Effects of Excess Dietary Phosphorus on Fecal Phosphorus Excretion and Water Extractable Phosphorus in Horses

• Phosphorus excretion was studied in horses fed excess phosphorus.
• Phosphorus intake, excretion, and water extractable phosphorus were compared.
• Excess phosphorus supplementation increased fecal phosphorus concentration.
• Water extractable phosphorus was increased in horses fed high levels of phosphorus.
• Increased water extractable phosphorus content may increase phosphorus runoff risk.
• Water extractable phosphorus may be useful for determining phosphorus runoff risk.

Phosphorus excretion was studied in horses fed excess phosphorus. The hypothesis of this study was that the concentration of phosphorus and water extractable phosphorus (WEP) in the feces of sedentary horses would reflect the level of dietary phosphorus. Eight Standardbred mares were divided into two groups and received diets of grass hay and grain. The high phosphorus (HP) group received 142 g/d of monosodium phosphate (NaH2PO4), formulated to provide 4.5-times dietary phosphorus requirement, or 65-g phosphorus per day. The low phosphorus (LP) group received 28 g of phosphorus per day in the basal diet. These amounts were based on horses consuming 2% of body weight per day as hay plus supplemental grain. After a 7-day diet adaptation, a 5-day collection was conducted. Animals were housed from 4 PM until 8 AM the following morning without bedding during the collection period. After the first period, horses underwent a 10-day washout and then groups were crossed over for a second 7-day adaptation and 5-day collection. Feces was collected daily, weighed, and a 10% of aliquot taken. At the end of each collection period, feces was composited for each horse and analyzed for nitrogen, potassium, phosphorus, and WEP. Fecal phosphorus and WEP content were greater in the HP group (8.1 ± 0.3 vs. 6.8 ± 0.3 g/kg, respectively) than the LP group (3.6 ± 0.3 vs. 2.1 ± 0.3 g/kg, respectively; P < .05). Overfeeding a phosphorus supplement increased phosphorus and WEP in the manure; WEP may be useful for determining phosphorus runoff risk.