# Effect of Lysine Supplementation in Low-Protein Diets on Nutrients Digestion, Growth Performance, Serum Biomarkers, and Production Performance of Female Blue Foxes (Alopex lagopus) in Fur-Growing Phase

## TL;DR

Adding lysine to low-protein diets improves digestion, growth, and fur quality in female blue foxes during fur growth.

## Contribution

Optimal lysine supplementation level (1.35%) in low-protein diets for blue foxes is identified to balance health, growth, and fur quality.

## Key findings

- Lysine supplementation improved nutrient digestibility and nitrogen retention in low-protein diets.
- Fur quality improved with longer underfur at higher lysine levels.
- Daily weight gain and serum biomarkers were comparable to normal-protein diets at 1.15–1.55% lysine.

## Abstract

Feeding animals with less protein can reduce costs and environmental impact, but it may also affect their growth and health. This study looked at whether adding extra lysine, an essential nutrient, to a low-protein diet could support the health and fur quality of female blue foxes during their fur-growing season. One group of foxes received a normal-protein diet, while the others were given a slightly lower-protein diet with different levels of added lysine. The results showed that adding lysine improved how well the foxes digested nutrients and helped their bodies use protein more effectively. It also supported better growth, especially in the middle of the fur-growing period. Blood tests showed improved nutritional status, and the quality of the fur improved with longer underfur. The best balance of health, growth, and fur quality was seen when lysine made up about 1.35% of the diet. These findings suggest that carefully adding lysine to low-protein diets can help maintain animal health and fur value, while also reducing feed costs and nitrogen waste—supporting more sustainable farming practices in the fur industry.

This study evaluated the effects of dietary lysine supplementation in low-protein diets on nutrient digestibility, nitrogen metabolism, growth performance, serum biomarkers, and pelt quality in female blue foxes (Alopex lagopus) during the fur-growing period. A total of 105 18-week-old female blue foxes were randomly assigned to seven groups (n = 15 per group). The control group received a standard-protein diet (28% dry matter, DM), while six experimental groups were fed low-protein diets (26% DM) supplemented with 0%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0% lysine, corresponding to total lysine levels of 0.75%, 0.95%, 1.15%, 1.35%, 1.55%, and 1.75% DM, respectively. Lysine supplementation at 1.35% and 1.55% DM significantly improved the digestibility of ether extract and amino acids, including aspartic acid, glycine, methionine, isoleucine, and tyrosine (p < 0.05). Nitrogen retention increased accordingly, indicating enhanced dietary utilization (p < 0.05). Daily weight gain, particularly from day 15 to day 30, was significantly higher in 1.15–1.55% lysine groups compared to low-lysine groups (p < 0.05), achieving growth performance comparable to the control (p > 0.05). Serum total protein and albumin concentration were significantly improved with increasing lysine levels in low-protein groups (p < 0.01), aligning with those of the control group (p > 0.05). Furthermore, high lysine supplementation significantly improved pelt quality, as evidenced by the increased underfur length and decreased guard hair/underfur in 1.35–1.75% DM (p < 0.05). These findings suggest that lysine supplementation in low-protein diets supports nutrient utilization, growth performance, and metabolic health status while reducing dietary protein content. The optimal dietary lysine range is 1.15% to 1.55% DM (corresponding to 0.4% to 0.8% in air-dry basis), with 1.35% DM (corresponding to 0.6% in air-dry basis) identified as the most suitable level for balancing growth, nitrogen excretion, and pelt quality in fur-growing female blue foxes.