158 MASS. EXPERIMENT STATION BULLETIN 2i2 



3. Mortality to Eight Weeks in Relation to Mortality in the Laying Houses. 



If vigor is a complex inherited trait, it is conceivable that families of sisters 

 should exhibit a greater degree of uniformity in vigor than the flock as a 

 whole. The probability exists also that the mortality rate in a family for the 

 first eight weeks might give a clue to the probable mortality rate in that 

 family after the surviving daughters are placed in the laying houses. There 

 is an element of unfairness in this comparison in that more daughters from 

 families with low chick mortality will actually be placed in the laying houses 

 giving this group something of a handicap over the high mortality group be- 

 cause larger nun^bers are subjected to the severe test of heavy egg 'aying. 

 Table 1 furnishes data on these important questions. 



Mortality rate in chicks shows an upward trend from 1923 to 192.5 with 

 records for 1926 not significantly different from those of 1925. Losses in the 

 laying houses for 1924 are excessive due to a severe epidemic of roup and are 

 of little value for this study. The most striking fact illustrated hy the table 

 is that 1925 shows the highest chick mortality and the lowest mortality in 

 tiie laying houses. Such a condition appears to be due to chance circum- 

 stances alone, however, because 1923 gave the smallest chick mortality and a 

 laying house mortality not significantly different from that of 1925. Table 1 

 indicates an upward trend in chick mortality but it does not show any rela- 

 tion between chick mortality to eight weeks and subsequent mortality in the 

 same families after placement in the laying houses. The records indicate 

 also that no change has taken place in mortality rate in the laying houses in 

 five years. 



4. Hatching Date in Relation to Morta,liti/. 



Observation and opinion rather generally agree that mortality rate in early 

 hatclied chicks is lower than in late hatched chicks. There is no conceivable 

 biological reason why this should be true. If such a difference actually 

 exists, two possible explanations seem pertinent. In the first place, there may 

 be selective mortality in embryos from the early hatches because the embry- 

 onic death rate is highest in the early set eggs. In the second place, early 

 iiatched chicks receive much closer attention when brooded artificially be- 

 cause weather conditions may be severe and because the caretaker has fewer 

 chicks to occupy his time. 



The very great importance of low mortality rate in the laying houses can- 

 not be over-emphasized in production-bred flocks. Early hatching is in rather 

 general favor, hence the value of knowing the relation of time of hatching 

 to vigor. 



Table 3 presents the mortality rate in chicks to eight weeks of age, from 

 1923 to 1926, and the mortality rate of pullets in the laying houses, from 1922 

 to 1926. These records are given by hatches, the dates being constant from 

 year to year, beginning March 25 and following at weekly intervals to about 

 May 15. 



On chart 1 the mean mortality rates to eight weeks by hatches for each 

 year and for the entire four years are presented graphically. This chart 

 shows that the mortality rate in chicks increases consistently with time of 

 hatching for the first three hatches of March 2.5, April 1 and April 8. In 

 1923, 1924 and 1926 a distinct decrease in mortality rate is shown for the 

 fourth hatch of April 15. When all chicks are considered over the four-year 



