from randombred and other populations and 

 were observed in selection experiments. 



It is important that we know which traits are genetically related to 

 which other traits and the direction and magnitude of these relation- 

 ships. Such knowledge can be used in breeding programs to make opti- 

 mum progress in all the important economic traits. A summary of gen- 

 etic correlations estimated from research in NE-6 is given in Table II. 



Biological Selection Indexes 



The problem of correlated responses to selection and of measuring 

 the aggregate phenotype of chickens was investigated by the formulation 

 of biological selection indexes and the theoretical evaluation of their 

 use as selection criteria as compared with the consequences of selection 

 for single traits. Directional selection for a single trait may result in 

 compensatory regression of other traits. By the use of biological selec- 

 tion indexes a balance of traits can be found which allows the maximum 

 increase in the most desirable traits witliout sacrificing other character- 

 istics of economic value. 



At the Pennsylvania Agricultural Experiment Station, egg number, 

 average egg weight, and body weight were measured (21, 22). The 

 term, biomass, was applied to these three traits considered together, 

 while the term, egg mass, was applied to egg number and egg weight. It 

 was found that egg mass used as a single criterion of selection would 

 reduce age at sexual maturity, increase egg number, increase specific 

 gravity of the eggs, increase reproductive potential, increase body weight 

 only slightly, and leave egg weight practically unchanged. 



Offspring from four lines of New Hampshires at the West Virginia 

 Agricultviral Experiment Station were weighed and measured for breast 

 angle at 10 weeks and at 6 months of age (10). Phenotypic correlations 



12 



