Reducing phosphorus excretion by horses

Abstract

Abstract Phosphorus is both a limited, non-renewable resource and also a major factor in water eutrophication. Therefore, dietary P intakes should only provide what an animal requires, reducing unnecessary addition of P to diets and excretion into the environment. Equine feeding programs commonly include inorganic P in excess of requirements, because organic P (e.g. phytate-P) is thought to be poorly digested by the horse. We have observed that 95% of phytate is degraded by the time it reaches the feces, suggesting that phytate-P availability, and thus true P digestibility, may be greater than believed. In other species but not in horses, it is recognized that fecal P excretion is important for maintaining P homeostasis, but can result in low estimates of P digestibility and subsequently greater P requirements. Ideally, P digestibility values used to calculate requirements should only reflect dietary P availability and not fecal P excreted as part of maintaining homeostasis. This research evaluated two factors that could affect P excretion, a change in physiological state (from lactation to post-lactation) and weight loss. We found that post-lactational mares tended to excrete more P than control mares (P = 0.082), but fecal P excretion exceeded P intake for both groups of mares, and mares also lost weight during the study. In the following study, horses fed to lose weight tended to excrete more P than horses fed to gain weight (P = 0.085), suggesting fecal P excretion can be altered by physiological state independent of dietary P availability and is likely the main homeostatic mechanism for P. Previous studies that have not accounted for homeostatic P excretion may have underestimated P digestibility. Thus, requirements could be revised, resulting in a significant reduction in the amount of inorganic P added to equine diets and in the amount of P excreted into the environment. This research suggests that fecal P excretion can be altered due to physiological changes in the horse independent of actual dietary P digestibility and is likely the main homeostatic mechanism for P. Previous studies that have overfed P or failed to account for homeostatic fecal P excretion may have underestimated dietary P availability.