Dietary protein: Layer performance and mineral use responses of laying performance and mineral utilization to graded levels of dietary crude protein in laying hens

The intricate relationship between nitrogen and the mineral metabolism in laying hens has been well-established. However, the changes in digestion, deposition, and utilization of minerals under low-protein diets remain to be fully elucidated. In this study, 504 Hy-line Brown laying hens, aged 49 weeks, were assigned to six groups (7 replicates with 12 hens each) and fed diets with crude protein (CP) levels of 15.0, 15.5, 16.0, 16.5, 17.0, and 17.5 % to explore mineral metabolism changes. The elevation of the dietary CP levels led to significant increases in the average daily feed intake, the total egg weight, and the egg yolk color (linear, P < 0.001, quadratic, P < 0.001). It also resulted in a decrease in the feed-to-egg ratio (linear, P < 0.001, quadratic, P < 0.001). The group that was administered 16.5 % CP exhibited the highest total egg weight and lowest feed-to-egg ratio (P < 0.001) compared to those of the other groups. A reduction in the dietary CP levels enhanced the apparent total intestinal digestibility (ATTD) of the CP (linear, P < 0.05; quadratic, P < 0.05). Specifically, the group that was administered 17.5 % CP exhibited a significant mRNA expression upregulation of duodenal mineral transporters, including the solute carrier family 34 member A2, the Na⁺/K⁺-ATPase α1 subunit, and the solute carrier family 9 member A2 (P < 0.05). Zinc deposition in the egg yolk was positively correlated with the dietary CP levels (linear, P = 0.002; quadratic, P = 0.010), whereas manganese deposition in the livers and egg yolks showed a negative correlation (linear, P < 0.001; quadratic, P < 0.001). The group administered 15.0 % CP tended to upregulate the expression of superoxide dismutase-2 mRNA (P = 0.069) in the liver, and showed significantly higher manganese deposition in the liver and egg yolk, compared with those of other groups (P < 0.001). A dietary CP level of 16.5 % was optimal for the maintenance of laying performance. A reduction of the CP to 15.0-15.5 % boosted the nutrient efficiency and manganese utilization and deposition but reduced the laying performance. The results of this study further promote the industrial application of low-protein diets in laying hens.