VIGOR IN PRODUCTION-BRED FLOCKS 



173 



has been responsible for about 22 per cent of all losses. During the last two 

 years reported it was controlled by vaccination with a bacterin. Thirty-five 

 per cent of all losses in the five-year period are due to unclassified causes. 

 These losses have not varied widely throughout the period reported. Reduc- 

 tion of these losses will depend largely upon the use of highly vigorous stock, 

 strict sanitary methods of management, and more improved methods of feed- 

 ing aided by recent discoveries. 



Changes in Vigor in Families. 



The fact should be clearly understood that the chief goal sought for in the 

 breeding operation reported here has been high fecundity. High vigor has 

 been considered, but fecundity has not been sacrificed to maintain vigor. Table 

 13 is given below to show how vigor of the birds has varied throughout the 

 five-year period reported. 



Table 13 — Percentage of Families with No Mortality 



Table 13 indicates a decrease in the percentage of families from pullet 

 mothers that have no loss in chicks to eight weeks of age, between 1923 and 

 1926. In the same period there has been no significant change in percentage 

 of families from hen mothers with zero mortality to eight weeks of age. When 

 all families are considered year by year, a significant decrease in the propor- 

 tion of perfect livability families is observed. Hen mothers exhibit a greater 

 proportion of perfect livability families than do pullet mothers for the entire 

 four years covered by these data. The total percentage of perfect livability 

 families from hen mothers is 35.61 and from pullet mothers, 22.66. 



When consideration is given to the relative percentages of perfect livability 

 of families in the laying houses, pullet mothers have an upward trend in per- 

 centage of perfect livability daughters from 1922 to 1926. In the same period 

 hen mothers also show improvement but to a lesser degree than pullet mothers. 

 In general, the statement may be made that the proportion of high vigor fam- 

 ilies has increased during the five-year period studied. 



