VIGOR IN PRODUCTION-BRED FLOCKS 



167 



mean winter clutch size as proposed by Hays and Sanborn (1927) offers the 

 advantages of being specific and not obscured by the element of time or by 

 the winter pause. Clutch size is also inherited in Mendelian fashion as re- 

 ported in the above publication. Birds carrying both dominant factors for 

 large clutch size will have a mean winter clutcii greater than 2.6, those lack- 

 ing one factor will show a clutch size between 2.2 and 2.6, while birds lack- 

 ing both factors will have a clutch size not far from 2.0. In these studies 

 the pullets have been divided into two classes: Those carrying both factors 

 for large clutch and those carrying none or only one factor for large clutcli. 



Table 8 — Relation of Intensity to Vigor 



In table 8 the proportion of high intensity pullets to those of low intensity 

 varies from year to year, yet the differences in mortality rate for the two 

 classes is not great. The smallest proportion of high intensity birds occurred 

 in the 1923 flock while the greatest proportion appeared in the 192-5 flock. On 

 tiiese two years the mortality rates were lowest of the five-year period. This 

 fact, together with the close agreement in total mortality rates for both high 

 and low intensity groups, would seem to indicate that intensity of production 

 is independent of vigor as measured l>y losses in the laying houses. 



10. The Relation of Broodiness to Mortality Rate in the Laying Houses. 



The broody trait is a recognized undesirable trait from tiie standpoint of 

 hig^h fecimdity. Hence, in order to raise the mean egg production of a flock 

 to a high level, the percentage of birds going broody the first laying year 

 must be greatly reduced from what might he considered nonnal in the gen- 

 eral purjiose breeds. 



In the Massachusetts Agricultural Experiment Station flock of Rhode 

 Island Reds the percentage of birds going broody their firsit laying year was 

 90 for the flock of 1913 and for the flock of 1926 the figure was 10. In this 

 time interval the number of broody periods has fallen from more than five to 

 less than two. There has not been a significant reduction in the mean length 

 of broody periods, however. (Hays and Sanborn 1926a). 



