LENGTH OF INCUBATION PERIOD 11 



The limited data are not conclusive regarding possible inherited factors affect- 

 ing the length of the incubation period. Sires and sons, sires and daughters, 

 and dams and sons showed insignificant correlations in length of incubation 

 period. Regression was non-linear between sires and daughters and between dams 

 and sons. On the basis of 100 degrees of freedom, there was a significant corre- 

 lation between dams and their daughters m length of incubation period. The 

 meaning of this relationship is not clear. If dams transmit factors affecting the 

 length of mcubation period to their daughters, it is difficult to understand why 

 such factors are not transmitted to their sons. It therefore seems logical to 

 assume that the data are too meager to furnish satisfactory evidence on the 

 inheritance phase of the problem. 



SUMMARY 



Data were secured during a three-year period on the length of incubation 

 required by 4730 Rhode Island Red chicks. A study of possible relationship 

 between length of the incubation period and sex, viability, and characters affect- 

 ing fecundity led to the following deductions: 



I. The length of the incubation period was not affected by weight of eggs in 

 430 dams studied. 



2 Length of the incubation period was slightly shorter in the earlier hatches. 



3. Length of the incubation period fluctuated over a period of about 56 hours 

 during the ncrmal hatching season. 



4. Fen ales predominated in the early emerging chicks. 



5. Mortality rates for six months increased consistently as the length of the 

 incubation period increased. 



6. There was no relation between the length of the incubation period and 

 the mortality rates for pullets in the laying houses between the ages of six and 

 eighteen months. 



7. No relation was shown between length of incubation period and body 

 weight of pullets at six months of age. 



8. Ccckerels emerging early were slightly heavier at six months of age than 

 late emerging ones. 



9. There was no important relation between length of incubation period and 

 body weight of pullets at sexual maturity. 



10. Very early emerging pullets are likely to be slightly earlier in sexual 

 maturity than late emerging ones. 



II. More eggs before March first may be expected from early emerging 

 pullets. 



12. There was a consistent decline in annual egg production as the length of 

 the incubation period increased. 



13. Early emerging pullets are likely to be more persistent layers than late 

 emerging pullets. 



14. No relation was found between length of the incubation period and 

 body weight at the end of the first laying year. 



15. There was no conclusive evidence that the length of the incubation period 

 is governed by inherited factors. 



16. Selection of early emerging chicks appears to be of considerable economic 

 importance. 



