As a major economic trait in chickens, egg weight (EW) receives widespread interests in breeding, production and consumption. However, limited information is available for underlying genetic architecture of longitudinal trend in EW. Herein, we measured EWs at nine time points from onset of laying to 60 week of age, and conducted comprehensive genome-wide association studies (GWAS) in 1,534 F2 hens derived from reciprocal crosses between White Leghorn and Dongxiang chickens. Egg weights at all ages except the first egg weight (FEW) exhibited high SNP-based heritability estimates (0.47 ~ 0.60). Strong pair-wise genetic correlations (0.77 ~ 1.00) were found among all EWs. Nine separate univariate genome-wide screens suggested 73 signals showing significant associations with longitudinal EWs. After multivariate and conditional analyses, four variants on three chromosomes remained independent contributions. The minor alleles at two loci exerted consistent and positive substitution effects on EWs, and other two were negative. The four loci together accounted for 3.84 % of the phenotypic variance for FEW and 7.29 ~ 11.06 % for EWs from 32 to 60 week of age. We obtained five candidate genes, of which NCAPG harbors a non-synonymous SNP (rs14491030) causing a valine-to-alanine amino-acid substitution. Genome partitioning analysis indicated a strong linear correlation between the variance explained by each chromosome and its length, which provided evidence that EW follows a highly polygenic nature of inheritance. Identification of significant genetic causes that together implicate EWs at different ages will greatly advance our understanding of the genetic basis behind longitudinal EWs, and would be helpful to illuminate the future breeding direction on how to select desired egg size.

An experiment was conducted to evaluate the effect of litter thickness on growth performance, immune status, environmental condition, and welfare quality in yellow broilers. In total, 1,800 one-day-old Suqin yellow broilers were raised for 21 d. On d 22, 1,600 birds of similar BW (404 ± 12 g) were randomly selected and placed into 20 indoor pens (8 birds/m², 10 m²/pen). These birds were assigned to a litter treatment of 4, 8, 12, and 16 cm. Each treatment was repeated in five pens. The results showed that a thicker litter was related to increased BW, daily weight gain, and daily feed intake (P < 0.001). Feed conversion ratio and mortality were unaffected by litter thickness (P = 0.320, P = 0.353, respectively). Absolute and relative liver weights showed a significant linear response to increasing litter thickness (P = 0.01, P = 0.001, respectively). The litter moisture content, air ammonia, and CO₂ content decreased, whereas the air dust content increased with increasing litter thickness (P < 0.001, P = 0.017, P = 0.033, P < 0.001, respectively). Litter thickness had no effect on gait, plumage damage, hock burn or breast skin crusting (P = 0.076, P = 0.964, P = 0.131, P = 0.401, respectively). Plumage cleanliness, foot pad dermatitis, hock swelling and breast blister varied significantly with litter thickness (P = 0.027, P = 0.011, P = 0.014, P = 0.042, respectively). The results of this study suggest that an increasing litter thickness has a beneficial effect on the growth performance, environmental condition and welfare of birds.

Energy and the cAMP-response element binding protein (CREB)/steroidogenic acute regulatory protein (StAR) signaling pathway play important roles in steroid hormone production and follicular development in hens. This present study aimed to investigate the effects of exogenous energy on the synthesis of steroid hormones and the expression characteristics of the CREB/StAR signaling pathway in theca cells of laying hen. The primary theca cells of small yellow follicles were randomly divided into 6 treatments and cultured in medium with glucose concentrations of 1, 1.5, 3, 4.5, 6, and 7.5 mg/mL for 48 h. It was found that growth was robust and cell outlines were clear when cells were cultured with 1, 1.5, 3, and 4.5 mg/mL glucose, but cell viability was diminished and cell density decreased after exposure to glucose at 6 and 7.5 mg/mL for 48 h. Cell viability showed an increasing and then decreasing quadratic response to increasing glucose concentration in culture (r² = 0.688, P < 0.001). The cell viability of theca cells cultured with 4.5 mg/mL glucose was greater than those cultured with 1, 1.5, 6, and 7.5 mg/mL glucose (P < 0.05). The concentration of estradiol in the medium containing 3 mg/mL glucose was higher than in medium containing 1, 1.5, and 6 mg/mL glucose (P < 0.05). There was an increasing and then decreasing quadratic correlation between progesterone concentrations and glucose concentrations (r² = 0.522, P = 0.002). The concentration of progesterone in medium with 4.5 mg/mL glucose was higher than in medium with 1 and 7.5 mg/mL glucose (P < 0.05). There was an increasing and then decreasing quadratic correlation between the relative expression of CREB1 (r² = 0.752, P < 0.001), StAR (r² = 0.456, P = 0.002), CYP1B1 (r² = 0.568, P < 0.001), and 3β-HSD (r² = 0.319, P = 0.018) in theca cells of laying hens and glucose concentrations after treatment with different glucose concentrations for 48 h. After treatment with 4.5 mg/mL glucose, the expression of StAR, CYP1B1, and 3β-HSD genes were increased compared to treatment with 1, 1.5, 3, 6, and 7.5 mg/mL glucose (P < 0.001). There was an increasing and then decreasing quadratic correlation between glucose concentrations and protein expression of CREB1 (r² = 0.819, P < 0.001), StAR (r² = 0.844, P < 0.001), 3β-HSD (r² = 0.801, P < 0.001), and CYP11A1 (r² = 0.800, P < 0.001) in theca cells of laying hens. The protein expression of CREB1, StAR, and 3β-HSD in theca cells cultured with 4.5 mg/mL glucose was higher than in other groups (P < 0.001). The results indicate that the appropriate glucose concentration (4.5 mg/mL) can improve the synthesis of steroid hormones in theca cells of laying hens through the upregulation of key genes and proteins in the CREB/StAR signaling pathway.

The aim of this study was to assess the effects of energy-restricted feeding during rearing on the performance, uniformity, and development of layer breeders at the initiation of the laying period. A total of 2400 8-week-old Rugao layer breeders were randomly assigned to one of five groups (480 pullets per group) with eight replicates and were fed one of five diets that were nutritionally equal with the exception of apparent metabolizable energy corrected for nitrogen (AME_n) content (2850, 2750, 2650, 2550, and 2450 kcal AME_n/kg) from 8 to 18 weeks of age. The daily amount of feed was restricted to the absolute quantity of the diet consumed by laying hens fed 2850 kcal AME_n per kg diet ad libitum (control). From 18 to 21 weeks of age, all hens were fed a basal diet ad libitum. The body weight (BW) of the laying pullets decreased linearly with increasing energy restriction (p < 0.001) but recovered within 3 weeks of ad libitum feeding (p = 0.290). A gradual increase in the degree of energy restriction resulted in a gradual decrease in average daily weight gain (ADG) and a gradual increase in the feed conversion ratio (FCR) and energy conversion ratio (ECR) from 8 to 18 weeks of age (p < 0.001, p < 0.001, p = 0.008). In contrast, the ADG and ADFI (p < 0.001, p < 0.001) gradually increased, while the FCR and ECR (p < 0.001, p < 0.001) gradually improved from 18 to 21 weeks of age. From 8 to 21 weeks of age, ECR improved (p = 0.005) with an increasing degree of energy restriction. The energy-restricted feeding for 6 weeks to the end of the trial improved BW uniformity (p < 0.05). The relative length and circumference of tarsus (p < 0.001, p < 0.001), and the relative weights and lengths of the small intestine, duodenum, jejunum, ileum, and caeca increased linearly (p < 0.001, p = 0.012, p < 0.007, p = 0.012, p = 0.040; p < 0.001, p = 0.003, p = 0.032, p = 0.029, p = 0.040) with increasing energy restriction at 18 weeks of age. After switching to ad libitum feeding for 3 weeks, the relative weights and lengths of the small intestine, duodenum, and jejunum of laying pullets increased linearly with increasing energy restriction (p < 0.001, p = 0.016, p = 0.011; p = 0.009, p = 0.028, p = 0.032). In conclusion, moderate energy restriction (85.97%, 2450 vs. 2850 kcal AME_n/kg) from 8 to 18 weeks of age and switching to ad libitum feeding from 18 to 21 weeks of age can be used to improve BW uniformity and stimulate the development of the duodenum and jejunum of native layer breeders at the initiation of the laying period without compromising BW.

This research aimed to assess body weight (BW) at the end of the brooding period in affecting Hyline Brown laying hens aged 6 to 72 wks from the aspects of growth performance, egg quality, sexual maturity, and productive performance. Pullets (6 wk old, n = 640) were sorted into two groups according to BW: normal (460.75 ± 10.82 g) and light (347.96 ± 6.27 g, 75.52% of normal weight), with eight replicates of 40 pullets each. All the hens, when reaching 6–72 wks in age, took the same diets by stages ad libitum. The heavier BW in the normal weight group (NWG) at 6 wks of age compared to lighter weight group (LWG) birds continued until 22 wks (p < 0.05). Similarly, a smaller coefficient of variation (CV) for BW of chicks in the LWG was detected at 19 (p < 0.01), 20 (p < 0.01), and 21 (p < 0.05) wks of age compared with that in the NWG. The body slope length and the shank circumference of pullets in the LWG were smaller than in the NWG at the age of 15 wks (p < 0.01), but the difference gradually disappeared by 20 wks of age (p > 0.05). The LWG presented raised ages of hens when producing the first egg and 5% eggs (p < 0.01), while the CV for the age at first egg decreased, compared with the CV in the NWG (p < 0.05). The total egg number (p < 0.05), laying rate (p < 0.05), and egg mass (p < 0.01) of laying hens in the LWG decreased at the age of 18–72 wks, and the CV for individual egg numbers (p < 0.05) increased compared with the CV in the NWG. Compared with the normal weight hens, the proportion of lighter weight hens laying more than 250 eggs at the age of 18–72 wks was significantly reduced (p < 0.05, 69.52% vs. 87.38%), while the proportion of hens laying less than 200 eggs was significantly increased (p < 0.05, 24.97% vs. 3.76%). In summary, BW by the end of the brooding period can be a good indicator reflecting individual differences among laying hens and may serve as an important phenotypic indicator for evaluating laying performance and early elimination of unqualified laying hens in layer production. Therefore, it is recommended that pullets weighing 25% or more below the normal flock weight at the end of the brooding period should be culled at this time.