Revisiting starch degradability in dairy ruminants: from feed processing to gut microbiota and metabolic regulation

Starch serves as the primary energy source for high-producing dairy ruminants, which include both dairy cows and dairy goats. Optimizing starch digestion is crucial for ensuring high milk production and maintaining animal health. This narrative review summarizes and discusses recent findings concerning the degradability of starch in these species. Dietary starch is classified into three distinct types based on the basis of its degradation characteristics: rumen degradable starch (RDS), which ferments in the rumen; rumen escape starch (RES), which is subsequently digested in the small intestine; and resistant starch (RS), which resists complete digestion and enters the large intestine. This review systematically links feed processing methods, which directly influence starch structure, to their subsequent effects on the gut microbiota composition and host metabolic regulation. Three key insights emerge from this synthesis of literature. First, processing techniques such as steam-flaking critically alter the ratio among the three starch types, thereby shifting the effective site of digestion. Second, the optimal application of RDS differs significantly between dairy cows and dairy goats, primarily because these species exhibit distinct digestive physiologies. Nutritionists must carefully account for these species-specific differences to effectively prevent metabolic disorders. Third, the primary site of starch digestion significantly reshaped the gut microbiota profile. While a proper balance supports beneficial bacteria, excessive RS reduces energy efficiency, whereas an overload of RDS can readily lead to severe rumen acidosis. Therefore, balancing the proportions of RDS, RES, and RS is vital for helping animals effectively manage the elevated energy demands experienced during peak lactation. Future research must focus on developing precise starch management strategies tailored to the specific needs of various ruminant species.