Phosphorus excretion by mares post-lactation.
Authors: Fowler Ashley L, Pyles Morgan B, Hayes Susan H, Crum Andrea D, Lawrence Laurie M
Journal: Journal of animal physiology and animal nutrition
Summary
# Editorial Summary The wide variation in published estimates of phosphorus digestibility in horses (ranging from −23% to 79%) likely reflects differences in how much phosphorus horses re-secrete into the gastrointestinal tract rather than differences in dietary intake alone. Fowler and colleagues investigated this by comparing phosphorus retention and digestibility in eight mares during the post-lactation period against eight control mares fed identical phosphorus levels, hypothesising that recently-weaned mares would retain more phosphorus due to reduced mammary demand. Contrary to expectations, post-lactational mares did not retain additional phosphorus compared to controls, though they tended to excrete more faecal phosphorus (p = 0.082), suggesting increased endogenous secretion into the GI tract—possibly driven by metabolic changes during mammary gland involution. Both groups excreted more phosphorus in faeces than they consumed and lost bodyweight during the trial period, indicating that tissue mobilisation during weight loss may have substantially influenced phosphorus secretion patterns. These findings underscore why accurate digestibility estimates require animals in positive phosphorus balance, and highlight that post-lactational status alone does not guarantee improved phosphorus retention in mares fed standard maintenance rations.
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Practical Takeaways
- •Phosphorus digestibility values from the literature are highly variable and may not reflect actual P retention; consider individual mare status (lactation history, body condition) when formulating mineral supplements
- •Post-lactational mares may have altered phosphorus metabolism during mammary gland involution—monitor P status and adjust dietary P if mares are losing weight or showing signs of mineral imbalance
- •Standard P digestibility coefficients may overestimate or underestimate true dietary P availability; focus on meeting minimum P requirements rather than assuming published digestibility values will accurately predict retention
Key Findings
- •Post-lactational mares tended to excrete more phosphorus than control mares (p = 0.082) despite similar P intake
- •Faecal P excretion exceeded P intake in both groups (p = 0.08), suggesting endogenous P secretion into the GI tract influenced digestibility estimates
- •True phosphorus digestibility estimates in horses range from -23% to 79%, with variation explained partly by differences in endogenous P secretion rather than intake alone
- •Weight loss during the study period may have influenced phosphorus secretion through tissue mobilization