Effects of Methionine Supplementation in Low-Protein Diets on Growth Performance, Fur Quality, Blood Indices, and Intestinal Microbiota of Blue Foxes (Vulpes lagopus) During the Fur-Growing Period

This study evaluated the effects of supplementing methionine to a low-protein diet on nutrient digestibility, nitrogen (N) metabolism, growth performance, serum biochemical parameters, fur quality, and intestinal microbiota composition in blue foxes (Vulpes lagopus) during the fur-growing period. Fifty 17-week-old blue foxes were randomly allocated to five experimental groups (n = 10 per group). The control group received a diet containing 28% crude protein (CP), while the experimental groups were fed a 22% CP diet supplemented with 0%, 0.35%, 0.55%, or 0.75% methionine on a dry matter (DM) basis, designated as M0, M1, M2, and M3, respectively. Results demonstrated that the final body weight (FW), total weight gain (TW), and average daily gain (ADG) of the M3 group were comparable to the control group (p > 0.05). Methionine supplementation significantly enhanced fur quality and stimulated hair follicle development (p < 0.05). Although the reduction in dietary protein level led to decreased N intake and fecal N excretion, the M2 and M3 groups exhibited significantly higher N retention compared to the control, M0, and M1 groups (p < 0.05). Regarding nutrient digestibility, the M2 and M3 groups showed higher DM digestibility (p < 0.05), while the M3 group maintained organic matter (OM) digestibility comparable to the control group (p > 0.05). The highest CP digestibility was observed in the M3 group (p < 0.05). Additionally, ether extract (EE) digestibility was significantly improved in the methionine-supplemented groups (M1–M3) relative to the control (p < 0.05). Serum analysis revealed dose-dependent increases in total protein (TP), albumin (ALB), and high-density lipoprotein (HDL) concentrations in the M2 and M3 groups. Conversely, low-density lipoprotein (LDL) levels were elevated in these groups compared to the control and M0 groups (p < 0.05). Liver function parameters were also significantly improved in the M2 and M3 groups (p < 0.05). Furthermore, methionine supplementation enhanced the diversity and richness of the intestinal microbiota and altered its composition at the phylum and genus levels. In conclusion, supplementing low-protein diets with methionine can maintain growth performance, improve fur quality, enhance nutrient utilization efficiency, and support intestinal microbiota homeostasis in blue foxes. The optimal supplementation level is 0.75%, resulting in a total dietary methionine concentration of 1.1% on a DM basis.