Stevia residue (SR), a typical by-product of the stevia industry, is rich in organic matter and has great potential as a feed resource. However, its high fiber content and low utilization efficiency limit its practical application in poultry production. To improve the nutritional value and application potential of SR, this study first optimized the fermentation conditions of SR using response surface methodology (RSM) with chlorogenic acid as the key optimization index and then investigated the effects of different doses of SR and fermented SR (FSR) on laying performance, egg quality, antioxidant capacity, and immunity in laying hens. Six fermentation parameters, including pH, solid-to-liquid ratio, temperature, inoculation quantity, brown sugar addition, and soybean meal addition, were first screened using single-factor experiments and then optimized with RSM. Subsequently, 560 laying hens were randomly divided into seven groups and fed a basal diet supplemented with 0, 0.5%, 1.0%, and 1.5% of either SR or FSR for 28 days. The results showed that the optimal fermentation conditions were a solid-to-liquid ratio of 75.6%, brown sugar addition of 2.7%, temperature of 25 °C, inoculation quantity of 3%, and fermentation time of 9 days. In the animal study, dietary 0.5% SR and FSR reduced average daily feed intake and eggshell strength, whereas the plasma total antioxidant capacity was enhanced in all (SR or FSR) supplemented groups (p < 0.05). Moreover, plasma immunoglobulin (Ig) A level was increased in the 1.0–1.5% SR and 1.5% FSR groups, and plasma IgY was elevated in the 1.0% SR group (p < 0.05). Our results suggested that SR fermentation was effectively optimized through RSM, and dietary FSR supplementation at 1.0% improved the health of laying hens, representing the optimal inclusion level.
He et al. (Mon,) studied this question.