VIGOR IN PRODUCTION-BRED FLOCKS 163 



period, the decrease in mortality rate of the fourth hatch brings their rate 

 very near to that of the first two hatches. No explanation of this apparent 

 inconsistency can be offered at this time. 



Considering the period as a whole there is a distinct increase in death rate 

 of chicks as the hatching date advances. Extensive data on these flocks show 

 that hatchability increases as the season advances. This increasing hatching 

 power is clearly not accompanied by increase of vigor as measured by death 

 rate in chicks up to eight weeks of age. 



When the relation of date of hatching to mortality rate in the laying 

 houses, as recorded graphically in chart 2, is considered, no significant rela- 

 tion between the two is observed. The data for 1923 show a slight increase 

 in mortality rates for pullets from succeeding hatches. Records for 1924 give 

 a rather distinct increase in mortality rates for later hatches. In 1925 mor- 

 tality rate goes down in the later hatches. In 1926 there is something of an 

 upward trend in mortality with the later hatching dates. 



The grand total line marks the relation of hatching date to mortality rate 

 in the laying houses for the entire flve-year period. This line has no sig- 

 nificant slope and indicates no relation between time of hatching between 

 March 25 and May 15 and mortality rate for pullets in the laying houses. 

 There is a suggestion in these data that mortality to eight weeks operates on 

 a selective basis eliminating less vigorous chicks in each family so that the 

 sunivors in each family exhibit about the same death rate in the laying 

 houses. ^ 



0. Relatwn of Age at First Egg to Mortality in the Laying Houses. 



The question whether or not breeding for very early sexual maturity reduces 

 vigor is a very pertinent one. This being the case, a study of large numbers 

 of fowls in the same flock over a period of years should furnish some infor- 

 mation. 



When the mean age at first egg is considered on respective years, no sig- 

 nificant change in age has occurred in the five-year period. The standard 

 deviation in age for these flocks amounts to 31.28 days (Hays and Sanborn, 

 l.f)26b) and is influenced both by heredity and by environment. In all prob- 

 ability the major portion of age differences in table 4 are caused by numer- 

 ous environmental influences. Both hen mothers and pullet mothers gave the 

 same mean age at first egg for their daughters each year. 



Eliminating the obviously abnormal year 1924, there has been no significant 

 change in mortality rate in the laying houses in five years. Since variations in 

 age at first egg are not accompanied by parallel changes in mortality rate in 

 the five-year period studied, these data indicate that age at first egg is not a 

 vigor trait in pullets. 



6. Relation of Genetic Early Maturity to Mortality Rate in the Laying 

 Houses. 

 Hays (1924) has shown that Rhode Island Red pullets which begin to lay 

 at 215 days or younger are early maturing by inheritance and that those 

 pullets starting to lay at a greater age than 215 days ate genetically late 

 maturing. In order to discover whether or not genetic early maturity is ac- 

 companied by low vigor as indicated by death rate in the laying houses, all 

 the pullets used in these studies are classified for maturity. Table 5 gives 

 the complete data: 



