title: Phosphorus balance and fecal losses in growing Standardbred horses in training fed forage-only diets.
abstract: This study examined the P balance and fecal P losses in growing Standardbred horses in training fed a forage-only diet with or without P supplementation and assessed the magnitude and proportion of the soluble, inorganic P (Pi) fraction in feces. Fourteen Standardbred horses (aged 20.0 ± 0.3 mo) adapted to ad libitum intake of grass forage containing 0.25% P were used in a crossover experiment investigating 2 dietary treatments with (high-P) and without (low-P) mineral supplementation for 6 d. Daily feed intake and refusals were weighed. Spot samples of feces were collected twice daily on d 4 to 6 and analyzed for total P and Pi. Acid-insoluble ash was used as a marker for total fecal output. Spot samples of urine were collected once on d 4 to 6 and analyzed for P and creatinine. Daily P intake was greater (P < 0.001) for the high-P diet (32.0 ± 0.6 g) than the low-P diet (17.5 ± 0.6 g), and the individual intake ranged from 13.3 to 38.4 g/d. Total fecal excretion of P was also greater (P < 0.001) for the high-P diet (30.3 ± 0.8 g/d) than the low-P diet (17.0 ± 0.8 g/d) whereas excretion in urine was less than 0.2 g/d on both diets. Using simple regression analysis, fecal endogenous P losses were estimated to be less than 10 mg/kg BW. Phosphorus retention was 1.6 ± 0.6 and 0.3 ± 0.6 g/d on the high- and low-P diets, respectively, but only that for the high-P diet was greater (P < 0.05) than 0 g/d. The proportion of Pi of total fecal P excretion was 100 ± 3% for the high-P diet and 87 ± 3% for the low-P diet (P = 0.005) and Pi increased linearly with P intake (y = 1.10x - 4.44; r(2) = 0.91; CV = 9.9%; P < 0.001). In conclusion, on this forage-only diet significant retention of P occurred at a daily P intake of 7.1 g/100 kg BW. Phosphorus was mainly excreted in feces and both total fecal P and Pi excretion had a strong relationship to P intake. More than 80% of total P appeared to be soluble. Fecal endogenous P losses were similar to those described previously in growing horses.
authors.affiliation: Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, 75007 Uppsala, Sweden.
authors.forename: G
authors.initials: G
authors.lastname: Ögren
authors.forename: K
authors.initials: K
authors.lastname: Holtenius
authors.forename: A
authors.initials: A
authors.lastname: Jansson
mesh_terms: D000821:Animal Feed / Q000032:analysis*; D000818:Animals; D004032:Diet / Q000662:veterinary; D019587:Dietary Supplements / Q000032:analysis; D005243:Feces / Q000737:chemistry; D006736:Horses / Q000254:growth & development / Q000378:metabolism*; D008297:Male; D008903:Minerals / Q000008:administration & dosage / Q000652:urine*; D016226:Phosphorus, Dietary / Q000008:administration & dosage / Q000378:metabolism*; D013053:Spectrophotometry / Q000662:veterinary
chemical_list: D008903:Minerals; D016226:Phosphorus, Dietary