Faecal phosphorus excretion from horses fed typical diets with and without added phytase

Phosphorus (P) returned to the environment in land-applied animal manures has potential for detrimentally affecting soil and water resources. Use of specific feed ingredients and/or the enzyme phytase in animal diets may favourably alter manure characteristics by increasing P mineralization in the gastrointestinal tract and, thus, the proportion of soluble, plant-available P in faeces. In the present study, faecal P excretion was characterized in eight yearling geldings (Equus caballus) fed four typical diets, with and without added phytase. The experimental design was a 4 × 4 Latin square in which horses were fed Coastal bermudagrass (Cynodon dactylon) hay at a rate of 0.015 × body weight (BW) daily plus sufficient quantity of whole oats (Avena sativa), alfalfa (Medicago sativa) cubes, a textured ‘sweet feed’ formulation or pelleted concentrate formulation to meet NRC requirements for digestible energy (DE) and crude protein (CP); phytase status represented a within-unit repeated measures factor. Apparent digestibility of P was not different, but faecal output of total P, water-soluble P and insoluble P differed (P < 0.001, <0.001 and = 0.002, respectively) among diets. Across all diets, there were no differences in apparent digestibility or faecal excretion of P when phytase was administered. However, diet × enzyme interactions were observed for faecal excretion of total P (P = 0.081) and insoluble P (P = 0.069) such that these were lower (P < 0.10) for the sweet feed-based diet when phytase was administered. Faecal output of total P, water-soluble P and insoluble P was 8.4, 3.0 and 5.4 g day−1; 10.1, 3.9 and 6.9 g day−1; 14.9, 5.3 and 9.6 g day−1; and 19.0, 7.9 and 11.1 g day−1, respectively for diets containing whole oats, alfalfa cubes, sweet feed and pelleted concentrate. Results indicate that typical diets for horses, formulated with commonly used feed ingredients to meet DE and CP requirements, can be expected to differ markedly with respect to quantity and solubility of P returned to the environment in faeces. As such, feed ingredient selection represents a viable approach to dietary nutrient management for meeting current and future regulations pertaining to environmental disposal of manure from equine operations. Added phytase showed limited potential for increasing the proportion of soluble, plant-available P in equine faeces.