Establishing a Linkage between Phosphorus Forms in Dairy Diets, Feces, and Manures

Effective manure management to efficiently utilize organic wastes without causing environmental degradation requires a clear understanding of the transformation of P forms from diet to manure. Thus, the objective of this study was to establish quantitative relationships between P forms in diets, feces, and manures collected from U.S. Northeastern and Mid‐Atlantic commercial dairy farms. Total P in diets ranged from 3.6 to 5.3 g kg −1 dry matter, while the feces had higher P than diets (5.7–9.5 g kg −1 ) and manures had lower P (2.5–8.9 g kg −1 ) than feces. The farms with total dietary P of 4.8 to 5.3 g P kg −1 had twofold higher concentrations of phytic acid (1647–2300 mg P kg −1 ) than farms with 3.6 to 4.0 g dietary P kg −1 (844–1100 mg P kg −1 ). Much of the phytic acid in diets was converted to inorganic orthophosphate in the rumen as indicated by a reduction in phytic acid percentage from diets (32%) to feces (18%). The proportion of orthophosphate diesters (phospholipids, deoxyribonucleic acid [DNA]) was twice as high in feces (6.2–10%) as diets (2.4–5.3%) suggesting the excretion of microbial residues in feces. Phosphonates (aminoethyl phosphonates and phosphonolipids) were not seen in diets but were detected in feces and persisted in manures, which suggests a microbial origin. These organic compounds (phytic acid, phospholipids, DNA) were decomposed on storage of feces in slurry pits, increasing orthophosphate in manures by 9 to 12% of total P. These results suggest that reducing dietary P and typically storing feces in dairy farms will result in manure with similar chemical forms (primarily orthophosphate: 63–77%) that will be land applied. Thus, both the reduction of dietary P and storage of manure on farm are important for controlling solubility and bioavailability of P forms in soils and waters.