## 4 Discussion  
### Phenotypic and Genetic Parameters  
The breeding population in this study underwent routine selection. Growth traits were measured at 6 weeks of age, and based on production requirements, individuals within a specific weight range were selected. Simultaneously, the top 5% of individuals with the longest BSL and NL were culled to enhance the uniformity of the breeding population. After routine selection, the coefficients of variation for growth phenotypic traits were relatively low (3.11-4.66%). In this study, multiple consecutive generations of Pekin duck populations were primarily focused on improving reproductive performance to enhance egg production efficiency. By implementing body size selection at six weeks of age, the uniformity of the population was increased. However, this approach also led to a decrease in the heritability of growth traits, such as body weight, body slope length, and neck length. The primary reasons for the decline in heritability include the reduction in phenotypic variation due to selective culling and the shift in selection pressure towards reproductive performance. Selective breeding may result in a decrease in genetic diversity, leading to underestimated heritability estimates for related traits [30]. Additionally, changes in selection pressure can affect the estimation of genetic variance [31]. In unselected chicken populations, growth traits tend to exhibit relatively high heritability [32]. Research showed that the heritability of growth rate in chicken populations remained above 0.4 even after challenging the birds with the La Sota strain of Newcastle Disease Virus, compared to a heritability of 0.55 prior to the challenge [33]. Based on pedigree and SNP information, the heritability estimates for reproductive traits and egg quality traits in this study population reached moderate levels (0.2-0.4). There was a strong negative correlation between AFE and EN25-44 WK, but a weak correlation between EWT and ESI. Similar results have been observed in related studies on other poultry populations [8; 34; 35; 36; 37; 38; 39]. Early onset of laying is usually associated with higher  
Figure 2: Predictive performance of BLUP, GBLUP, and ssGBLUP models for growth, reproductive, and egg quality traits in Pekin ducks: (**a**) predictive accuracy for each trait using different models; (**b**) predictive unbiasedness for each trait using different models. The predictive performance of BLUP, GBLUP, and ssGBLUP is represented by green, orange, and red bars, respectively. BW, body weight at 6 weeks of age; BSL, body slope length; NL, neck length; AFE, age at first egg; EN25-44 WK, egg number from 25 to 44 weeks of age; EWT, average egg weight at 40 weeks of age; ESI, average egg shape index at 40 weeks of age. reproductive efficiency because these individuals enter the peak laying period earlier [38]. A smaller age at first egg (AFE) indicates earlier sexual maturation in individuals [40], which is associated with certain key genetic variations or the regulation of endocrine axes [41]. During the peak laying period from the onset of laying to 44 weeks of age, breeding female ducks are in their highest egg production phase. A smaller AFE allows individuals to lay more eggs within the fixed laying cycle (25-44 weeks of age), thereby increasing the number of eggs produced during this period (EN25-44 WK). Consequently, there is a strong negative correlation between AFE and EN25-44 WK. This finding is consistent with our previous research and other related studies [34; 42; 43]. In addition, egg weight is mainly influenced by the volume and mass of albumen and yolk, whereas the shape index is the ratio of the egg's length to its diameter. These traits have different genetic and physiological bases, resulting in a weak correlation between the egg weight and shape index [39]. In the studied population, a weak genetic correlation was observed between growth traits at 6 weeks of age and both egg quality traits at 40 weeks of age and egg production traits from 25 to 44 weeks of age.