Walnut cake meal improves amino acids, fatty acid composition and flavor of egg yolk via the microbiota–yolk metabolites crosstalk in Jingfen-1 laying hens

Egg production is important for both human nutrition and its economic contribution. However, regions with a shortage of soybean meal (SM) may not meet the nutritional needs of the laying hen industry. Walnut cake meal (WM), a by-product of walnut processing, can be reused in laying hen production. In this study, we evaluated the feasibility of using WM to reduce the demand for SM in the laying hen industry. A total of 144 47-week laying hens (with similar performance) were randomly assigned to two groups (12 hens in each replicate and six replicates for each group). One group was fed a maize-SM diet (SM group), and the other was fad a maize-WM diet (WM group). The diets of both groups had similar crude protein and ME contents. After replacing SM with WM, the laying rate, egg weight, and feed efficiency did not significantly change (P > 0.05). WM increased (P < 0.05) the polyunsaturated fatty acids content, particularly linoleic acid, as well as umami and essential amino acids, in the eggs. WM diet also enhanced the flavor of eggs by enriching multiple volatile organic compounds that smell of sweet and herb-like foods. In addition, probiotic bacteria such as Cyanobacteriota and Prevotella, were enriched in the cecal microbiota of laying hens fed WM. Moreover, targeted metabolomics revealed the enrichment of butyric acid, 4-methylvaleric acid, isoleucine, and valine in the cecal digesta of laying hens fed with WM. Metagenomic sequencing revealed genes in the cecal microbiota associated with the synthesis of these enriched metabolites. Increased isoleucine and 4-methylvaleric acid in the digestive system contributed to the enrichment of fatty acids and amino acids in the yolks, whereas elevated flavor substances in the yolk could be associated with more amino acids in the intestine of laying hens. In conclusion, WM can reduce the need for soybean meal, improve cecal metabolism and egg quality, and ultimately achieve sustainable agriculture.