Early feeding of starter is an effective strategy for modulating gastrointestinal microbiota in newborn calves. However, the effects of starter nutrient composition on calf gut microbiota vary significantly. Although both fiber and starch are essential for early calf nutrition, each has distinct advantages and disadvantages. This study investigated how high-starch and high-fiber diets influence growth performance, hindgut health, and microbiota in calves. Two groups of calves were fed experimental starters with distinct nutrient compositions: a high-starch (HS, n = 8) diet containing 40.4% starch and 13.3% NDF, and a high-fiber (HF, n = 7) diet containing 18.8% starch and 30.5% NDF (DM basis). The experiment spanned calf ages 14 to 63 d, with weekly BW and body size measurements using calibrated scales and measuring tape. Serum was collected via jugular venipuncture for metabolic marker analysis. At trial end, animals were slaughtered to collect hindgut tissues and contents for immunoblotting, metagenomic sequencing, and metabolite analysis. We identified a fundamental trade-off that HS feeding shaped a Bifidobacterium-dominated enterotype, correlating with superior growth performance. In contrast, HF feeding selected for a Bacteroides-dominated, more mature microbiota and significantly enhanced gut barrier integrity by upregulating key tight junction proteins (ZO-1, Claudin-1, and E-cadherin). Multiomics integration revealed that this trade-off was underpinned by different microbial metabolic pathways. The HS hindgut was enriched in enzymes and metabolites for carbohydrate and AA fermentation, driving growth. Conversely, the HF hindgut exhibited enhanced enzymatic capacity for fiber degradation (e.g., starch phosphorylase) and a metabolic profile favoring arginine biosynthesis and acetate production, which supported barrier function. This functional divergence was further evidenced in distinct short-chain fatty acid (SCFA) profiles. The HF group exhibited significantly elevated acetate and a trend for higher total SCFA concentration, whereas the HS group showed increased branched-chain fatty acids (isovalerate) and a trend toward higher butyrate and valerate proportions. Our findings provide a mechanistic model linking dietary carbohydrate source to a fundamental choice between growth optimization and gastrointestinal health in preweaning calves, offering novel insights for targeted nutritional strategies.
Du et al. (Sun,) studied this question.