VIGOR IN PRODUCTION-BRED FLOCKS 



169 



11. Relation of Perslatenry in Layinr; to Mortalifi/ Rate in the Laying 

 Hoii.tes. 



High persistency in laying is an important and desirable trait from the 

 standpoint of fecundity. In ordinary poultry practice the date of onset of 

 complete molt is made use of as an indicator of persistency. The importance 

 of late melting as a desirable fecundity trait has been recognized for several 

 decades and is constantly made use of in culling operations. Persistency be- 

 ha-ves in Mendelian fashion in ittheritance and high persistency is linked with 

 early sexual maturity (Hays, 1927). 



The dividing line between genetically low and genetically high persistency 

 appears to come at aibout 31.5 day;-. Birds have therefore been classed as 

 high in persistency if they continue to lay for 315 days or more from the time 

 of first pullet egg and low in persistency if they lay for a shorter period than 

 31.5 days. In order to classify the families of sisters used in these studies 

 it has been necessary to obtain the mean persistency of the surviving daugh- 

 ters of each family. If this mean family persistency falls below 315 days 

 the entire group of sisters is classed as low in persistency even though some 

 individuals may actually lay for 315 days or longer. This method is at best 

 only an approximate separation of the papulation into high and low per- 

 sistency classes. 



In studying the relation of persistency to vigor as measured by mortality 

 rates in the laying houses under such a classification as is made in table 10, 

 the assumption must be that surviving sisters in each family represent a true 

 sample of all the sisters of the family with regard to persistency. Actual 

 persistency records can only be secured on surviving sisters juid their records 

 must be used to classify all sisters. 



Table 10 — Relation of Persistency to Vigor 



Reference to table 10 brings out the fact that no important change has 

 taken place in five years in the percentage of genetically highly persistent 

 birds. When the percentage of high birds is calculated l)y families the figures 



