ISSUE FOCUS 58 FEED & ADDITIVE MAGAZINE November 2025 environmental pathogens is high. During this period, any stress, nutritional imbalance, or microbial disturbance can tip the balance toward disease. The Gut Microbiome: A Key Player in Health In parallel with immune development, the calf’s gut microbiome—composed of bacteria, viruses, fungi, and other microorganisms—is still taking shape. In early life, this microbial community is unstable and sensitive to change. When it is disrupted (a condition known as dysbiosis), harmful microbes can overgrow, compromising digestion and immune function. Recent research by Teagasc within the HoloRuminant project found that dysbiosis in the hindgut microbiome can occur before clinical scour appears. In this study, more than half of the calves developed scour even though they had received sufficient colostrum and antibodies (called adequate passive transfer of immunity). This shows that immune protection alone is not enough; supporting microbiome development is equally vital (Figure 1). PREVENTING DYSBIOSIS: WHAT WORKS The good news is that the microbiome can recover if the calf is properly supported. Management practices that protect microbial community balance can help reduce the risk of scour and improve overall calf performance. Key strategies include: • Colostrum management: Feed clean, high-quality colostrum (>50g/L IgG; BRIX ≥22%) as soon as possible after birth—ideally within two hours. Continue feeding transition milk for the first few days to extend immune protection. Transition milk is the nonsaleable milk produced by the cow between the 2nd and 8th milking. • Nutritional consistency: Provide high-quality whole milk or a milk replacer made with dairy-based ingredients. Maintain consistent feeding schedules, feed adequate volumes of milk and avoid sudden changes in diet. • Hygiene and housing: Keep pens dry, clean, and well-ventilated to reduce exposure to pathogens. Separate sick calves and clean and disinfect feeding equipment regularly. • Judicious antibiotic use: Use antibiotics only under veterinary guidance. Overuse can harm beneficial gut bacteria, delaying microbiome recovery. • Hydration support: During any signs of diarrhoea, maintain fluid balance with oral electrolytes between milk feedings and never skip milk meals. Image 1. Calf with scour, resting after receiving oral rehydration solution Photo: Sabine Scully March 2022 100 80 60 40 20 0 0 1 2 3 4 5 6 Microbiome stability (%) Calf immune system (%) Maternal antibodies (%) Calf Age (weeks) High scour risk period Percentage (%) Calf Immunity, Material Antibodies & Microbiome Stability Figure 1
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