By Ev Thomas and Bill Mahanna

Adapted from “Field to Feedbunk” column, Hoards Dairyman, Oct 10, 2011

Dairy cows need phosphorus for milk production, growth and pregnancy ... anything more than needed for these purposes is excreted. Dairy cattle feeding studies suggest 55% to 65% of dietary P is excreted in the manure with annual excretion averaging about 90 pounds of P2O5 per cow.

Phosphorus doesn't volatilize, and it doesn't leach unless soil levels become extremely high. The P in dairy manure is highly plant-available, so most of this excreted phosphorus is available for crop uptake and for increasing soil levels. It was once thought that for good reproductive efficiency, cows needed a lactating ration P concentration of 0.50% to 0.60%. However, research has shown that dairy cows don't need more than 0.40% P to support high milk production and good reproductive performance.

Feeding more phosphorus than necessary is expensive and raises manure's P content. A seemingly small change in ration P can result in a big difference in the amount of phosphorus applied to cropland. For instance, feeding a ration with 0.40% P results in annual manure output of 90 pounds of P2O5 per cow, while a ration with 0.45% P results in 105 pounds. Fifteen pounds of P2O5 may not sound like much, but multiplied by the number of cows on a farm and accumulating year after year, it really adds up.

Cows and crops are similar in that neither uses more P than is needed. Apply the recommended rate of P2O5 (as commercial fertilizer or dairy manure), and both alfalfa and corn harvested for silage will take up about 80 pounds per acre. This recommendation assumes alfalfa hay yield of 5 tons per acre and corn silage yield of 20 tons per acre. Apply twice or three times this much phosphorus and the crops will still take up about the same amount, 80 pounds per acre, while the excess will remain in the tillage layer to build soil fertility.

About 20 years ago, the Miner Institute dairy farm (Chazy, N.Y.) showed practical ways to reduce phosphorus. Only when they lowered ration P concentration from the 0.45% to 0.50% range to the 0.38% to 0.40% range did they make real progress in the farm's P balance. How much of a difference did these changes make? The P content of Miner Institute's dairy manure, which contained 1.8 pounds of P2O5 per ton (100 percent dry matter basis) before they started reducing P inputs, fell to 0.9 pounds per ton after reducing both fertilizer and ration P. Concentrations of other crop nutrients in the dairy manure changed little.

Reducing ration P concentration provided much more flexibility in manure management and saved money on both ends: They spent less money spent for phosphorus in the dairy rations and less money spent for fertilizer because the manure needed to meet the crop's phosphorus requirements supplied twice as much nitrogen, potassium and minor nutrients.

One species has four legs and feeds above the ground, while the other has a root system and gets its nutrition from below the ground, but both have this in common: They need only so much phosphorus for efficient production.

The foregoing is provided for informational purposes only. Please consult with your nutritionist or veterinarian for suggestions specific to your operation. Product performance is variable and subject to a variety of environmental, disease, and pest pressures. Individual results may vary.