Abstract
The present study was performed to analyse the status of the egg size increase in the pullet year, using the three year's data of White Leghorns consisting of two strains (WL-D, WL-E), Barred Plymouth Rocks (BPR), New Hampshires (NH) and Rhode Island Reds (RIR). Egg weight and egg number were recorded individually every day throughout the pullet year.
The regression of egg weight increase on months of age during the first six months of laying was found to be linear on the logarithmic scale in all the breeds used (Fig. 1). The rate of increase in the mean monthly egg weight in most breeds was the maximum (7.6%) in the 2nd-month after the first egg and it decreased rapidly thereafter. Generally, the egg from the hen of early matuation presents a higher increasing rate, but in this case it takes a long interval from the first laying to reach the mean annual egg weight. When age at the first egg was pooled, this interval was about five months (Figs. 2 and 3).
The highest correlation coefficients (0.83-0.99) of average each monthly egg weight to the mean annual egg weight were found in the 4th-to 5th-month after the first laying (Table 2). Correlation coefficient between the mean egg weight in the spring (March-May) and that in the summer (June-August) was estimted to be the highest (0.77-0.96) in most breeds. The correlations of mean egg weight for respective 10 days of the first 51st-to 60th-day and of the 61st-to 70th-day to the mean annual weight were estimated to be above 0.7 in all cases (Table 3).
From these results, more consideration placing emphasis upon the average weight of eggs laid for voluntary 10 days during the period from the 40th-to 80th-day after the first egg and upon that laid during a month in the next spring (March or April) in which the eggs are set for hatching may lead to effective improvement of the egg size.
