title: Phosphorus concentration and solubility in dairy feces: variability and affecting factors.
abstract: Recent data from phosphorus (P) feeding trials have demonstrated that P concentration in dairy feces is directly affected by P levels in diets and that farm P surpluses as well as potential environmental losses can be reduced through dietary manipulation. The current study was conducted to examine the variability of fecal P under farm conditions and to elucidate factors affecting the concentration and solubility of fecal P. Feed and fecal samples from >30 commercial dairies in the Northeast and Mid-Atlantic regions were analyzed. Dietary P concentrations ranged from 3.45 to 5.78 g/kg of feed DM (DM), and P determined in acid digests (TP) of feces from 5.84 to 12.84 g/kg of fecal DM. On average, 50% of fecal TP was water soluble; of the latter, 83% was inorganic (Pi). Across-farm variability (n=33) had CV averaging 18.9% for fecal TP and >20% for Pi and total P (Pt) in water extracts. Within-farm variability based on multiple samples per herd had the same magnitude as across-farm and was independent of sample numbers from individual farms (n=7 to 30). Of all fecal parameters determined, pH and DM had the lowest variability (CV <10%), water-soluble Pi, Pt, and Ca the highest (CV of 20 to 30%), and total P, Ca, and Mg determined by acid digests were intermediate (CV 10 to 20%). Water-soluble Pi concentrations determined in dried-ground fecal samples were lower than in wet samples. The drying-grinding process changes Pi solubility and the change is not linear. This study confirms that dietary P concentration is the dominating factor affecting fecal P excretion; however, Ca concentration, DIM, and fecal pH also made small, but statistically significant contributions, although some of the mechanisms remain to be thoroughly investigated.
authors.affiliation: Center for Animal Health and Productivity, School of Veterinary Medicine, University of Pennsylvania Kennett Square, 19348, USA.
authors.forename: L
authors.initials: L
authors.lastname: Chapuis-Lardy
authors.forename: J
authors.initials: J
authors.lastname: Fiorini
authors.forename: J
authors.initials: J
authors.lastname: Toth
authors.forename: Z
authors.initials: Z
authors.lastname: Dou
mesh_terms: D000821:Animal Feed / Q000032:analysis*; D000818:Animals; D002417:Cattle / Q000378:metabolism; D004785:Environmental Pollutants / Q000032:analysis*; D004787:Environmental Pollution / Q000517:prevention & control; D005243:Feces / Q000737:chemistry*; D005260:Female; D010758:Phosphorus / Q000008:administration & dosage / Q000032:analysis* / Q000378:metabolism; D016226:Phosphorus, Dietary / Q000008:administration & dosage / Q000378:metabolism*; D012995:Solubility
chemical_list: D004785:Environmental Pollutants; D016226:Phosphorus, Dietary; D010758:Phosphorus