Animal Molecular Breeding 2024, Vol.14, No.5, 297-306 http://animalscipublisher.com/index.php/amb 298 Additionally, traits such as laying rate and clutch behavior are critical in free-range or cage-free systems, where hens must lay eggs in nests rather than on the floor. Research indicates that selecting for nest acceptance (NAL) and laying frequency in these systems can improve efficiency. These behavioral traits can be improved through selective breeding to reduce the occurrence of floor eggs, thus enhancing production (Becot et al., 2021). Moreover, eggshell quality is another key trait impacting egg production performance. Eggshell strength is essential for the storage and transportation of eggs, reducing breakage. By using QTL mapping, researchers have identified genetic loci that contribute to stronger eggshells, which can be leveraged in breeding programs to improve shell thickness, strength, and consistency (John-Jaja et al., 2017). 2.2 Heritability and genetic basis of egg production traits The heritability of egg production traits varies based on the specific trait and chicken breed. Studies indicate that traits such as egg weight, AFE, and body weight have moderate to high heritability, meaning these traits can be effectively improved through selective breeding. Heritability estimates for egg weight typically range between 0.3 and 0.5, allowing for significant improvements over a short period. AFE, with heritability estimates of 0.2 to 0.4, is also a key target for improving early egg production performance (Das et al., 2016). Genetic correlations between traits play an essential role in poultry breeding. For instance, there is a positive genetic correlation between egg weight and body weight, indicating that selecting hens for larger eggs may also lead to heavier body weight. However, increased body weight can delay the age of first egg, highlighting the need to balance trait selection. Simultaneous selection for traits such as egg weight and body weight ensures that the health and productivity of hens are maintained while improving egg quality (John-Jaja et al., 2017). Behavioral traits like nest acceptance (NAL) also have moderate heritability in cage-free systems. Studies show that while behavioral traits tend to have lower heritability than physical traits, improvements through selective breeding are still possible. NAL has a heritability estimate of 0.13 to 0.26, indicating that it is feasible to reduce floor laying and improve productivity in non-cage environments through genetic selection (Becot et al., 2021). 2.3 Phenotypic variation and environmental influence Egg production traits in hens are influenced not only by genetics but also by significant environmental factors. As shown in Figure 1, traits such as egg production, egg weight, body weight change, and feed conversion ratio exhibit noticeable individual variation, indicating the importance of environmental factors like housing systems, feed quality, temperature, and lighting. For example, differences in body weight (Figure 1e and 1f) and feed intake directly impact egg weight (Figure 1b) and laying rate (Figure 1a). Additionally, the distribution of feed conversion ratio (Figure 1g) and body weight change (Figure 1h) suggests varying feed efficiency under different environmental conditions. Proper management of feeding, especially adjusting nutrition at different production stages, can optimize egg weight, eggshell quality, and overall production efficiency. (Anene et al., 2020). In free-range or cage-free systems, hens may exhibit greater behavioral diversity, affecting egg-laying patterns and clutch size. In these systems, environmental management, such as optimizing nest design, can help reduce floor laying and improve productivity. Environmental conditions, including temperature and lighting, also play a crucial role in determining egg-laying performance. High temperatures or improper lighting can lead to lower egg production, decreased eggshell quality, and health issues in hens. Therefore, maintaining optimal environmental conditions is essential for improving both egg production and hen welfare (Becot et al., 2021). Temperature and lighting also influence the phenotypic expression of egg production traits. For instance, excessively high temperatures can reduce the number of eggs laid and weaken eggshell quality. Adequate control of environmental factors, such as maintaining a consistent temperature and proper lighting schedules, can help improve both the quantity and quality of eggs produced.
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