Recommended concentrations of dietary phosphorus (P) for lactating dairy cows have been reduced in recent years in order to reduce the excretion of P in manure. This reduction in manure P has resulted in less P spread on dairy farm land thus reducing the amount of P moving with water to adjacent land and streams. The target concentration of dietary P for lactating Holstein cows is now between 0.32 to 0.38% of dietary dry matter depending on milk yield. Researchers in Wisconsin and Pennsylvania reported that feeding a diet below this lower concentration (0.32%) resulted in less milk production. Diets fed to milking cows can still be in excess of target P values if several feeds are included in the diet that contain a high P concentration such as brewers grains (0.59%), whole cottonseeds (0.60%), hominy (0.65%), distillers grains (0.83%), corn gluten feed (1.00%), wheat midds (1.02%), cottonseed meal (1.15%), wheat bran (1.18%), and rice bran (1.78%). The inclusion of these feeds in the ration may be attractive because of their lower market price from time to time.
Movement of P from manure depends upon its solubility. We conducted a study to try to reduce the solubility of P in feces by feeding more calcium (Ca). The theory was that the extra Ca in the diet would bind with the P to form a less soluble complex (hydroxylapatite or whitlockite) resulting in feces containing P in a less soluble form, yet the same concentration of total P. Lactating Holstein cows were fed diets of 0.38% P that also contained either 0.64% or 0.95% Ca (dry matter basis). The source of Ca was either calcium carbonate or calcium chloride. Milk production averaged 76 lb during the 63-day study. Feces were collected from the cows and dried. Fecal samples were washed with water ten consecutive times to simulate long-term effects of a wet environment on P movement. Phosphorus was measured in the water extract after each washing. The extent of P extracted was reduced from an average of 48% to 38% when the dietary concentration of Ca was increased from 0.64 to 0.95%. When applying this to the cows in this study, soluble P is decreased from 16 to 11 grams per cow per day. When projected over a year’s time for a 500-cow dairy, an extra 2010 pounds of P would remain on farm land rather than leaving through leaching. Calcium in the carbonate and chloride form were both effective but the calcium carbonate is the preferred form due to better fat-corrected milk production and market price for calcium carbonate. X-ray diffraction analysis of the fecal samples verified that more of the fecal P was in the unavailable form form when cows were fed more Ca. Production and composition of milk as well as feed intake were not affected by feeding more Ca. Digestibility of nutrients including P was not changed. Solubility of P in dairy cow feces can be reduced preemptively by increased dietary supply of Ca when cows are fed the recommended dietary concentration of P.