Impact of Dietary Zinc Chloride Hydroxide and Zinc Methionine on the Faecal Microbiota of Healthy Adult Horses and Ponies.
Authors: Paßlack Nadine, van Bömmel-Wegmann Sarah, Vahjen Wilfried, Zentek Jürgen
Journal: Journal of equine veterinary science
Summary
# Editorial Summary: Zinc Supplementation and Equine Gut Microbiota Whilst zinc supplements are routinely prescribed in equine practice to enhance hoof and skin quality or bolster immune function, their effects on the intestinal microbiota remain poorly characterised. Paßlack and colleagues conducted a crossover trial in eight ponies and two horses, cycling through three dietary zinc levels (maintenance at 4 mg/kg BW⁰·⁷⁵/day, plus 120 and 240 mg/kg dry matter) supplied as either zinc chloride hydroxide or zinc methionine, with faecal samples analysed via 16S rDNA sequencing at the end of each four-week treatment period. Increasing zinc intake progressively reduced microbial richness regardless of compound type, whilst high-dose supplementation (240 mg/kg DM) specifically decreased Bacteroidetes and Fibrobacteres populations and reduced faecal acetate and total short-chain fatty acid concentrations—indicators of diminished fermentative capacity. The response pattern differed between inorganic and organic zinc sources, suggesting variable bioavailability in equines and implying that zinc chloride hydroxide may exert more pronounced effects on bacterial taxonomy than methionine-bound zinc. Although the study animals remained clinically healthy throughout, these microbiotal shifts warrant cautious interpretation, particularly given the critical role of commensal bacteria and volatile fatty acids in maintaining digestive resilience and barrier function; practitioners should reconsider blanket high-dose zinc supplementation protocols pending further investigation into long-term consequences and individual susceptibility.
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Practical Takeaways
- •High-dose zinc supplementation (240 mg/kg DM) may compromise gut microbial diversity and fermentation capacity in horses and ponies; consider whether supplementation exceeds maintenance requirements (4 mg/kg BW0.75/day)
- •Organic zinc sources (methionine) appear better tolerated than inorganic forms (chloride hydroxide) for maintaining microbiota composition, suggesting choice of zinc compound matters for gastrointestinal health
- •Monitor horses on elevated zinc supplementation for subtle signs of compromised gut function, as microbiota changes occurred without overt clinical disease in this study
Key Findings
- •Dietary zinc supplementation at 240 mg/kg DM decreased faecal microbiota richness independently of zinc compound type
- •High zinc intakes reduced relative abundance of Bacteroidetes and Fibrobacteres phyla and decreased faecal acetate and total short-chain fatty acids
- •Zinc chloride hydroxide showed more pronounced effects at bacterial order, family and genus levels compared to zinc methionine, indicating differences in bioavailability between organic and inorganic zinc sources
- •Despite all animals remaining clinically healthy throughout the study, high dietary zinc levels decreased fermentative activity of the faecal microbiota with potential implications for gut health