14 MASS. EXPERIMENT STATION BULLETIN 307 



ability decreased, and vigor reduced. No significant gain in uniformity of 

 annual egg production was observed under inbreeding. The deduction is made 

 from these studies that only flocks made up of very diverse foundation stocks 

 will tolerate inbreeding. As a rule, for ordinary improved flocks, much greater 

 progress will be made by selection on the basis of individual fecundity characters 

 and the avoidance of matings closer than cousins in the ilock. 



Breeding For Egg Size 



Egg size depends indirectly upon a great many inherited characters. Some 

 of these characters are age at first egg, body weight at first egg, and intensity 

 of laying. Hays (1930a) showed the correlation between age at first egg and 

 mean egg weight to January 1 to be +.4280 + .0148 in Rhode Island Reds. 

 The correlation between body weight at first egg and egg weight to January 1 

 was +.4314 + .0147. Hays (1930b) presented evidence that egg size is not 

 linked with either factor for early sexual maturity and this fact suggests that the 

 reason why there is a significant correlation between age at first egg and winter 

 egg size is due to the direct relation between age and weight at first egg. Hays 

 (1930b) demonstrated a specific linkage between gene A for small egg size and 

 gene I' for highest intensity. In view of these findings, it is evident that poultry- 

 men should take great care in selecting breeders for high intensity that they do 

 not carry the gene for small egg size. 



Parkhurst (1933) reviews the effect of nutrition on egg size and shows that in 

 general either inadequate food supply or lack of proper protein in the ration may 

 reduce egg size. Parkhurst's data indicate that birds fed rations deficient in 

 vitamin D lay small eggs, that the addition of milk to the ration increases egg 

 size, but that minerals added to the ration do not afTect egg size. The differences 

 observed in egg size in all cases are rather small. 



A number of workers including Jull, Parkhurst, and (according to Jull, 1932) 

 Atwood and Clark, Thompson, Asmundson, and Marble present data indicating 

 that egg weight and number of eggs laid in a year are independent entities. But 

 the experience of breeders of heavy laying flocks indicates rather universally 

 that egg size will decrease as the mean production of the flock increases unless 

 careful attention is given to egg size. Hays (1929a) pointed out that very heavy 

 winter laying in Rhode Island Reds is accompanied by small egg size. A number 

 of other workers including Lippincott, Jull, and Parkhurst report that early 

 sexual maturity, which is related to annual egg production, is associated with 

 small egg size. Hadley and Caldwell, Atwood, Jull and Rynehart have sub- 

 stantiated the findings with Rhode Island Reds at this station — that heavy 

 body weight is associated with large egg size. 



In considering the question of breeding for large egg size, it becomes necessary 

 to lay down certain standards for egg size. Hays (1929a) has assumed that 

 pullet eggs weighing 56.7 grams each or 24 ounces to the dozen in February are 

 satisfactory. On such a basis all eggs laid the first year will average about 23.5 

 ounces per dozen. Egg weight in such birds will increase about 7 per cent in the 

 second laying year, or from 1.5 to 2 ounces per dozen. 



Working on this basis, Hays (1929a) has reported two major genes as largely 

 responsible for the differences in egg weight. Gene A produces an egg size in 

 February of considerably less than 24 ounces per dozen; gene B alone gives an 



