| Soils receiving dairy manure (mixture of feces with urine) over long periods of time can be a non-point source of phosphorus (P) that potentially can degrade water quality. My research objectives were to evaluate the combined effect of physiological state and diet on dairy-feces P solubility and to test the hypothesis that fecal P solubility can be decreased by increasing available calcium (Ca) relative to magnesium (Mg) in the diet of lactating dairy cows. Feed and fecal samples were collected from animals in different physiological states and experimental multiparous cows (24) were fed eight diets with a 2x2x2 factorial arrangement involving two dietary Ca sources, two dietary Ca and Mg concentrations. Feed and fecal samples were collected and chemically analyzed for nutritive parameters, total and water-extractable P, Ca and Mg. The combined effect of physiological state and diet yielded fecal samples with different water-extractable P (WEP), despite comparable dietary P concentrations. Dietary treatments had little practical effects on animal performance parameters, digestibility of nutrients, and overall P balance of lactating dairy cows; however addition of CaCl2 had a tendency to decrease dry matter intake (DMI). Fecal samples with higher Ca and Mg concentrations showed reduced WEP; possibly, high Ca and Mg concentrations mutually suppressed dissolution of Ca,Mg-P forms by the common ion effect. This finding was supported by consecutive extraction data, SEM solid-state analysis and by XRD results from ashed fecal samples where hydroxyapatite (HAP), HAP plus Ca,Mg-P, and Ca,Mg-P were the P forms found in ashed fecal samples for high, intermediate, and low dietary available Ca, respectively. Increasing Ca concentration in the diet of lactating dairy cows preemptively reduced P release from incubated feces-soil mixtures. This effect was most pronounced in soils with low P retention capacity. No further P stabilization effect of high available dietary Ca was observed over a 42 week period in soil-feces incubations. This lack of a time effect suggests that Ca, Mg, and P interactions in the gastro intestinal tract (GIT) may be the major determinant of the subsequent environmental fate of fecal P; formation of stable Ca-P forms are determined in the GIT and changes in P forms upon application of feces to soils are unlikely. |
