Effect of stocking density on behavioral traits, blood biochemical parameters and immune responses in meat ducks exposed to heat stress

High stocking density (HSD) and heat stress (HS) caused by climate change can lower blood homeostasis and negatively impact the behavioral traits of animals. The objective of this study was to explore the influence of stocking densities on behavioral traits, blood parameters, immune responses, and stress hormones in meat ducks (Cherry valley, Anas platyrhynchos) exposed to HS. A total of 320 meat ducks were assigned to four groups with different stocking densities using a randomized complete block design. The ducks were then reared for 42 days. The assigned density groups were as follows: (1) control group (CON, three birds m−2, normal environmental heat conditions); (2) low stocking density (LSD, three birds m−2, heat stress conditions); (3) medium stocking density (MSD, four birds m−2, heat stress conditions); and (4) high stocking density (HSD, six birds m−2, heat stress conditions). To induce HS, the environment of the poultry house was set to a temperature of 34±2∘C with a relative humidity of 70 % from 11:00 to 16:00 for the finisher period (from day 22 to day 42 of the rearing period). Concentrations of blood triacylglycerol, total cholesterol, low-density lipoprotein cholesterol (LDL-C), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) were higher in the HS groups compared with the CON group, with HSD showing the highest levels (P < 0.05). The concentrations of high-density lipoprotein cholesterol (HDL-C) and glucose were lower in the HSD groups than in the CON group (P<0.05). Red blood cell (RBC) and platelet (PLT) counts were lower in HS groups compared with the CON group, with the HSD group displaying the lowest counts (P<0.05). Blood pH values were also higher in the HS groups than in the CON group, with the highest values observed in the HSD group (P<0.05). Concentrations of blood pCO2, HCO3, and TCO2 were higher in the HS groups than in the CON group, with HSD showing the lowest levels (P<0.05). The concentration of PO2 was higher in CON than in any of the HS groups, with the lowest levels found in the HSD group (P<0.05). The concentrations of blood IgG and corticosterone were increased in the HS groups compared with the CON group (P<0.05). Animal behavioral trait scores were also higher in HS groups than in the CON group (P<0.05); these scores were the highest in the HSD group. Overall, animal behavioral traits, blood biochemical parameters, and immune responses in meat ducks exposed to heat stress were highest in the HSD group, but not significantly different between the LSD or MSD groups.