THE SIGNIFICANCE OF BODY WEIGHT 

 IN BREEDING FOR EGG PRODUCTION 



By F. A. Hays, Research Professor of Poultry Husbandry 



INTRODUCTION 



Body weight in flocks bred for egg production is considered to be important 

 from the standpoint of egg size, persistency, and mortality rate in the laying 

 houses. The opinion is rather prevalent that larger birds within the flock are 

 likely to carry a greater physiological reserve to enable them to carry on more 

 successfully all of the complex functions associated with high egg production. 



The literature concerned with the mode of inheritance of body weight has 

 been rather fully reviewed by Maw (1935) and need not be reviewed here. In 

 general, available evidence indicates that body weight in domestic fowl depends 

 upon a series of multiple factors. The inheritance of body weight diff^erences 

 within breeds has not been extensively studied although strain differences are 

 believed to occur. 



The relation between body weight and egg weight has been studied by Hadley 

 and Caldwell (1920), Asmundson (1921), Atwood (1923), Jul! (1924), Parkhurst 

 (1926), Upp and Thompson (1927), Rhynehart (1929), Hanschildt (1929), 

 and Funk and Kempster (1934). All of these workers report a significant rela- 

 tion between body weight and egg weight. The Blue Book of the Canadian 

 National Poultry Record Association for 1929 reported a significant relation 

 between body weight at the end of the year and mean egg weight in White Leg- 

 horns, Barred Plymouth Rocks, White Wyandottes, and Rhode Island Reds. 

 Hays (1930) stated that the largest birds laid the largest eggs up to January 

 first of the first laying 3'ear. Knox (1934) found a significant correlation between 

 initial body weight and average egg weight and between average body weight 

 and egg weight for the first laying year. Funk (1935) noted a significant correla- 

 tion between mean annual body weight and mean annual egg weight in White 

 Plymouth Rocks. 



Hays and Sanborn (1933) stated that the most persistent layers were slightly 

 smaller at first egg than were the less persistent layers in Rhode Island Reds. 

 The difference was small, however, and the fact should be kept in mind, as Hays 

 (1933) pointed out, that weight at first egg is directly dependent upon age at 

 first egg in the birds used. 



Body weight at the close of the laying year was found to be somewhat greater 

 in the highly persistent birds. Weights taken at the close of the laying year 

 are known to represent the true body weight more nearly than those taken at 

 the beginning of the first laying year when the birds differ widely in age and 

 maturity. 



Body weight at sexual maturity was not correlated with laying-house mortality 

 in Hays and Sanborn's (1928) studies. Since the relation between maximum 

 body weight of the first laying year and subsequent mortality has not been studied, 

 there is a possibility that adult size may affect mortality. 



The relation between body weight and annual egg production has been con- 

 sidered by a larger number of workers and has been summarized by Jull (1932). 

 The evidence strongly indicates that body weight and total egg production are 

 independent. Knox (1934) reported no correlation between initial or final and 

 average body weight and number of eggs laid. The fact should be noted, however, 



