Effects of forage phosphorous content on faecal phosphorous excretion and possible markers of low phosphorous intake in foals fed forage-only diets.
Authors: Ögren Gunnilla, Ragnarsson Sveinn, Jansson Anna
Journal: Journal of animal physiology and animal nutrition
Summary
# Editorial Summary Estimating phosphorus requirements in growing horses requires understanding how much phosphorus is lost through faeces independent of dietary intake, yet published data on foals consuming forage-only diets remain limited. Ögren and colleagues housed six foals on timothy-based grass haylages with three different phosphorus concentrations (1.9, 2.1, and 3.0 g/kg dry matter) over repeated 17-day periods, collecting total faecal output to quantify endogenous losses and examining whether serum bone resorption markers (CTx) or faecal phosphorus concentration could serve as practical indicators of phosphorus status. Endogenous faecal phosphorus losses proved low in foals—comparable to adult horses—and plasma CTx concentrations remained unchanged across the three dietary phosphorus levels, suggesting this biomarker is unreliable for detecting short-term phosphorus insufficiency in young stock. Whilst a strong mathematical correlation existed between phosphorus intake and faecal phosphorus content (r² = 0.75), the relationship was imprecise enough that clinicians cannot confidently use faecal phosphorus concentration to assess intake when foals are receiving marginal or sub-requirement phosphorus diets. For practitioners formulating forage-based rations for growing horses, these findings underscore the importance of direct forage analysis and consistent supplementation strategies rather than relying on blood biomarkers or faecal markers to identify phosphorus inadequacy.
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Practical Takeaways
- •Foals can be safely managed on forage-only diets with phosphorous content close to or slightly below estimated requirements without apparent acute bone metabolism changes
- •Faecal phosphorous content cannot reliably be used to assess phosphorous intake in young horses, particularly in forage-only systems near marginal P levels
- •Plasma CTx is not a practical tool for detecting subclinical phosphorous deficiency in foals over short timeframes
Key Findings
- •Faecal endogenous phosphorus losses in foals are low and comparable to adult horses, with no difference detected between diets containing 1.9, 2.1, and 3.0 g P/kg DM
- •Plasma CTx concentration did not differ between diets and cannot be used as a marker of short-term low phosphorous intake in foals
- •A correlation exists between phosphorous intake and faecal phosphorous content (r² = 0.75), but regression analysis indicates both underestimation and overestimation of intake are likely when using faecal P content as an assessment tool