Inorganic, soluble faecal phosphorus excretion in growing horses fed forage-only diets

The inorganic and soluble fraction of phosphorus (P) in horse manure can runoff to nearby water and contribute to eutrofication. The aim of this study was to investigate the magnitude of inorganic, soluble P fraction (Pi) in feces and if this fraction is affected by two levels and sources of P intake in growing horses. Fourteen Standard-bred horses (aged 20±0.3 months) were fed grass forage ad libitum and two dietary treatments: with (H= high) and without (L= low) P supplementation (monocalcium phosphate) for 6 days in a cross-over design. Grab samples of feces were collected twice per day on days 4-6 and analyzed for P. Acid insoluble ash (AIA) was used as a marker for faecal output. Daily feed intake and leftovers were weighed. In feed and feces total P content (Pt) was analyzed by ICP atomic emission spectrometry. Pi in faecal samples was analyzed by colorimetric method. Statistical analyses were done using GLM procedure and Tukey's Test in SAS. Daily P intake was 32.0±1 g and 17.5±1 g on diet H and L, respectively (P<0.0001), corresponding to 130% and 70% of the NRC requirement. Total faecal excretion of P was 30.3±1 g and 17.0±1 g on diet H and L, respectively (P<0.0001). Pi of Pt faecal excretion was 40.2±1% for diet H and 34.6±1% for diet L (P=0.0055) and Pi increased linear with P intake (y = 0.44x - 1.78, R2=0.91). The result indicates that the inorganic, soluble fraction of P in feces from forage fed horses made more than one third of the total P excretion and that the proportion increased when P supplement was offered.