4 MASS. EXPERIMENT STATION BULLETIN 420 



Bostian and Dearstyne (1944) studied the results of selective breeding of 

 White Leghorns for high livability over a five-year period and concluded that 

 selective breeding was effective in reducing mortality from all causes in pullets 

 between the ages of three and twelve months. Inbreeding was shown to increase 

 the mortality rate. 



Bryant and Johnson (1944) bred White Leghorns for high and low mortality 

 from all causes. They report an insignificant difference in mortality rates between 

 the two lines for the first eight weeks, when all dead chicks were included. The 

 differences in death rates in pullets for the period from eight to twenty weeks of 

 age was also of questionable significance. During the adult period, from five to 

 seventeen months, the mortality rates for pullets from all causes were 25.66 

 percent and 36.34 percent respectively for the low and high mortality lines. 

 The significance of the difference in adult mortality rates when all dead birds 

 are included is not reported. The incidence of the leukosis complex was 9.06 

 percent in the high mortality line compared with 2.99 percent in the low mor- 

 tality line. 



Waters and Prickett (1944) were successful in developing a strain of White 

 Leghorns that was free from the avian leukosis complex as long as the birds were 

 kept isolated. Through contact exposure with other birds a high incidence of 

 the complex appeared. 



Nelson and Thorp (1943) pointed out that ocular lymphomatosis should not 

 be considered as dissimilar to visceral or nerve types of the complex; and there- 

 fore birds with the eye type of the leukosis complex should be considered dan- 

 gerous disseminators in the flock until conclusive evidence to the contrary is 

 produced. 



Materials and Methods 



This report covers eight generations hatched from 1935 to 1942 and includes 

 all pedigreed experimental stock. The major portion of the birds were pure- 

 bred Rhode Island Reds. The control group and the high and low mortality 

 lines included only Rhode Island Reds. Other experimental birds were chiefly 

 Rhode Island Reds, but a few crosses between Rhode Island Reds and Barred 

 Plymouth Rocks and between Rhode Island Reds and Buff Oprington were 

 included. The study was divided into two phases: mortality records to the age 

 of 18 months and probable causes of death from 8 weeks to 18 months of age. 



All stock was reared together and pullets and cockerels were removed from the 

 range at about 6 months of age. Sexes were separated at 8 weeks so that the 

 period of observation included: (1) First 8 weeks when the sex was unknown; 

 (2) growing period from 8 weeks to 6 months when the sexes were known; (3) 

 an adult period from 6 months to 18 months of age. 



The control line was selectively bred for characters affecting fecundity and an 

 attempt was made to include low mortality rate in the family as one of the 

 characters to select for. High mortality rate from all causes was the sole basis 

 of selection in the high mortality line without inbreeding. Low mortality rate 

 was the sole basis of selection in the low mortality line. All males and all females 

 were retained to the age of 18 months in the last six generations of the high and 

 low mortality lines. 



Necropsies on as large a proportion as possible of sick and dead birds through- 

 out the period were made by the Department of Veterinar\- Science. Unfor- 

 tunately a considerable number of dead birds were not submitted for necropsy, 

 among them a significant number that appear to have died from cannibalism. 



