Partial periods for genetic evaluation of egg production and egg quality attributes in two varieties of Japanese quails

Introduction

Japanese quail (Coturnix coturnix japonica) farming has emerged as an important component of the Indian poultry sector due to its low initial investment, rapid generation turnover, early sexual maturity, and high egg-laying capacity. With growing domestic and export demand for quail eggs and meat, sustained genetic improvement has become indispensable for enhancing productivity and profitability in resource-constrained production systems. Accurate estimation of genetic parameters, particularly heritability and genetic correlations, forms the cornerstone of any successful breeding programme targeting economically important traits. In laying birds, egg production efficiency is influenced by multiple factors, including age at sexual maturity, body weight at first egg, cumulative egg number, and egg weight across different phases of the production cycle. Conventional selection strategies in layer breeding have primarily focused on increasing total egg number and average egg weight. However, egg production is not uniform throughout the laying cycle, and significant genetic variation exists between early, mid, and late production periods. Partitioning the production trajectory into partial periods allows breeders to exploit this temporal heterogeneity, accelerate genetic progress, and achieve higher selection response per unit time. Beyond quantity, egg quality (both external and internal) is a critical determinant of consumer acceptance, table-egg marketability, and hatchability in breeding operations.

Traits such as shell strength, albumen quality, yolk proportion, and pigmentation directly affect economic returns and reproductive success. Several studies in chickens have demonstrated that intense selection for production traits can inadvertently deteriorate certain egg quality attributes, underscoring the need to incorporate quality parameters into multi-trait selection indices. Extensive literature exists on genetic parameter estimates for egg production and quality in chicken layers, but comparable information in Japanese quail remains limited and fragmented. Understanding the genetic architecture of production and quality traits, as well as the nature and magnitude of their genetic correlations, is essential for designing balanced breeding objectives that simultaneously improve egg number, egg weight, and quality without undesirable antagonistic effects. The present investigation was therefore undertaken at the Central Avian Research Institute (CARI), Bareilly, India, using a pedigreed Japanese quail population maintained under a one-to-one sire-dam mating system. The primary objectives were to estimate heritability of egg production and egg quality traits during the early laying phase (12-16 weeks) and to quantify phenotypic and genetic correlations among these traits, thereby providing a scientific foundation for the development of an efficient selection programmer in Japanese quail.

Materials and Methods Experimental Location, Population, and Pedigree Management

The study was conducted during 2021-2022 at the Experimental Quail Farm, ICAR-Central Avian Research Institute (CARI), Izatnagar, Bareilly, Uttar Pradesh, India. Two genetically distinct lines of Japanese quail (Coturnix coturnix japonica), CARI-BROWN (meat-type) and CARI-SUNEHERI (egg-type), were used. A base population consisting of 200 sires and 200 dams from each line was randomly mated to produce the third generation of a fully pedigreed experimental population. To ensure accurate pedigree recording, individual pair mating (One Sire: One Dam) was practised in California cages (25 cm length × 25 cm width × 15 cm height). Pedigreed fertile eggs were collected twice daily from breeder flocks aged 8-11 weeks. Each egg was individually numbered with sire and dam identification and set in pedigree hatching boxes. Standard incubation conditions were maintained (37.5 °C, 60-65% RH). On the day of hatching, chicks were wing-banded with numbered tags, and complete three-generation pedigree records were maintained.

Selection and Management of Birds for Trait Recording

From the progeny, 100 sires and 100 dams per line that contributed the maximum number of viable offspring were selected as parents of the recording generation. From their progeny, 200 female quail per line (one daughter per dam; total N=400) were randomly selected at hatching for phenotypic evaluation. Chicks were brooded in battery cages up to 5 weeks of age under standard management practices (temperature: 34 °C in week 1, gradually reduced by 3 °C/week; 16L: 8D photoperiod from week 3). At 5 weeks of age, selected females were shifted to individual laying cages (30 × 25 × 40 cm) and maintained under a uniform 16 h light: 8 h dark photoperiod throughout the recording period (6-18 weeks of age).

Recording of Production Traits: Age at Sexual Maturity (ASM) was recorded as the number of days from hatch to the laying of the first egg. Daily egg production was recorded individually from 6 to 18 weeks of age. Part-period egg number was computed biweekly and cumulatively between 12 and 16 weeks of age.

Egg Sampling and Quality Evaluation

At 12 weeks of age, 200 eggs (100 eggs per line) were randomly collected on the same day from the recorded females (one egg per female from 100 sire-dam families per line) for quality analysis within 24 h of lay.

External egg quality traits:  Egg weight (EW, g): Measured using a precision electronic balance (0.01 g accuracy).

Statistical Analyses

Data were checked for normality using the Shapiro-Wilk test in JMP v.14.0 (SAS Institute Inc.). Least-squares means for fixed effects were estimated using the mixed model procedure. Variance components and heritability were estimated by restricted maximum likelihood (REML) using SPSS software version 28 with a single-trait animal model

Where, y=vector of observations, b=vector of fixed effects (line: CARI-BROWN and CARI-SUNEHERI; hatch: 1 st , 2 nd and 3 rd ), a=vector of direct additive genetic effects ~ N(0, Aσ²ₐ), e=vector of residual effects ~ N(0, Iσ²ₑ), X and Z₁= corresponding incidence matrices, A=numerator relationship matrix derived from three-generation pedigree.

Heritability estimates

Heritability estimates are summarised in Table 3. ASM exhibited low heritability in both lines (h²=0.12-0.15±0.11-0.12). Heritability of part-period egg number increased with age, from low at 8 weeks (h²=0.17-0.18) to moderate-to-high between 10 and 16 weeks (h²=0.31-0.53±0.13-0.17), peaking at 16 weeks. Egg weight showed moderate heritability (h²=0.34-0.37±0.13-0.48). Most egg quality traits displayed moderate-to-high heritability (h²=0.25-0.52), except shape index, shell weight, and yolk diameter (h²=0.12-0.29).

Genetic and phenotypic correlations

Genetic (above diagonal) and phenotypic (below diagonal) correlations among biweekly egg production traits are shown in Table 4. Strong negative genetic correlations were observed between ASM and early egg number in both lines (r_g=-0.78 to -0.95±0.01-0.25), indicating that selection for reduced ASM would substantially increase early production. Part-period egg numbers from 10 to 16 weeks exhibited strong positive genetic correlations (r_g=0.40-0.99±0.04-0.30), confirming high genetic uniformity during the ascending and peak phases. Genetic and phenotypic correlations among key egg quality traits are presented in Table 5. Albumen height and Haugh unit were almost perfectly correlated both genetically (r_g=0.85-0.90±0.01-0.03) and phenotypically (r_p=0.96-0.97). Yolk height and yolk diameter showed very high positive genetic correlations (r_g=0.92-0.96±0.10-0.30). Egg weight displayed moderate positive genetic correlations with yolk weight (r_g=0.46-0.52) but weak negative correlations with shape index (r_g=-0.31 to -0.42).

Discussion

The markedly earlier sexual maturity of the CARI-BROWN line reflects its historical selection for rapid body weight gain and meat production, whereas the egg-specialised CARI-SUNEHERI line prioritised higher egg mass at the expense of delayed maturity. The 4-7 d range in ASM observed here falls within the broad variation previously reported for Japanese quail 42-56 days (Lotfi et al., 2012) [9] and underscores the substantial additive genetic component controlling this trait, despite its low heritability. Heritability of part-period egg number increased progressively from sexual maturity to peak production, reaching moderate-to-high values (0.31-0.53) between 10 and 16 weeks. This pattern aligns with earlier findings in both quail and chickens, where additive genetic variance becomes more pronounced after the onset of lay. The peak heritability at 12-16 weeks highlights this early production phase as the most effective selection criterion for lifetime egg yield in Japanese quail, offering the dual advantage of high genetic response and shortened generation interval a critical consideration in resource-constrained production systems. Moderate heritability of egg weight and most internal and external quality traits (except shape index and shell weight) confirms ample scope for direct genetic improvement. The near-perfect genetic correlation between albumen height and Haugh unit (0.85-0.90) reinforces albumen height as the primary driver of interior egg quality, consistent with reports in chickens and quail. Strong genetic associations among yolk dimensions and weight further indicate that selection for larger yolk reserves beneficial for embryonic nutrition, hatchability, and early chick growth can be achieved efficiently. The consistently strong negative genetic correlation between ASM and early egg production (-0.78 to -0.95) across both lines provides robust evidence that reducing age at first egg will accelerate early-laying performance without compromising total output. Similarly, the high positive genetic correlations among part-period egg numbers from 10 to 16 weeks imply that selection on any single period within this window will elicit substantial correlated gains throughout the early laying phase, enabling rapid cumulative genetic progress.

Conclusion

The profitability of Japanese quail enterprises hinges on early sexual maturity, sustained egg production, and superior egg quality. This study demonstrates that the dual-purpose CARI-BROWN line achieves significantly earlier age at sexual maturity (48.6 vs. 52.6 days) and higher early-phase egg number than the egg-specialised CARI-SUNEHERI line, whereas both lines exhibit comparable performance during peak production (10-18 weeks) and largely similar egg quality profiles at 12 weeks of age. Most importantly, moderate-tohigh heritability estimates for part-period egg number (h²=0.31-0.53) and key egg quality traits (notably egg weight, albumen height, Haugh unit, and yolk parameters), combined with strong favourable genetic correlations particularly the near-perfect association between albumen height and Haugh unit (r_g ≥ 0.85) and the consistently strong negative correlation between age at sexual maturity and early egg production (r_g=-0.78 to -0.95) reveal substantial additive genetic variation exploitable through selection. These findings establish the 12-16-week production window as the most heritable and genetically correlated phase for predicting lifetime egg yield, while confirming that selection for reduced age at sexual maturity will simultaneously accelerate early lay without compromising total output or egg quality. The strong positive genetic interrelationships among egg number periods and among internal quality components further indicate that multi-trait selection will produce balanced, cumulative genetic gains across economically critical traits. In practical terms, incorporation of age at sexual maturity, cumulative egg number at 12-16 weeks, and Haugh unit/albumen height into selection indices for both CARI-BROWN and CARI-SUNEHERI lines will enable rapid, simultaneous improvement of production efficiency and egg market value. Such a targeted breeding strategy offers a highly effective and resource-efficient pathway to enhance the productivity, sustainability, and commercial competitiveness of Japanese quail farming systems worldwide.

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