Minimizing phosphorus (P) intake to concentrations that will not compromise milk production or animal health has had only limited success in reducing P in manure. Elevated soil P from manure-impacted soils has been associated with non-point source of pollution to water bodies. Conditions found in manure amended soils (high Ca, P, and pH) should favor calcium phosphate stability, but no crystalline phosphate minerals have been identified. The objectives of this study were i) evaluate P, Ca and Mg release from three groups of dairy cows in different physiological stages ii) asses the in vitro (ruminal and post-ruminal) release of P, Ca and Mg from feeds and compare it with that of the manure for the three groups. Manure was collected from heifers, close-up, and lactating dairy cows. Cumulative and individual P release from 10 successive water extractions (1:50 manure:water ratio) shows significant difference (P < 0.0001) among groups. Phosphorus release is significantly associated with Ca and Mg release (P < 0.0001). Water extractable P after 10 successive extractions is unrelated to total P in manure; heifer‘s manure released the most P as a percentage of TP in manure.
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