4 MASS. EXPERIMENT STATION BULLETIN 411 



A study of table 1 indicates that egg size increased very rapidly from October 

 to March, with the most rapid rate of increase during the months of October, 

 November, and December. After this period there was a slowing up in the rate 

 of increase which was most pronounced during the month of March. These 

 data are illustrated graphically in figure 1 together with the data reported earlier 

 (Hays, 1930). In the earlier studies, when egg size in the flock was small, egg 

 weight was highest in February, lowest in April, and remained at a low level 

 through August. In the later studies, when large egg size prevailed in the flock, 

 egg size showed the greatest decline in May, began to increase in June, and had 

 returned completely to the spring maximum by the end of September. 



As already indicated, egg size plays a very important part in meeting market 

 demands. The average behavior of egg size in pullets hatched in March and 

 April is well illustrated in table 1 and figure 1. The fact is very evident that in 

 order to balance the period of about two months when egg size falls below 24 

 ounces to the dozen, breeders must use earlier-hatched pullets or old hens. 

 When egg size has attained the high level indicated in table 1 (24.7 ounces to 

 the dozen for the year), the period of small eggs is rather short. On the other 

 hand, if egg size is not much above the 24-ounce level during the early spring 

 months, there will be a long period when eggs weigh less than 24 ounces to the 

 dozen. 



The data in table 1 also indicate that if a flock is to be e-^^aluated for egg size 

 this should be done at about the age of eleven months. If egg weight records 

 are made at this age, the maximum for the pullet la^'ing year will be recorded 

 aiid, furthermore, variability in egg size will be near a minimum. The fact is 

 also very evident that the 24-ounce standard applies only for limited periods 

 during the pullet laying year. 



Monthly variability in egg size was determined by calculating the standard 

 deviation of the individual monthly mean egg weights. Variability in egg size 

 was high during October, November, and December. In January, February, 

 March, and April variability was lower but constant for the four months. In 

 May there was a further decline in variability that persisted through August. 

 In September the variability returned to the level of early spring. In general, 

 these data appear to indicate that the variability in egg size is greatest when egg 

 size is increasing rapidly and lowest when egg size reaches its maximum in early 

 spring. 



Atwood has also used standard deviation as a measure of variability in egg 

 weight as related to season of the year. His data, however, are not comparable 

 with ours because the records did not begin until December and because a differ- 

 ent basis was used in calculating the standard deviation. Atwood used the in- 

 dividual egg weights as the unit, instead of the individual monthly mean egg 

 weight, which was the unit used in this study. 



Individual Variability in March Egg Weight 



The month of March of the pullet laying year offers a close approach to the 

 period of maximum egg weight in the first year for the Rhode Island Reds used 

 in this study. During March the mean age of the pullets is approximately eleven 

 months. March is also a very important month from the standpoint of repro- 

 duction and it offers the breeder an opportunity to select for maximum egg size 

 in breeding pullets that are hatched in March and April. 



The standard deviation in egg weight for the month of March was calculated 

 on each individual pullet of the total 765. These values have been summarized 



