Effects of stocking density on ovarian development and maturation during the rearing period in Shan-ma ducks

Stocking density critically affects the growth and subsequent performance of animals in modern poultry production. This study investigated the effects of stocking density on ovarian development, ovarian maturation, and the mRNA expression of key genes in the reproductive axis during the rearing period of Shan-ma ducks. The experiments involved 180 healthy 7-wk-old Shan-ma ducks and randomly divided into low stocking density (LSD; n = 30, density = 5 birds/m²), medium stocking density (MSD; n = 60, density = 10 birds/m²) and high stocking density groups (HSD; n = 90, density = 15 birds/m²), for rearing. After examining ovarian development and measuring hormone levels in the plasma and expression levels of key regulatory genes in the reproductive axis at 19 wk of rearing, analysis of the gonad index analysis, reflecting stocking density, uncovered statistically significant differences. The gonad index of the LSD group was significantly higher than those of the MSD and HSD groups (P < 0.01), while no significant difference was observed between the MSD and HSD groups. pre-ovulatory follicles (POFs) and small yellow follicles (SYFs) development was only apparent in the LSD group, with the large white follicles (LWFs) number of this group being significantly higher than that of the MSD group (P < 0.05). The blood levels of E2 (estradiol), P4 (progesterone), and T (testosterone) were significantly higher in the LSD group than in the MSD and HSD groups (P < 0.05 or 0.01). Also, the levels of both P4 and T were significantly higher in the MSD group than in the HSD group (P < 0.01). The gene expression levels of GnRHR, FSH, AMHR, and FSHR were significantly increased in the LSD group compared to the MSD and HSD groups (P < 0.05 or 0.01), while the expression levels of GnIHR and GDF9 were significantly decreased in the LSD and MSD groups compared to the HSD group (P < 0.05 or 0.01). Steroid biosynthesis pathway genes such as StAR, CYP11A1, 3β-HSD, CYP19A1, and BMP15 were significantly downregulated at greater stocking densities (P < 0.05 or 0.01). Likewise, the protein expression of StAR, 3β-HSD, and CYP19A1 was also significantly decreased (P < 0.05 or 0.01). These results demonstrate that both medium and high stocking densities suppressed the expression of the key reproduction-promoting factors, while the expression level of the key reproductive inhibitory factors was enhanced. Therefore, rates of ovarian development and maturation could be reduced by a high stocking density leading to a delay in reproduction performance during the rearing period of Shan-ma ducks.