Oviposition time of 419 hens from a White Leghorn population of two lines divergently selected for high and low yolk-albumen ratio was observed at four stages of egg production period for a duration of about 30 days in each stage. A total of 1,498 records were used in the analyses of this study. The clutch traits for heritability estimation include; intra-clutch mean lag of oviposition time (LAG), mean delay of pause day between clutches (DELAY), the rate of internal laying (IP), the number of clutches (CN), the average length of clutches (CL), and the average size of clutches (CS). LAG and DELAY were estimated by the multiphasic model (LUC et al., 1995). Heritabilities of these clutch traits were estimated by a sire-dam nested model and intra-sire regression of daughter on dam. Means of LAG were 1.04 hours and 1.32 hours for high and low lines, respectively. Corresponding heritability estimates were 0.41, 0.32, 0.62, and 0.55, 0.51, 0.62 for sire component and combined components of sire and dam, and intra-sire regression coefficient of daughter on dam, respectively. Heritability estimates of DELAY were not different from zero in both lines using the two methods. Medium and high heritability estimates of CN, CL and CS were found for both high and low lines in the two methods, except for regression estimates in the high line. Heritability estimates in the low line were higher than that in the high line for most traits, except for DELAY and IP.

Intra-clutch changes in egg composition and shell quality were examined using two laying hen strains (High line: 40 hens, Low line: 39 hens) which were developed by a divergent selection for yolk-albumen ratio. Oviposition time was recorded every 30 minutes from 5: 30 to 18: 30 for a duration of 90 days at about 270 days of age. The eggs of individual hens were collected every day from three consecutive clutches.Only the hens that formed the clutches determined apparently by their oviposition time were used for the analyses.Statistical analyses were performed separately for the four clutch size groups.The lag in oviposition time was the longest for the last egg of the clutch (terminal egg). There was no difference in the oviposition interval between lines. The changes in egg weight of consecutive eggs within a clutch showed that the first egg of the clutch was the heaviest and then the weight gradually decreased.The decrease in weight the low line was estimated to be 3.0-4.0g, or 6% of the 1st egg, while the decrease in weight in the high line was 1.5-2.0g (4% of the first egg). The yolk weight of the 2nd egg was heavier than that of other eggs in the same clutch, and decreased by 1.0-1.5g in both lines. Albumen weight showed a remarkable decrease from the first egg to the 3rd or the 4th eggs of the clutch, then the decrease tended to be smaller.The decrease in the low line, which had a larger amount of albumen, was estimated to be larger.Thick albumen weight tended to decrease as the serial number within a clutch increased.However, thin albumen weight increased slightly in eggs laid at later positions of the clutch.Egg-shell weight, thickness and strength showed a clear increase in the terminal egg of the clutch.(Jpn. Poult. Sci., 34: 273-281, 1997)

Four mathematical models (compartmental (C), modified compartmental (MC), Wood (W), and Adams-Bell (AB)) were fitted to different patterns of egg production data in laying hens to compare the fits of these models. The egg production data were obtained from the two lines which were founded on a basis of divergent selection for egg quality traits. Egg production data of individual hens were classified in six patterns. The average hen-day rates of lay were calculated for every 10 days from the first egg for each pattern. The comparison of fits was based on the R2 adjusted for degrees of freedom and Akaike's information criterion (AIC). The fits of C, MC and AB models were similar for the egg production data showing a general pattern of curve. However, the model parameters could not be estimated by these three models for the data whose patterns showed abrupt decreases after the peak of egg production. The data in which the period from the first egg to the peak of egg production was short and linearly decreased after reaching the peak showed poor fits of the models. The parameters of all patterns could be estimated by the W model, but the fits were lower than those of the others.

A study was carried out to examine the effects of lines on the clutch traits and the prediction of egg production in laying hens. Data of oviposition time were recorded at four stages from the 120th day to the 330th day of egg production for two years. The duration of observation in each stage was 30 days. The four lines were abbreviated as H,

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